Patent Publication Number: US-11650742-B2

Title: Accessing stored metadata to identify memory devices in which data is stored

Description:
FIELD OF THE TECHNOLOGY 
     At least some embodiments disclosed herein relate to memory systems in general and more particularly, but not limited to accessing stored metadata to identify memory devices of a memory system in which data is stored. 
     BACKGROUND 
     Various types of memory devices can be used to store data in the main memory of a computer system. One type of volatile memory device is a dynamic random access memory (DRAM) device. Various types of non-volatile memory device can include a NAND flash memory device or a non-volatile random access memory (NVRAM) device. 
     In an operating system, memory management is responsible for managing the main memory of the computer system. The memory management tracks the status of memory locations in the main memory (e.g., a memory status of either allocated or free). Memory management further determines the allocation of memory among various processes running on the operating system. When memory is allocated to a process, the operating system determines the memory locations that will be assigned to the process. 
     In one approach, an operating system uses paged allocation to divide the main memory into fixed-sized units called page frames. A virtual address space of a software program is divided into pages having the same size. A hardware memory management unit maps pages to frames in physical memory. In a paged memory management approach, each process typically runs in its own address space. 
     In some cases, a memory management unit (MMU) is called a paged memory management unit (PMMU). The MMU manages all memory references used by the operating system and performs the translation of virtual memory addresses to physical addresses. The MMU typically divides a virtual address space, which is the range of addresses used by the processor, into pages. 
     In some approaches, the MMU uses a page table containing page table entries to map virtual page numbers to physical page numbers in the main memory. In some cases, a cache of page table entries called a translation lookaside buffer (TLB) is used to avoid the need to access a page table stored in the main memory when a virtual address is mapped. When using virtual memory, a contiguous range of virtual addresses can be mapped to several non-contiguous blocks of physical memory. 
     More generally, a computer system can have one or more memory sub-systems. A memory sub-system can be a memory module, such as a dual in-line memory module (DIMM), a small outline DIMM (SO-DIMM), or a non-volatile dual in-line memory module (NVDIMM). A memory sub-system can be a storage system, such as a solid-state drive (SSD), or a hard disk drive (HDD). A memory sub-system can include one or more memory components that store data. The memory components can be, for example, non-volatile memory components and volatile memory components. Examples of memory components include memory integrated circuits. Some memory integrated circuits are volatile and require power to maintain stored data. Some memory integrated circuits are non-volatile and can retain stored data even when not powered. Examples of non-volatile memory include flash memory, Read-Only Memory (ROM), Programmable Read-Only Memory (PROM), Erasable Programmable Read-Only Memory (EPROM) and Electronically Erasable Programmable Read-Only Memory (EEPROM) memory, etc. Examples of volatile memory include Dynamic Random-Access Memory (DRAM) and Static Random-Access Memory (SRAM). In general, a computer system can utilize a memory sub-system to store data at the memory components and to retrieve data from the memory components. 
     For example, a computer system can include one or more memory sub-systems attached to the computer system. The computer system can have a central processing unit (CPU) in communication with the one or more memory sub-systems to store and/or retrieve data and instructions. Instructions for a computer can include operating systems, device drivers, and application programs. An operating system manages resources in the computer and provides common services for application programs, such as memory allocation and time sharing of the resources. A device driver operates or controls a particular type of device in the computer; and the operating system uses the device driver to offer resources and/or services provided by the type of device. A central processing unit (CPU) of a computer system can run an operating system and device drivers to provide the services and/or resources to application programs. The central processing unit (CPU) can run an application program that uses the services and/or resources. For example, an application program implementing a type of application can instruct the central processing unit (CPU) to store data in the memory components of a memory sub-system and retrieve data from the memory components. 
     An operating system of a computer system can allow an application program to use virtual addresses of memory to store data in, or retrieve data from, memory components of one or more memory sub-systems of the computer system. The operating system maps the virtual addresses to physical addresses of one or more memory sub-systems connected to the central processing unit (CPU) of the computer system. The operating system translates the memory accesses specified at virtual addresses to the physical addresses of the memory sub-systems. 
     A virtual address space can be divided into pages. A page of virtual memory can be mapped to a page of physical memory in the memory sub-systems. The operating system can use a paging technique to access a page of memory in a storage device via a page of memory in a memory module. At different time instances, the same virtual page of memory in a memory module can be used as a proxy to access different physical pages of memory in the storage device or another storage device in the computer system. 
     A computer system can include a hypervisor (or virtual machine monitor) to create or provision virtual machines. A virtual machine is a computing device that is virtually implemented using the resources and services available in the computer system. The hypervisor presents the virtual machine to an operating system as if the components of virtual machine were dedicated physical components. A guest operating system runs in the virtual machine to manage resources and services available in the virtual machine, in a way similar to the host operating system running in the computer system. The hypervisor allows multiple virtual machines to share the resources of the computer system and allows the virtual machines to operate on the computer substantially independently from each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar elements. 
         FIG.  1    illustrates an example computer system having a memory sub-system, in accordance with some embodiments. 
         FIG.  2    shows a mobile device that accesses different types of memory in a memory module using a memory bus, in accordance with some embodiments. 
         FIG.  3    illustrates an example computer system that stores metadata used to access memory devices in a memory sub-system, in accordance with some embodiments. 
         FIG.  4    shows a memory module configured for memory bus access by a host computer system to volatile and non-volatile memory of the memory module, in accordance with some embodiments. 
         FIG.  5    shows a host operating system accessing a memory module using memory bus access, in accordance with at least some embodiments. 
         FIG.  6    shows a method for managing memory for processes in an address space of a computer system based on stored metadata that associates virtual address ranges for the processes in the address space with physical addresses for memory devices in the computer system, in accordance with some embodiments. 
         FIG.  7    is a block diagram of an example computer system in which embodiments of the present disclosure can operate. 
     
    
    
     DETAILED DESCRIPTION 
     At least some embodiments herein relate to accessing stored metadata to identify memory devices of a memory system in which data is stored. In various embodiments as discussed herein, the metadata can be stored and accessed by various types of computer systems. In one example, the computer system is a system-on-chip (SoC) device that stores metadata for managing memory usage by one or more processes running on the SoC device. In one example, a mobile device uses a SoC device to manage allocation of main memory for one or more applications that are running on the mobile device. 
     Prior computer systems often use different types of memory devices for storing data. One type of memory device typically used is DRAM, which is generally considered to provide fast read and write access. DRAM is commonly used for storing data in the main memory of a computer system. 
     Other memory devices, such as flash memory, are typically considered to be slower than DRAM. For example, read or write access latency for a DRAM is typically significantly less than read or write access latency for flash memory. As a particular example, write access latency for some memory devices can be tens or even hundreds of times greater than for DRAM devices. 
     In prior computer systems that use different types of physical memory devices for storing data in main memory, a technical problem exists in which the processor does not have an awareness of how memory for various processes is actually mapped to the memory devices. For example, the processor may allocate a virtual address range to a process. However, the processor is unaware of how the virtual address range is mapped to the different memory devices. 
     In one example, if the virtual address range for a process is mapped to physical memory devices (e.g., flash memory) that are significantly slower than other memory devices (e.g., DRAM), then the process can be forced to run slowly due to an inability to rapidly access data from main memory that is needed for continuing execution of the process. For example, the process may need a response from main memory in order to continue data computations or other processing (e.g., a response that includes data for a read access request made by a processor to main memory to obtain data needed during execution by the process). If the data needed from main memory is actually stored in a slow physical memory device, then the processing is significantly delayed while waiting for the response. 
