Patent Publication Number: US-2023161714-A1

Title: Method and system for direct memory access

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/EP2020/064854, filed on May 28, 2020, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD 
     The present disclosure relates to a system and method for a computing system. In particular, the present disclosure relates to a system and method for performing direct memory access operations to a computing system. 
     BACKGROUND 
     Direct memory access (DMA) allows a device or subsystem in a computing system to directly read and write data into a computing system. DMA may be implemented in a computing system by devices such as the graphical processing units or sound cards or additional processing cores in multicore systems. DMA frees up computational resources in the computing system. In particular DMA operations may be performed while programs run on the main processor, simultaneously. 
     Remote direct memory access (RDMA) allows one computing system to read or write data to the memory of another computing system across a network. RDMA can improve network performance allowing higher throughput, lower latency networking over systems which do not implement RDMA. 
     SUMMARY 
     The present disclosure provides a method for a computing system such as may be used to perform direct memory access operations to a computing system. 
     Further implementation forms are apparent from the description and the figures. 
     According to a first aspect, a method for a computing system is provided. The method includes receiving a data packet comprising data to be written to the computing system and address data comprising an address in a set of addresses of a first address space of the computing system, determining, an address in a second address space of the computing system identified with the address of the data packet and writing the data to the computing system on the basis of the determination. A first subset of the set is identified with a first subset of addresses of the second address space and a second subset of the set is identified with a second subset of addresses of the second address space. The first subset of addresses of the second address space is associated with a region of memory of the computing system and the second subset of addresses of the second address space is associated with a region of a data storage area. 
     The method according to the first aspect provides a method of writing data to a computing system on the basis of a determination of an address in a second address space of the computing system from an address in a first address space. The second address space is partitioned into subsets which include a subset of addresses associated to memory and a subset associated to a region of a data storage area. The data is written to the memory or region of the data storage area in the event that the determined address is in one of those subsets. 
     According to a second aspect an apparatus for a computing system is provided. The apparatus is arranged to identify an address in a set of addresses in a first address space of a computing system from a data packet comprising data to be written to the computing system and address data comprising the address, determine an address in a second address space of the computing system identified with the address of the data packet and write the data to the computing system on the basis of the determination. A first subset of the set is identified with a first subset of addresses of the second address space and a second subset of the set is identified with a second subset of addresses of the second address space. The first subset of addresses of the second address space is associated with a region of memory of the computing system and the second subset of addresses of the second address space is associated with a region of a data storage area. 
     In an implementation determining the address in the second address space comprises accessing an address translation table and determining the address on the basis of the address translation table. 
     In a further implementation the method comprises determining an address from a third subset of addresses in the second address space associated with a region of the data storage area, when the address of the data packet is not identified with an address in the first or second subset of the second address space and writing the data to the region of the data storage area associated to the determined address. 
     This implementation of the method provides a method of selecting an address from a pool of addresses of the region of the data storage area to map an unmapped address in the first address space 
     In a further implementation the method comprises updating the address translation table to identify the address in the data packet with the determined address from the third subset. 
     This implementation provides a method of performing a direct memory access for a previously unmapped address to an address in the third subset in a stall-free manner. 
     In a further implementation the second address space is a physical address space of the computing system. 
     In a further implementation the first address space is a virtual address space. 
     In a further implementation determining an address from a third subset of addresses in the second address space comprises accessing stored address data for one or more addresses in the third subset and determining an address in the third subset from the stored address data. 
     This implementation provides a method of selecting an address from a pool of addresses to map an unmapped address in the first address space to. 
     In further implementation of the method determining an address from the third subset of addresses in the second address space comprises communicating a request to identify an address from the third subset, to the computing system and receiving a response comprising address data for an address in the third subset from the computing system. 
     This implementation provides an alternative method for determining an address from a pool of available addresses to map an unmapped address to. 
     In an implementation the method comprises removing a determined address from the third subset of addresses of the second address space. 
     The method according to this implementation provides a method of removing addresses from a pool that have been allocated to a previously unmapped address in the first address space. 
     In a further implementation the method comprises determining a number of addresses of the second address space in the third subset and replenishing the third subset when the number of addresses falls below a threshold number. 