     Various embodiments of the present disclosure provide a technological solution to one or more of the above technical problems. In some embodiments, a computer system stores data regarding the latency of memory devices used by the computer system (e.g., memory devices used to provide main memory). In one example, the latencies of various memory regions that are visible to a processor of the computer system are known (e.g., as represented by information collected and/or aggregated in stored metadata, as discussed below). 
     In some embodiments, the processor, an operating system, and/or an application (as programmed by a software designer) can initiate and/or perform actions by the computer system to avoid significant process delays due to slow memory access. For example, a high priority process that needs fast memory response can be configured to run in DRAM. 
     In another example, a priority for an application executing on a mobile device can be monitored. When the priority for the application increases (e.g., changes from low to high), then the processor and/or operating system can automatically transfer the application out of an address range of main memory that corresponds to a slow memory device, and move the application to a new address range that corresponds to a fast memory device. 
     In one example, memory device types include DRAM, NVRAM, and NAND flash. The priority of a process is determined by the processor (e.g., based on data usage patterns by the process). Based on stored metadata regarding address range mapping to these memory device types, the processor allocates the process to an address range having an appropriate memory latency. For example, the processor can determine whether a process has a low, intermediate, or high priority. Based on determining that the process has an intermediate priority, software and/or data associated with the process are stored in an address range corresponding to physical storage in the NVRAM memory device type, which has an intermediate latency. 
     In one example, the NVRAM device type is a 3D XPoint memory. In one example, the NVRAM device type can be resistive random-access memory, magnetoresistive RAM, phase-change RAM, and/or ferroelectric RAM. In one example, an NVRAM chip is used as main memory of a computer system (e.g., NVDIMM-P). In one example, an NVRAM device is implemented using non-volatile 3D XPoint memory in a DIMM package. 
     In another example, if the processor and/or operating system are not configured to automatically transfer the application to a different address range in response to a priority change, the software code of the application itself can be configured to read one or more values from the stored metadata. Based on the read values, the application itself can manage data storage so that data is preferentially stored in address ranges that correspond to faster memory devices. In one example, the application can determine relative latencies of available memory devices in a computer system based on reading or otherwise being provided access to the stored metadata. In one example, the stored metadata specifies what data is on which memory device of various different memory devices having different latencies. By specifying the memory device in this manner, the application can determine a latency of access for particular data depending on the memory device being used to store the data. 
     In one example, an application on a mobile device reads stored metadata when requesting an allocation of main memory by an operating system (e.g., executing on a system-on-chip device). In one example, the application makes a request for an address range in main memory that corresponds to a particular type of memory device and/or a particular latency associated with memory read or write access. 
     In one example, the application reads or otherwise accesses the stored metadata to determine which memory is fast, and which memory is slow. In a first context of the mobile device, the application makes a request for allocation of fast memory. In a second context of the mobile device, the application makes a request for allocation of slow memory. In one example, in response to the detection of a predetermined context, the application initiates or makes a request for a change in allocation of memory. In one example, the application determines a change in context based on an updated query made to the stored metadata (e.g., by the processor), and/or data (e.g., operating characteristics of the mobile device) provided to the application by the processor of the computer system. 
     In one embodiment, a computer system includes a first memory device (e.g., DRAM) and a second memory device (e.g., NVRAM or NAND flash), and one or more processing devices (e.g., a CPU or system on a chip (SoC)). The computer system further includes memory containing instructions configured to instruct the one or more processing devices to: access memory in an address space maintained by an operating system, the accessing including accessing the first memory device and the second memory device using addresses in the address space; store metadata that associates a first address range of the address space with the first memory device, and a second address range of the address space with the second memory device; and manage, by the operating system based on the stored metadata, processes including a first process and a second process, where data for the first process is stored in the first memory device, and data for the second process is stored in the second memory device. 
     In one embodiment, a computer system uses memory device types including DRAM, NVRAM, and NAND flash. In one example, the DRAM is faster than the NVRAM, which is faster than the NAND flash. The computer system is configured so that all three different types of memory can be accessed directly by a processor of the computer system using a virtual memory address. In one example, the processor communicates with a memory management unit to implement a virtual to physical address mapping system. 
     In one embodiment, an application is not pre-programmed or otherwise configured to manage or handle optimization of memory allocation based on different types of memory devices. For example, this may occur for legacy software programs. In this type of situation, an operating system can be configured to manage optimization of memory allocation for the application. 
     In one example, the operating system detects or otherwise determines one or more characteristics of an application. Based on the characteristics, the operating system uses the stored metadata to assign one or more address ranges in main memory to the application. In one example, the characteristics are determined based on information provided by the application itself (e.g., when the application is launched on a mobile device). In another example, the characteristics are provided by a computing device other than the computer system on which the application is being executed. In one example, a central repository is used to store and update a database or table of characteristics for applications. In one example, the central server provides an indication to the operating system regarding a type of physical memory to use. 
     In one embodiment, the operating system determines a context associated with a computer system and/or execution of an application. Based on this context, the operating system uses the stored metadata to assign one or more address ranges in main memory to the application. 
     In one embodiment, stored metadata is used for identifying devices in which data is stored. A memory sub-system has multiple physical memory devices (e.g., DRAM, NVRAM, and NAND flash) that can be addressed by a processor (e.g., SoC) in a memory address space. The metadata is used to specify which memory address regions are mapped to which physical memory devices. The metadata can be loaded into the DRAM and/or the processor (e.g., loaded into a cache of the processor) to determine what data is on which device, and/or used to estimate the latency of access for the respective data. 
     In one embodiment, an application is executing on a mobile device having a processor that uses a main memory. The application requests that an operating system of the mobile device allocate a portion of main memory for use by the application. The allocated memory is in a logical/virtual memory space (e.g., memory addresses as seen by the programmer and by the execution units of the processor are virtual). In one embodiment, the virtual memory addresses are mapped to real/physical memory by page tables. A portion of the mapping data in the page tables is cached in a buffer of the processor. In one example, the buffer is a translation lookaside buffer (TLB). 
     In one embodiment, a computer system includes DRAM, NVRAM, and NAND flash memory devices. A processor of the computer system randomly accesses main memory by address. Addresses within the main memory correspond to physical locations of data storage on these three types of memory devices. In one example, each of the devices is accessed by the processor using a synchronous memory bus. In one example, the DRAM is synchronous dynamic random access memory (SDRAM) having an interface synchronous with a system bus carrying data between a CPU and a memory controller hub. 
       FIG.  1    illustrates an example computing environment  100  having a memory sub-system  110 , in accordance with some embodiments. The memory sub-system  110  can include media, such as memory components  109 A to  109 N. The memory components  109 A to  109 N can be volatile memory components, non-volatile memory components, or a combination of such. In some embodiments, the memory sub-system  110  is a memory module. Examples of a memory module include a DIMM and an NVDIMM. In some embodiments, the memory sub-system  110  is a hybrid memory/storage sub-system. In general, the computing environment  100  can include a computer system  120  that uses the memory sub-system  110 . For example, the computer system  120  can write data to the memory sub-system  110  and read data from the memory sub-system  110 . 
     The computer system  120  can be a computing device such as a mobile device, IoT device, desktop computer, laptop computer, network server, or such computing device that includes a memory and a processing device. The computer system  120  can include or be coupled to the memory sub-system  110  so that the computer system  120  can read data from or write data to the memory sub-system  110 . The computer system  120  can be coupled to the memory sub-system  110  via a physical host interface. As used herein, “coupled to” generally refers to a connection between components, which can be an indirect communicative connection or direct communicative connection (e.g., without intervening components), whether wired or wireless, including connections such as electrical, optical, magnetic, etc. Examples of a physical host interface include, but are not limited to, a serial advanced technology attachment (SATA) interface, a peripheral component interconnect express (PCIe) interface, universal serial bus (USB) interface, Fiber Channel, Serial Attached SCSI (SAS), a double data rate (DDR) memory bus, etc. The physical host interface can be used to transmit data between the computer system  120  and the memory sub-system  110 . The computer system  120  can further utilize an NVM Express (NVMe) interface to access the memory components  109 A to  109 N when the memory sub-system  110  is coupled with the computer system  120  by the PCIe interface. The physical host interface can provide an interface for passing control, address, data, and other signals between the memory sub-system  110  and the computer system  120 . 
       FIG.  1    illustrates memory sub-system  110  as an example. In general, the computer system  120  can access multiple memory sub-systems via a shared communication connection, multiple separate communication connections, and/or a combination of communication connections. In one example, each memory sub-system  110  can be a different type of memory device that is randomly accessed by processing device  118  over a memory bus. 
     The computer system  120  includes the processing device  118  and a controller  116 . The processing device  118  can be, for example, a microprocessor, a central processing unit (CPU), a processing core of a processor, an execution unit, etc. In some instances, the controller  116  can be referred to as a memory controller, a memory management unit, and/or an initiator. In one example, the controller  116  controls the communications over a bus coupled between the computer system  120  and one or more memory sub-systems  110 . 
     In general, the controller  116  can send commands or requests to the memory sub-system  110  for desired access to memory components  109 A to  109 N. The controller  116  can further include interface circuitry to communicate with the memory sub-system  110 . The interface circuitry can convert responses received from memory sub-system  110  into information for the computer system  120 . 
     The controller  116  of the computer system  120  can communicate with controller  115  of the memory sub-system  110  to perform operations such as reading data, writing data, or erasing data at the memory components  109 A to  109 N and other such operations. In some instances, the controller  116  is integrated within the same package of the processing device  118 . In other instances, the controller  116  is separate from the package of the processing device  118 . The controller  116  and/or the processing device  118  can include hardware such as one or more integrated circuits and/or discrete components, a buffer memory, a cache memory, or a combination thereof. The controller  116  and/or the processing device  118  can be a microcontroller, special purpose logic circuitry (e.g., a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), etc.), or another suitable processor. 
     The memory components  109 A to  109 N can include any combination of various different types of non-volatile memory components and/or volatile memory components. An example of a non-volatile memory component includes a negative-AND (NAND) type flash memory. In one example, each of the memory components  109 A to  109 N can include one or more arrays of memory cells such as single level cells (SLCs) or multi-level cells (MLCs) (e.g., triple level cells (TLCs) or quad-level cells (QLCs)). In some embodiments, a particular memory component can include both an SLC portion and a MLC portion of memory cells. Each of the memory cells can store one or more bits of data (e.g., data blocks) used by the computer system  120 . 
     Although non-volatile memory components such as NAND type flash memory are one example, the memory components  109 A to  109 N can be based on any other type of memory such as a volatile memory. In some embodiments, the memory components  109 A to  109 N can be, but are not limited to, random access memory (RAM), read-only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), phase change memory (PCM), magneto random access memory (MRAM), Spin Transfer Torque (STT)-MRAM, ferroelectric transistor random-access memory (FeTRAM), ferroelectric RAM (FeRAM), conductive bridging RAM (CBRAM), resistive random access memory (RRAM), oxide based RRAM (OxRAM), negative-or (NOR) flash memory, electrically erasable programmable read-only memory (EEPROM), nanowire-based non-volatile memory, memory that incorporates memristor technology, and a 3D XPoint array of non-volatile memory cells. A cross-point array of non-volatile memory can perform bit storage based on a change of bulk resistance, in conjunction with a stackable cross-gridded data access array. Additionally, in contrast to many flash-based memories, cross-point non-volatile memory can perform a write in-place operation, where a non-volatile memory cell can be programmed without the non-volatile memory cell being previously erased. Furthermore, the memory cells of the memory components  109 A to  109 N can be grouped as memory pages or data blocks that can refer to a unit of the memory component used to store data. 
     The controller  115  of the memory sub-system  110  can communicate with the memory components  109 A to  109 N to perform operations such as reading data, writing data, or erasing data at the memory components  109 A to  109 N and other such operations (e.g., in response to commands scheduled on a command bus by controller  116 ). The controller  115  can include hardware such as one or more integrated circuits and/or discrete components, a buffer memory, or a combination thereof. The controller  115  can be a microcontroller, special purpose logic circuitry (e.g., a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), etc.), or another suitable processor. The controller  115  can include a processing device  117  (processor) configured to execute instructions stored in local memory  119 . In the illustrated example, the local memory  119  of the controller  115  includes an embedded memory configured to store instructions for performing various processes, operations, logic flows, and routines that control operation of the memory sub-system  110 , including handling communications between the memory sub-system  110  and the computer system  120 . In some embodiments, the local memory  119  can include memory registers storing memory pointers, fetched data, etc. The local memory  119  can also include read-only memory (ROM) for storing micro-code. While the example memory sub-system  110  in  FIG.  1    has been illustrated as including the controller  115 , in another embodiment of the present disclosure, a memory sub-system  110  may not include a controller  115 , and can instead rely upon external control (e.g., provided by an external host, or by a processor or controller separate from the memory sub-system). 
     In general, the controller  115  can receive commands or operations from the computer system  120  and can convert the commands or operations into instructions or appropriate commands to achieve the desired access to the memory components  109 A to  109 N. The controller  115  can be responsible for other operations such as wear leveling operations, garbage collection operations, error detection and error-correcting code (ECC) operations, encryption operations, caching operations, and address translations between a logical block address and a physical block address that are associated with the memory components  109 A to  109 N. The controller  115  can further include host interface circuitry to communicate with the computer system  120  via the physical host interface. The host interface circuitry can convert the commands received from the computer system into command instructions to access the memory components  109 A to  109 N as well as convert responses associated with the memory components  109 A to  109 N into information for the computer system  120 . 
     The memory sub-system  110  can also include additional circuitry or components that are not illustrated. In some embodiments, the memory sub-system  110  can include a cache or buffer  121  (e.g., DRAM or SRAM) and address circuitry (e.g., a row decoder and a column decoder) that can receive an address from the controller  115  and decode the address to access the memory components  109 A to  109 N. 
     The computing environment  100  includes a metadata component  113  in the computer system  120  that stores metadata used to identify memory devices in which data is stored (e.g., as discussed in various embodiments above). A portion of metadata component  113  can reside on computer system  120  and/or memory sub-system  110 . In one example, a portion of the metadata is stored in local memory  119  and/or buffer  121 . In one example, a portion of the metadata is alternatively and/or additionally stored in a cache of controller  116  (e.g., stored in a translation lookaside buffer). 
     In one example, the memory sub-system  110  can provide access for computer system  120  to data in different types of memory devices via a DDR or other type of synchronous memory bus. In one embodiment, the access is provided to data in NVRAM on a DIMM and to data in a DRAM. In one example, data is made accessible in a random access memory address space of the computer system  120  for access during host read/write requests made over the DDR memory bus. 
     In one example, computer system  120  sends a page-in request (for access to a page) to controller  115 . In response to receiving the page-in request, controller  115  moves a page from a slow media such as a non-volatile memory device to a volatile memory device (e.g., DRAM on memory sub-system  110 ). 
     In one example, computer system  120  sends a page-out request to controller  115 . In response to receiving the page-out request, controller  115  moves data out of volatile memory (e.g., DRAM on memory sub-system  110 ) to non-volatile memory via buffer  121 . 