     This implementation replenishes the pool of addresses to ensure there are enough addresses to withstand numerous requests from unmapped addresses. 
     In a further implementation replenishing the third subset comprises determining the availability of an address in the second address space on the basis of a criterion and including the address in the third subset on the basis of the determination. 
     In a further implementation the criterion comprises a criterion of usage of the address by the computing system. 
     This implementation provides a criterion based on usage of an address for determining the availability of an address for replenishing the pool of addresses. This implementation of the method ensures that addresses which are being underused by the computing system are recycled and used in the pool of available addresses for mapping unmapped addresses. 
     In a further implementation the address of the data packet is a destination address for a direct memory access (DMA) request. 
     In a further implementation the data packet is a destination address for a remote direct memory access (RDMA) request. 
     In a further implementation the method comprises accessing one or more further address translation tables, each table comprising entries for each address in the set; and updating the entries of the one or more further address translation tables corresponding to the address of the data packet to identify the address of the data packet with the determined address. 
     In a further implementation the address translation table is stored on the computing system. 
     In a further implementation the address translation table is stored on a device performing a direct memory access or remote direct memory access request to the computing system. 
     These and other aspects of the present disclosure will be apparent from and the embodiment(s) described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    shows a computing system, according to an example of the present disclosure. 
         FIG.  2 A  is a schematic diagram showing a remote direct memory access request, according to an example of the present disclosure. 
         FIG.  2 B  is a schematic diagram showing a remote direct memory access request, according to an example of the present disclosure. 
         FIG.  3    shows a flow diagram of a method for a computing system, according to an example of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments are described below in sufficient detail to enable those of ordinary skill in the art to embody and implement the systems and processes herein described. It is important to understand that embodiments can be provided in many alternate forms and should not be construed as limited to the examples set forth herein. 
     Accordingly, while embodiments can be modified in various ways and take on various alternative forms, specific embodiments thereof are shown in the drawings and described in detail below as examples. There is no intent to limit to the particular forms disclosed. On the contrary, all modifications, equivalents, and alternatives falling within the scope of the appended claims should be included. Elements of the example embodiments are consistently denoted by the same reference numerals throughout the drawings and detailed description where appropriate. 
     The terminology used herein to describe embodiments is not intended to limit the scope. The articles “a,” “an,” and “the” are singular in that they have a single referent, however the use of the singular form in the present document should not preclude the presence of more than one referent. In other words, elements referred to in the singular can number one or more, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, items, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, items, steps, operations, elements, components, and/or groups thereof. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein are to be interpreted as is customary in the art. It will be further understood that terms in common usage should also be interpreted as is customary in the relevant art and not in an idealized or overly formal sense unless expressly so defined herein. 
       FIG.  1    is a block diagram of an apparatus  100 . The apparatus  100  comprises a computing system  110 . The apparatus  100  may be used with the methods and systems described herein. The computing system  110  comprises a central processing unit (CPU)  120 . The CPU  120  comprises logic to read and write data to memory and execute processes on the computing system  110 . The CPU  120  is connected to further components of the computing system  110  via a bus  130 . The bus  130  facilitates transfer of data between interconnected components of the computing system  110 . 
     The computing system  110  comprises a storage device  140 . The storage device  140  may comprise any type of non-transitory storage device configured to store data, programs, and other information and to make the data, programs, and other information accessible via the bus  130 . The storage device  140  may comprise, for example, a solid state drive, hard disk drive, a magnetic disk drive, or an optical disk drive. The storage device  140  is connected to other components of the computing system  110  via the bus  130 . 
     The computing system  110  comprises a physical memory  150 . The memory  150  may comprise any type of non-transitory system memory such as static random access memory (SRAM), dynamic random access memory (DRAM), synchronous DRAM (SDRAM), read-only memory (ROM), or a combination thereof. The memory  150  comprises a plurality of memory cells. Each memory cell has an address that identifies the location of the memory cell in the memory  150 . The range of discrete addresses of the memory  150  is referred to as the physical address space. 