     In some embodiments, the controller  116  and/or the processing device  118  in the computer system  120  includes at least a portion of the metadata component  113 . For example, the controller  116  and/or the processing device  118  can include logic circuitry implementing the metadata component  113 . For example, the processing device  118  (processor) of the computer system  120  can be configured to execute instructions stored in memory for performing operations that identify in which devices data is stored for the metadata component  113  as described herein. In some embodiments, the metadata component  113  is part of an operating system of the computer system  120 , a device driver, or an application (e.g., an application executing on computer system  120 ). 
     In some embodiments, the controller  115  and/or the processing device  117  in the memory sub-system  110  includes at least a portion of the metadata component  113 . For example, the controller  115  and/or the processing device  117  can include logic circuitry implementing the metadata component  113 . 
     In one example, a central processing unit (CPU) can access memory in a memory system connected to the CPU. For example, the central processing unit (CPU) can be configured to access the memory based on a query to stored metadata of metadata component  113 . 
       FIG.  2    shows a mobile device  200  that accesses different types of memory in a memory module  205  using a memory bus  203 , in accordance with some embodiments.  FIG.  2    shows a computer system having different types of memory. The computer system of  FIG.  2    includes a mobile device  200 , and a memory module  205  connected to the mobile device  200  via memory bus  203 . The memory module  205  is an example of the memory sub-system  110  illustrated in  FIG.  1   . 
     The mobile device  200  includes processing device  118 , which can be a central processing unit or a microprocessor with one or more processing cores. The mobile device  200  can have a cache memory  211 . At least a portion of the cache memory  211  can be optionally integrated within the same integrated circuit package of the processing device  118 . 
     The memory module  205  illustrated in  FIG.  2    has multiple types of memory (e.g.,  221  and  223 ). For example, memory of type A  221  (e.g., DRAM) is faster than memory of type B  223  (e.g., NVRAM). For example, the memory bus  203  can be a double data rate bus. In general, several memory modules (e.g.,  205 ) can be coupled to the memory bus  203 . 
     The processing device  118  is configured via instructions (e.g., an operating system and/or one or more device drivers) to access a portion of memory in the computer system using metadata component  113 . For example, memory of type B  223  (e.g., NVRAM) of the memory module  205  can be accessed or memory of type A  221  (e.g., DRAM) of the memory module  205  can be accessed. In one embodiment, memory of type B  223  of the memory module  205  is accessible only through addressing the memory of type A  221  of the memory module  205 . 
     A controller  227  can be provided in the memory module  205  to manage data access to the memory of type A  221  and the memory of type B  223 . In one embodiment, controller  227  multiplexes access to DRAM or NVRAM by mobile device  200  and memory module  205  when transferring data to or from buffer  121 . In one example, memory bus  203  provides a host DDR channel as the DDR interface between mobile device  200  and memory module  205 . In one example, once a page is retrieved from NVRAM memory into buffer  121 , the page can be loaded for access by the mobile device via a conventional DDR4 slot (e.g., a host DDR channel). 
     In general, the memory sub-systems (e.g.,  205 ) can include media, such as memory (e.g.,  221 , . . . ,  223 ). The memory (e.g.,  221 , . . . ,  223 ) can include volatile memory, non-volatile memory (NVM), and/or a combination of such. The processing device  118  can write data to each of the memory sub-systems (e.g., memory module  205 ) and read data from the memory sub-systems (e.g., memory module  205 ) directly or indirectly. 
     In one embodiment, memory module  205  provides memory bus access to non-volatile memory or volatile memory by using buffer  121 . In one example, memory module  205  is a DIMM coupled to a mobile device  200  via a DDR bus. The storage media is, for example, cross-point memory. 
     In one embodiment, the mobile device communicates with the memory module via a communication channel for read/write operations (e.g., using a DDR4 bus). The mobile device can have one or more Central Processing Units (CPUs) to which computer peripheral devices, such as the memory module, may be attached via an interconnect, such as a computer bus (e.g., Serial AT Attachment (SATA), Peripheral Component Interconnect (PCI), PCI eXtended (PCI-X), PCI Express (PCIe)), a communication portion, and/or a computer network. 
     In one embodiment, the memory module can be used to store data for a processor in the non-volatile or volatile storage media. The memory module has a host interface that implements communications with the mobile device using the communication channel. In one embodiment, the memory module  205  has a controller  227  running, for example, firmware to perform operations responsive to communications from the processing device  118 . In one example, the memory module includes volatile Dynamic Random-Access Memory (DRAM) and NVRAM. The DRAM and NVRAM store data accessible by the processing device  118  in a memory address space. 
     As illustrated, the computer system of  FIG.  2    is used to implement a mobile device. The processing device  118  can read data from or write data to the memory sub-systems (e.g.,  205 ). 
     A physical host interface can be used to transmit data between the processing device  118  and the memory sub-system (e.g.,  205 ). The physical host interface can provide an interface for passing control, address, data, and other signals between the memory sub-system (e.g.,  205 ) and the processing device  118 . 
     In general, a memory sub-system (e.g., memory module  205 ) includes a printed circuit board that connects a set of memory devices, such as memory integrated circuits, that provides the memory (e.g.,  221 , . . . ,  223 ). The memory (e.g.,  221 , . . . ,  223 ) on the memory sub-system (e.g.,  205 ) can include any combination of the different types of non-volatile memory devices and/or volatile memory devices. 
     In some implementations, the memory (e.g.,  221 , . . . ,  223 ) can include, but are not limited to, random access memory (RAM), read-only memory (ROM), dynamic random access memory (DRAM), static random access memory (SRAM), synchronous dynamic random access memory (SDRAM), phase change memory (PCM), magneto random access memory (MRAM), negative-or (NOR) flash memory, electrically erasable programmable read-only memory (EEPROM), and/or a cross-point array of non-volatile memory cells. 
     A memory sub-system (e.g., memory module  205 ) can have a controller (e.g.,  227 ) that communicates with the memory (e.g.,  221 , . . . ,  223 ) to perform operations such as reading data, writing data, or erasing data in the memory (e.g.,  221 , . . . ,  223 ) and other such operations, in response to requests, commands or instructions from the processing device  118 . The controller (e.g.,  227 ) can include hardware such as one or more integrated circuits and/or discrete components, a buffer memory, or a combination thereof. The controller (e.g.,  227 ) can be a microcontroller, special purpose logic circuitry (e.g., a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), etc.), or another suitable processor. The controller (e.g.,  227 ) can include one or more processors (processing devices) configured to execute instructions stored in local memory. 
     The local memory of the controller (e.g.,  227 ) can include an embedded memory configured to store instructions for performing various processes, operations, logic flows, and routines that control operation of the memory sub-system (e.g.,  205 ), including handling communications between the memory sub-system (e.g.,  205 ) and the processing device  118 , and other functions described in greater detail below. Local memory of the controller (e.g.,  227 ) can include read-only memory (ROM) for storing micro-code and/or memory registers storing, e.g., memory pointers, fetched data, etc. 
     While the example memory sub-system  205  in  FIG.  2    has been illustrated as including controller  227 , in another embodiment of the present disclosure, a memory sub-system (e.g.,  205 ) may not include a controller (e.g.,  227 ), and can instead rely upon external control (e.g., provided by a processor or controller separate from the memory sub-system (e.g.,  205 )). 
     In general, the controller (e.g.,  227 ) can receive commands, requests or instructions from the processing device  118  in accordance with a standard communication protocol for the communication channel (e.g.,  203 ) and can convert the commands, requests or instructions in compliance with the standard protocol into detailed instructions or appropriate commands within the memory sub-system (e.g.,  205 ) to achieve the desired access to the memory (e.g.,  221 , . . . ,  223 ). For example, the controller (e.g.,  227 ) can be responsible for operations such as address translations between a logical address and a physical address that are associated with the memory (e.g.,  221 , . . . ,  223 ). The controller (e.g.,  227 ) can further include host interface circuitry to communicate with the processing device  118  via the physical host interface. The host interface circuitry can convert the commands received from the processing device  118  into command instructions to access the memory devices (e.g.,  221 , . . . ,  223 ) as well as convert responses associated with the memory devices (e.g.,  221 , . . . ,  223 ) into information for the processing device  118 . 
     The memory sub-system (e.g.,  205 ) can also include additional circuitry or components that are not illustrated. In some implementations, the memory sub-system (e.g.,  205 ) can include a cache or buffer (e.g., DRAM) and address circuitry (e.g., a row decoder and a column decoder) that can receive an address from the controller (e.g.,  227 ) and decode the address to access the memory (e.g.,  221 , . . . ,  223 ). 
     In one example, the memory bus  203  has one or more connectors to provide the memory sub-system (e.g.,  205 ) with power and/or communicate with the memory sub-system (e.g.,  205 ) via a predetermined protocol; and the memory sub-system (e.g.,  205 ) has one or more connectors to receive the power, data and commands from the processing device  118 . In one example, the processing device  118  can execute one or more operating systems to provide services, including memory access in which a portion of memory (e.g., a page stored in NVRAM) in the computer system is accessed using synchronous memory access. 
       FIG.  3    illustrates an example computer system  300  that stores metadata  320  used to access memory devices in a memory sub-system  302 , in accordance with some embodiments. The memory devices accessed in memory sub-system  302  include DRAM  304 , NVRAM  306 , and NAND flash  308 . In one embodiment, computer system  300  alternatively and/or additionally stores metadata  322  in DRAM  304  that is used to access the memory devices. 
     In one embodiment, a processing device  310  of computer system  300  accesses memory in an address space. In one example, the memory is main memory used by processing device  310  when executing one or more applications. The processing device  310  accesses different memory devices using addresses in the address space. 
     In one embodiment, metadata  320 ,  322  associates a first address range of the address space with a memory device (e.g., DRAM  304 ) and a second address range of the address space with a different memory device (e.g., NVRAM  306  or NAND flash  308 ). In one example, the latency of DRAM  304  is less than the latency of NVRAM  306  and NAND flash  308 . 
     The applications executing on processing device  310  include application  312 , which is configured to include a memory type  314 . When the application  312  is initially launched, application  312  provides memory type  314  to processing device  310  along with a request for an allocation of memory in the main memory of computer system  300 . 
     In response to the request for the allocation of memory, processing device  310  makes a query to metadata  320  and/or sends a query to metadata  322 . Based on a result from one or both of these queries, processing device  310  allocates an address range in the address space to application  312 . 
     In one embodiment, application  312  makes a request to processing device  310  for an indication of latency associated with the memory devices. Processing device  310  accesses metadata  320 ,  322  to obtain a result, and based on this result provides the indication of latency to application  312 . In response to receiving the indication of latency, application  312  makes a request for an allocation of memory corresponding to a specific one of the memory devices, a memory device corresponding to memory type  314 , or a request for an allocation of memory that has performance characteristics meeting at least one or more predetermined thresholds and/or requirements. 
     In one embodiment, metadata  322  stores data that associates an address range in a virtual address space with physical addresses in the memory devices of memory sub-system  302 . In one example, metadata  322  stores address range  324  for NVRAM, and address range  326  for NAND flash. In one example, address range  324  maps a virtual or logical address of processing device  310  to a physical address of NVRAM  306 . In one example, address range  326  maps a virtual or logical address of processing device  310  to a physical address of NAND flash  308 . In one embodiment, metadata  320  or  322  stores one or more address ranges mapping addresses of processing device  310  for data stored in DRAM  304 . 
     In one embodiment, metadata  322  is stored as part of page table  328 , which provides a mapping of virtual addresses to physical addresses for a memory management unit  316  of computer system  300 . Processing device  310  provides a virtual address to memory management unit  316 , which accesses a translation lookaside buffer  318  to obtain a physical address in one of the memory devices of memory sub-system  302 . 
     In one embodiment, translation lookaside buffer  318  is a cache that stores a portion of the data from page table  328 . In one example, buffer  318  stores a portion of metadata  322 . In one embodiment, a portion of metadata  320  stored on computer system  300  is copied to translation lookaside buffer  318  for access by memory management unit  316  when accessing a memory device in memory sub-system  302 . 
     In one embodiment, processing device  310  provides memory characteristics of the different memory devices to application  312 . Application  312  makes a request for an allocation of memory based on the provided memory characteristics. 
     In one embodiment, processing device  310  receives a requested latency from application  312 . An address range is allocated to the application  312  based on the requested latency. 
     In one embodiment, processing device  310  determines a priority associated with application  312 . The address range allocated to application  312  is based on the determined priority. In one example, a faster memory device type is selected for use with the determined priority. Processing device  310  uses metadata  320 ,  322  to select an address range that physically stores data in a memory device of the selected faster memory device type. 
     In one embodiment, processing device  310  determines a change in priority of application  312 . In one example, based on an increase in priority of application  312 , processing device  310  changes a memory allocation that is used for application  312  in the address space. In one example, in response to the increase in priority, processing device  310  accesses metadata  320 ,  322  to determine an address range that corresponds to a faster physical memory device. 
     In one embodiment, processing device  310  determines a priority of application  312  based on observing characteristics associated with data access by application  312  in the address space. The observed characteristics can be used for allocating memory usage for application  312 . In one embodiment, processing device  310  determines one or more latencies associated with physical memory devices. Metadata  320 ,  322  stores data regarding the determined one or more latencies, which can be used by processing device  310  when initially allocating and/or changing an allocation of main memory. 
       FIG.  4    shows a memory module  401  configured for memory bus access by a host computer system (not shown) to volatile memory  402  and non-volatile memory  404 , in accordance with some embodiments. Memory module  401  is an example of memory sub-system  302  or memory module  205 . In one example, memory module  401  is a hybrid DIMM. Volatile memory  402  is for example DRAM. 
     Memory module  401  uses multiplexer  408  to provide access to volatile memory  402  and non-volatile memory  404  by memory controller  416 . Memory controller  416  is coupled to host interface  406  for handling read/write access by a host system. In one embodiment, multiplexer  408  is controlled based on signals received from memory controller  416  in response to receiving read or write commands from the host system via host interface  406 . 
     In one example, a host system accesses a memory space (e.g., DRAM memory address space) on the memory module  401  (e.g., a DIMM). The DIMM exposes itself to the host as a channel of DRAM. In one embodiment, a hypervisor of the host system controls data movement on the DIMM. For example, a request is made for moving memory blocks in and out of the DRAM address space and exposing the DRAM pages to software running on the host. The software is, for example, executing in a virtual machine (VM). 
     In one example, a page in/out control path is provided for a driver to request a page that is currently in DRAM or in NVRAM. In one example, the NVRAM has a much larger capacity than the DRAM. 
     In one example, memory module  401  is implemented as a DIMM. The non-volatile memory  404  is provided by 3D XPoint memory packages. In one example, pages of data obtained from the 3D XPoint memory are copied in and out of a buffer (page in/page out). 
     In one example, the host system has read/write access to any DRAM or NVRAM address using normal DDR4 timing. For example, the host can generate arbitrary traffic per DDR4 rules during those times. 
     In one example, the full DDR address space of the non-volatile memory  404  is exposed to the host system. According to various embodiments, a controller (e.g., controller  116 ) of computer system  120  can operate in the same way (e.g., same read/write and refresh timing cycles) as it would for access to a conventional DRAM. 
       FIG.  5    shows a host operating system  241  accessing a memory module  502  using a memory bus, in accordance with at least some embodiments. Memory module  502  includes a buffer  410 . Buffer  410  is an example of buffer  121 . In one example, buffer  410  stores metadata  322  and/or at least a portion of page table  328 . Commands and data are received from a host operating system  241  via host interface  406 . In one example, host operating system  241  executes on computer system  120  or  300 . 
     In one embodiment, a device driver  247  (e.g., a back-end driver) is configured for memory access via a hypervisor  245 . In one example, the system of  FIG.  5    is implemented in a computer system of  FIGS.  1 - 3   . 
     In one example, the host operating system  241  runs on the processing device  118  of the computer system of  FIG.  1  or  2   , or processing device  310  of  FIG.  3   . The host operating system  241  includes one or more device drivers (e.g.,  247 ) that provide memory services using the memory (e.g.,  221 , . . . ,  223 ) of memory sub-systems, such as the memory module  205  or memory sub-system  302 . 
     In one embodiment, back-end driver  247  maintains a mapping table  246 . For example, the driver  247  maintains mapping table  246  to include a mapping for pages of data stored in DRAM  304 , NVRAM  306 , and NAND flash  308 . 
     In one embodiment, the host operating system  241  includes a hypervisor  245  that provisions a virtual machine  249 . The virtual machine  249  has virtual hardware implemented via the resources and services provided by the host operating system  241  using the hardware of a computing system of  FIGS.  1 - 3   . For example, the hypervisor  245  can provision virtual memory as part of the virtual machine  249  using a portion of the memory (e.g.,  221 , . . . ,  223 ) of memory sub-systems, such as the memory module  205 . 
     The virtual machine  249  allows a guest operating system  243  to provide resources and/or services to applications (e.g.,  251 , . . . ,  253 ) running in the guest operating system  243 , in a way as the operating system  243  running on a physical computing machine that has the same or similar set of hardware as provisioning in the virtual machine. The hypervisor  245  manages the mapping between the virtual hardware provisioned in the virtual machine and the services of hardware in the computing system managed by the host operating system  241 . 
     A device driver  248  (e.g., a front-end driver) communicates with back-end driver  247 . Driver  247  and driver  248  can communicate for memory ballooning when additional DDR capacity (e.g., capacity in DRAM or NVRAM) is available. 
       FIG.  5    illustrates an instance in which a virtual machine  249  is provisioned by the hypervisor  245 . In general, the hypervisor  245  can provision several virtual machines (e.g.,  249 ) that can run the same guest operating system  243 , or different guest operating systems. Different sets of users and/or application programs can be assigned to use different virtual machines. 
     In some instances, the host operating system  241  is specialized to provide services for the provisioning of virtual machines and does not run other application programs. Alternatively, the host operating system  241  can provide additional services to support other application programs, such as applications (e.g.,  251 , . . . ,  253 ). 
     In one embodiment, the device driver  247  can be configured to request page-in of a page from slower memory (e.g., NVRAM) to faster memory (e.g., DRAM) for use by the virtual machine  249 . This request can be made in response to a request from an application (e.g., application  312  of  FIG.  3   ). After requesting the page, the page is made available in the faster memory by loading and/or transferring the page of data from the slower memory to the faster memory. In one example, processing device  310  moves the page from slower memory to faster memory based on address range information stored as metadata  320 ,  322 . In one example, the slower memory can be the non-volatile memory  404  in the memory module  401  and the faster memory be the volatile memory  402  in the same memory module  401 . 
     In one embodiment, the transfer of data (e.g., performed in response to a page-in request by the host operating system  241 ) is performed within a same memory sub-system, such as within the same memory module  401 , to avoid or reduce congestion in communication channels connected to the processing device  118 , such as the memory bus  203 . For example, data can be copied from the slower memory  223  (e.g., NVRAM or NAND flash) in the memory module  205  to the faster memory  221  (e.g., DRAM) in the memory module  205 , under the control of controller  227  in the memory module  205  in response to one or more commands, requests, and/or instructions from the device driver  247 . 
     In one embodiment, the hypervisor  245  not only requests the device driver  247  to access a memory (e.g.,  221 , . . . ,  223 ) in a memory sub-system (e.g., memory module  205 ), but also provides the device driver  247  with information that can be used in managing pages in the memory (e.g.,  221 , . . . ,  223 , . . . , or  225 ) to be used. In one example, the provided information includes stored metadata  320  or  322 . 
     In one example, driver  247  is a memory mode driver used to access a memory address space in memory module  502  (e.g., a DIMM). Driver  247  has control over which pages are in volatile memory of the DIMM at any one time. In one approach, for example, the memory address space is exposed to the guest operating system  243 . In this hypervisor environment, the guest operating system  243  sees the full storage capacity of the non-volatile memory (e.g., NVRAM and DRAM) in the DIMM. 
     In one example, only a number of pages that are in the DDR DRAM are actively paged-in via the host operating system  241 . If there is a guest access to a page that is not present, a page fault path in a memory management unit (MMU) of the host system triggers the driver  247  to cause loading (page in) of a page. In one example, the page gets loaded in through control registers. Once the page is actually present in the DDR DRAM, then the driver  247  can set up MMU mapping (via mapping table  246 ) so that a guest application can directly read and write that data. 
     In one example, a front-end driver of a guest and a back-end driver of a host communicate regarding access to the memory address space. In one example, when deciding that pages are stale (e.g., not being used frequently based on a predetermined threshold), a request is made that a portion of data that is currently mapped in the DDR memory address space be pushed back out to the NVRAM memory (e.g., via an SRAM buffer) to make space available in the DRAM memory for other pages to be paged in. The back-end driver  247  communicates the page out request to move data from the DDR DRAM to the NVRAM memory. 
     In one embodiment, back-end driver  247  operates as a memory mode driver. Until driver  247  loads, there is no access to the NVRAM memory capacity of memory module  502 . During this operation as a memory mode driver, the guest operating system  243  sees the memory as normal, and the driver  247  reserves DRAM pages on the memory module for page-in and page-out operations. 
     The driver  247  exposes the NVRAM memory to the guest operating system  243  and maintains the page mapping (e.g., in mapping table  246 ). For example, the driver  247  maintains the mapping between pages that are currently in the DRAM and pages that are on the NVRAM memory. 
     In one example, the driver  247  sets up memory management unit mapping tables at the host system to map any pages that are currently stored in DRAM. A page fault path from the guest can be used if there is an access outside of a mapped page to trigger a page-in request. A page-out request can be performed to maintain some memory space in the DRAM. 
     In one embodiment, operation is not restricted to memory mode. Driver  247  can also be operated as a block mode driver for which NVRAM memory is exposed as block mode storage. 
     In one embodiment, the memory module  502  maintains its own mapping table including a list of pages that are in an SRAM buffer (not shown). The memory module  502  can return a page-in completion signal to the host system once a page has been moved to the SRAM buffer. These permit reducing the latency for the host system to access those particular page(s). The driver  247  ensures that until its mapping is set up, the host will not access that page(s) until the page-in request completes. 
     In one embodiment, driver  247  implements a page out operation. In one example, this operation is triggered as a thread. This operation trades free pages back out of the DRAM memory and changes the mapping of valid pages. 
       FIG.  6    shows a method for managing memory for processes in an address space of a computer system based on stored metadata that associates virtual address ranges for the processes in the address space with physical addresses for memory devices in the computer system, in accordance with some embodiments. For example, the method of  FIG.  6    can be implemented in the system of  FIGS.  1 - 3   . 
     The method of  FIG.  6    can be performed by processing logic that can include hardware (e.g., processing device, circuitry, dedicated logic, programmable logic, microcode, hardware of a device, integrated circuit, etc.), software (e.g., instructions run or executed on a processing device), or a combination thereof. In some embodiments, the method of  FIG.  6    is performed at least in part by one or more processing devices (e.g., processing device  310  of  FIG.  3   ). 
     Although shown in a particular sequence or order, unless otherwise specified, the order of the processes can be modified. Thus, the illustrated embodiments should be understood only as examples, and the illustrated processes can be performed in a different order, and some processes can be performed in parallel. Additionally, one or more processes can be omitted in various embodiments. Thus, not all processes are required in every embodiment. Other process flows are possible. 
     At block  601 , an operating system maintains memory in an address space. The memory is accessed including accessing a first memory device and a second memory device using addresses in the address space. In one example, the operating system executes on processing device  310  of  FIG.  3   . In one example, the first memory device is DRAM  304 , and the second memory device is NVRAM  306 . In one example, the first memory device is NVRAM  306 , and the second memory device is NAND flash  308 . 
     At block  603 , metadata is stored that associates a first address range of the address space with the first memory device. The metadata also associates a second address range of the address space with the second memory device. In one example, the stored metadata is metadata  320  and/or  322  of  FIG.  3   . In one example, the first address range is address range  324 , and the second address range is address range  326 . 
     At block  605 , processes running in a computer system are managed based on the stored metadata. The processes include a first process and a second process. Data for the first process is stored in the first memory device, and data for the second process is stored in the second memory device. In one example, data for the first process is stored in address range  324 , and data for the second process is stored in address range  326 . In one example, data for the first process is stored in an address range of metadata  320 ,  322  that corresponds to physical memory storage in DRAM  304 . In one example, the computer system is computer system  120  or  300 . 
     In one embodiment, a method comprises: accessing, by a processing device (e.g., processing device  310  of  FIG.  3   ) of a computer system, memory in an address space, wherein memory devices of the computer system are accessed by the processing device using addresses in the address space; storing metadata (e.g., metadata  320  and/or  322 ) that associates a first address range of the address space with a first memory device (e.g., DRAM  304 ), and a second address range of the address space with a second memory device (e.g., NVRAM  306 ), wherein a first latency of the first memory device is different from a second latency of the second memory device; and allocating, based on the stored metadata, the first address range to an application (e.g., application  312 ) executing on the computer system. 
     In one embodiment, allocating the first address range to the application is performed in response to a request by the application. 
     In one embodiment, the method further comprises: in response to a first request by the application, providing an indication that the first latency is greater than the second latency; receiving a second request made by the application based on the indication; and in response to receiving the second request, allocating the second address range to the application. 
     In one embodiment, the first latency is less than the second latency, and the metadata is stored in the first memory device. 
     In one embodiment, the computer system uses a memory bus to access the first memory device and the second memory device, and wherein the metadata is stored in the second memory device. 
     In one embodiment, the metadata is stored in the first memory device, and the method further comprises loading at least a portion of the metadata into a buffer (e.g., translation lookaside buffer  318 ), wherein the processing device queries the buffer to determine a physical address corresponding to a virtual address in the first address range. 
     In one embodiment, the computer system is a system-on-chip device, and the buffer is a translation lookaside buffer. 
     In one embodiment, the method further comprises: providing, to the application, memory characteristics of the first memory device and the second memory device; wherein allocating the first address range to the application is in response to a request made by the application based on the provided memory characteristics. 
     In one embodiment, the method further comprises receiving a requested latency from the application, wherein allocating the first address range to the application is further based on the requested latency. 
     In one embodiment, the method further comprises determining a priority associated with the application, wherein allocating the first address range to the application is further based on the priority. 
     In one embodiment, the first latency is less than the second latency; prior to allocating the first address range to the application, the application is allocated to the second address range; and allocating the first address range to the application is performed in response to determining an increase in a priority associated with the application. 
     In one embodiment, determining the increase in the priority associated with the application is based on one or more observations regarding data access by the application in the address space. 
     In one embodiment, the method further comprises determining, by the processing device, latencies associated with the memory devices, wherein storing the metadata further comprises storing the determined latencies. 
     In one embodiment, a system comprises: a first memory device; a second memory device; at least one processing device; and memory containing instructions configured to instruct the at least one processing device to: access memory in an address space maintained by an operating system, the accessing including accessing the first memory device and the second memory device using addresses in the address space; store metadata that associates a first address range of the address space with the first memory device, and a second address range of the address space with the second memory device; and manage, by the operating system based on the stored metadata, processes including a first process and a second process, wherein data for the first process is stored in the first memory device, and data for the second process is stored in the second memory device. 
     In one embodiment, the first process has a first priority, the second process has a second priority, and the first memory device is selected to store the data for the first process in response to determining that the first priority is higher than the second priority. 
     In one embodiment, the first process corresponds to a first application; the instructions are further configured to instruct the at least one processing device to receive a request from the first application that indicates a type of memory to use for storing data; and the first memory device is selected to store the data for the first process based on the indicated type of memory. 
     In one embodiment, the system further comprises a buffer to store the metadata, wherein the operating system receives a virtual address in the first address range from the first process, and accesses the buffer to determine a physical address of the first memory device corresponding to the virtual address. 
     In one embodiment, a read latency of the first memory device is less than a read latency of the second memory device, and the instructions are further configured to instruct the at least one processing device to store the metadata in the first memory device. 
     In one embodiment, the system further comprises a memory management unit (e.g., memory management unit  316 ) configured to, when accessing the stored data for the first process, map a virtual address in the first address range to a physical address in the first memory device. 
     In one embodiment, a non-transitory machine-readable storage medium stores instructions which, when executed on at least one processing device, cause the at least one processing device to at least: access memory in an address space, wherein memory devices of a computer system are accessed by the at least one processing device using addresses in the address space; store metadata that associates a first address range of the address space with a first memory device, and a second address range of the address space with a second memory device; provide, to an application executing on the computer system, first data indicating that a first latency of the first memory device is less than a second latency of the second memory device; in response to providing the first data to the application, receive a request from the application that second data associated with the application be stored in the first memory device; in response to a request by the application to store the second data, query the stored metadata to provide a result; and store, based on the result, the second data in the first memory device. 
       FIG.  7    is a block diagram of an example computer system in which embodiments of the present disclosure can operate.  FIG.  7    illustrates an example machine of a computer system  600  within which a set of instructions, for causing the machine to perform any one or more of the methodologies discussed herein, can be executed. In some embodiments, the computer system  600  can correspond to a host system (e.g., the computer system  120  of  FIG.  1   ) that includes, is coupled to, or utilizes a memory sub-system (e.g., the memory sub-system  110  of  FIG.  1   ) or can be used to perform the operations of a metadata component  113  (e.g., to execute instructions to perform operations corresponding to the metadata component  113  described with reference to  FIGS.  1 - 6   ). In alternative embodiments, the machine can be connected (e.g., networked) to other machines in a LAN, an intranet, an extranet, and/or the Internet. The machine can operate in the capacity of a server or a client machine in client-server network environment, as a peer machine in a peer-to-peer (or distributed) network environment, or as a server or a client machine in a cloud computing infrastructure or environment. 
     The machine can be a personal computer (PC), a tablet PC, a set-top box (STB), a Personal Digital Assistant (PDA), a cellular telephone, a web appliance, a server, a network router, a switch or bridge, an Internet of Things (IOT) device, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while a single machine is illustrated, the term “machine” shall also be taken to include any collection of machines that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein. 
     The example computer system  600  includes a processing device  602 , a main memory  604  (e.g., read-only memory (ROM), flash memory, dynamic random access memory (DRAM) such as synchronous DRAM (SDRAM) or Rambus DRAM (RDRAM), static random access memory (SRAM), etc.), and a data storage system  618 , which communicate with each other via a bus  630  (which can include multiple buses). 
     Processing device  602  represents one or more general-purpose processing devices such as a microprocessor, a central processing unit, or the like. More particularly, the processing device can be a complex instruction set computing (CISC) microprocessor, reduced instruction set computing (RISC) microprocessor, very long instruction word (VLIW) microprocessor, or a processor implementing other instruction sets, or processors implementing a combination of instruction sets. Processing device  602  can also be one or more special-purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), network processor, or the like. The processing device  602  is configured to execute instructions  626  for performing the operations and steps discussed herein. The computer system  600  can further include a network interface device  608  to communicate over the network  620 . 
     The data storage system  618  can include a machine-readable storage medium  624  (also known as a computer-readable medium) on which is stored one or more sets of instructions  626  or software embodying any one or more of the methodologies or functions described herein. The instructions  626  can also reside, completely or at least partially, within the main memory  604  and/or within the processing device  602  during execution thereof by the computer system  600 , the main memory  604  and the processing device  602  also constituting machine-readable storage media. The machine-readable storage medium  624 , data storage system  618 , and/or main memory  604  can correspond to the memory sub-system  110  of  FIG.  1   . 
     In one embodiment, the instructions  626  include instructions to implement functionality corresponding to a metadata component  113  (e.g., the metadata component  113  described with reference to  FIGS.  1 - 6   ). While the machine-readable storage medium  624  is shown in an example embodiment to be a single medium, the term “machine-readable storage medium” should be taken to include a single medium or multiple media that store the one or more sets of instructions. 
     CLOSING 
     The disclosure includes various devices which perform the methods and implement the systems described above, including data processing systems which perform these methods, and computer readable media containing instructions which when executed on data processing systems cause the systems to perform these methods. 
     The description and drawings are illustrative and are not to be construed as limiting. Numerous specific details are described to provide a thorough understanding. However, in certain instances, well-known or conventional details are not described in order to avoid obscuring the description. References to one or an embodiment in the present disclosure are not necessarily references to the same embodiment; and, such references mean at least one. 
     Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not other embodiments. 
     In this description, various functions and operations may be described as being performed by or caused by software code to simplify description. However, those skilled in the art will recognize what is meant by such expressions is that the functions result from execution of the code by one or more processors, such as a microprocessor, Application-Specific Integrated Circuit (ASIC), graphics processor, and/or a Field-Programmable Gate Array (FPGA). Alternatively, or in combination, the functions and operations can be implemented using special purpose circuitry (e.g., logic circuitry), with or without software instructions. Embodiments can be implemented using hardwired circuitry without software instructions, or in combination with software instructions. Thus, the techniques are not limited to any specific combination of hardware circuitry and software, nor to any particular source for the instructions executed by a computing device. 
     While some embodiments can be implemented in fully functioning computers and computer systems, various embodiments are capable of being distributed as a computing product in a variety of forms and are capable of being applied regardless of the particular type of machine or computer-readable media used to actually effect the distribution. 
     At least some aspects disclosed can be embodied, at least in part, in software. That is, the techniques may be carried out in a computing device or other system in response to its processor, such as a microprocessor, executing sequences of instructions contained in a memory, such as ROM, volatile RAM, non-volatile memory, cache or a remote storage device. 
     Routines executed to implement the embodiments may be implemented as part of an operating system, middleware, service delivery platform, SDK (Software Development Kit) component, web services, or other specific application, component, program, object, module or sequence of instructions referred to as “computer programs.” Invocation interfaces to these routines can be exposed to a software development community as an API (Application Programming Interface). The computer programs typically comprise one or more instructions set at various times in various memory and storage devices in a computer, and that, when read and executed by one or more processors in a computer, cause the computer to perform operations necessary to execute elements involving the various aspects. 
     A machine readable medium can be used to store software and data which when executed by a computing device causes the device to perform various methods. The executable software and data may be stored in various places including, for example, ROM, volatile RAM, non-volatile memory and/or cache. Portions of this software and/or data may be stored in any one of these storage devices. Further, the data and instructions can be obtained from centralized servers or peer to peer networks. Different portions of the data and instructions can be obtained from different centralized servers and/or peer to peer networks at different times and in different communication sessions or in a same communication session. The data and instructions can be obtained in entirety prior to the execution of the applications. Alternatively, portions of the data and instructions can be obtained dynamically, just in time, when needed for execution. Thus, it is not required that the data and instructions be on a machine readable medium in entirety at a particular instance of time. 
     Examples of computer-readable media include but are not limited to recordable and non-recordable type media such as volatile and non-volatile memory devices, read only memory (ROM), random access memory (RAM), flash memory devices, solid-state drive storage media, removable disks, magnetic disk storage media, optical storage media (e.g., Compact Disk Read-Only Memory (CD ROMs), Digital Versatile Disks (DVDs), etc.), among others. The computer-readable media may store the instructions. 
     In general, a tangible or non-transitory machine readable medium includes any mechanism that provides (e.g., stores) information in a form accessible by a machine (e.g., a computer, mobile device, network device, personal digital assistant, manufacturing tool, any device with a set of one or more processors, etc.). 
     In various embodiments, hardwired circuitry may be used in combination with software and firmware instructions to implement the techniques. Thus, the techniques are neither limited to any specific combination of hardware circuitry and software nor to any particular source for the instructions executed by a computing device. 
     Although some of the drawings illustrate a number of operations in a particular order, operations which are not order dependent may be reordered and other operations may be combined or broken out. While some reordering or other groupings are specifically mentioned, others will be apparent to those of ordinary skill in the art and so do not present an exhaustive list of alternatives. Moreover, it should be recognized that the stages could be implemented in hardware, firmware, software or any combination thereof. 
     In the foregoing specification, the disclosure has been described with reference to specific exemplary embodiments thereof. It will be evident that various modifications may be made thereto without departing from the broader spirit and scope as set forth in the following claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense. 
     Various embodiments set forth herein can be implemented using a wide variety of different types of computing devices. As used herein, examples of a “computing device” include, but are not limited to, a server, a centralized computing platform, a system of multiple computing processors and/or components, a mobile device, a user terminal, a vehicle, a personal communications device, a wearable digital device, an electronic kiosk, a general purpose computer, an electronic document reader, a tablet, a laptop computer, a smartphone, a digital camera, a residential domestic appliance, a television, or a digital music player. Additional examples of computing devices include devices that are part of what is called “the internet of things” (IOT). Such “things” may have occasional interactions with their owners or administrators, who may monitor the things or modify settings on these things. In some cases, such owners or administrators play the role of users with respect to the “thing” devices. In some examples, the primary mobile device (e.g., an Apple iPhone) of a user may be an administrator server with respect to a paired “thing” device that is worn by the user (e.g., an Apple watch). 
     In some embodiments, the computing device can be a computer or host system, which is implemented, for example, as a desktop computer, laptop computer, network server, mobile device, or other computing device that includes a memory and a processing device. The host system can include or be coupled to a memory sub-system so that the host system can read data from or write data to the memory sub-system. The host system can be coupled to the memory sub-system via a physical host interface. In general, the host system can access multiple memory sub-systems via a same communication connection, multiple separate communication connections, and/or a combination of communication connections.