     In examples described herein the storage device  140  may also be referred to as a swap device. In  FIG.  1    the storage device  140  is a physically separate entity to the physical memory  150 . In other examples, the function of the storage device  140  as a swap device may be performed by a cache in the physical memory  150  of the computing system  110 , or similar. In other examples, a remote storage device that is physically separate from the computing system  110  may perform the equivalent functions of the storage device  140  as a swap device. 
     Generally, only system software, such as the BIOS, which is executed on start-up, and an operating system, directly accesses the physical memory  150 . For other processes, the computing system  110  maintains a virtual address space. The virtual address space is similar to the physical address space however the addresses do not correspond to locations in the physical memory  150 . The virtual address space gives the appearance of a contiguous address space to a process. A mapping of virtual addresses to physical address is stored in a data structure called a page table. Each entry of the page table may be referred to as a page table entry. A page, memory page, or virtual page is a fixed-length contiguous block of virtual memory, described by a single page table entry. A frame is a fixed-length contiguous block of physical memory on to which pages are mapped. 
     As well as providing the appearance of a contiguous address space, virtual addressing also allows creation of virtual partitions of the memory  150  in to two disjointed areas. A first area referred to as kernel space is reserved for protected processes such as the BIOS and operating system. A second area referred to as user space is allocated to other processes. The computing system  110  maintains separation of the kernel space and user space by preventing processes that execute in user space from addressing the kernel space. 
     The CPU  120  shown in  FIG.  1    comprises a memory management unit (MMU)  160 . The MMU  160  performs address translations to map virtual addresses of pages that are addressed by a process to the physical address of corresponding frames in the memory  150 . The MMU  160  also performs virtual memory management for processes running on the computing system  110 . When a page is moved between different physical memory locations the MMU  160  manages the corresponding page table entries, updating the page table as is needed. 
     Data stored at virtual addresses is moved between the physical memory  150  and other storage such as the storage device  140  using a virtual memory management process called paging. When a process requests a page in the virtual address space, the MMU  160  determines if the requested page is available in the memory  150  by performing an address translation. When the page is available the physical address is returned and the computation executes on the CPU  120 . When the page is not available in the memory  150  the MMU  160  returns a page fault. Software running on the operating system, referred to as a paging supervisor, accesses the storage device  140 , restores the frame corresponding to the virtual address of the page that caused the page fault and updates the page table in the MMU  160  with the new mapping between the virtual address and the physical address where the page has been restored to in the memory  150 . 
     Paging allows the computing system to allocate a contiguous virtual addresses range to a process that exceeds space available in the physical memory  150  by extending the virtual address space into a secondary storage device such as the storage device  140 . However, when all frames are in use in physical memory  150 , the operating system selects a frame to reuse for the page that the process requires. The paging supervisor may use a page replacement algorithm such as Least Recently Used (LRU) or First In First Out (FIFO) to determine which memory location in the memory  150  to free up for the requested page. The paging supervisor may page out or “swap out” a page, according to the page replacement algorithm, from the memory  150  to the storage device  140 . The paging supervisor updates the page table such that the page requested by the process points to the freed up location in memory. The region of the storage device  140  that is reserved for this purpose is referred to as the swap space. In some instances, pages may be locked or “pinned” in the memory  150  to prevent the page being swapped out to the storage device  140 . 
     The computing system  110  further comprises a direct memory access (DMA) device  170 . The DMA device  170  may be a disk drive, a graphics card, sound card or other hardware device. In other examples the DMA device  170  may be a further processing core similar to the CPU  120 . The DMA device  170  is connected to the bus  130  and may interact with the other components of the computing system  110  via the bus  130 . The DMA device  170  may perform DMA requests to the memory  150 . A DMA request to the memory  150  from the device  170  is an operation, such as a data write operation, which is executed directly to a location in the memory  150 , independently of the CPU  120 . Without DMA, when the CPU  120  is using programmed input/output, it is fully occupied for the entire duration of the read or write operation, and is unavailable to perform other work. DMA allows the CPU  120  to perform other operations while the DMA request originating from the device  170  is being processed. DMA is useful, for example, for performing a large data transfer between the device  170  and memory  150 . Once the DMA operation to the memory  150  is complete, the device  170  sends an interrupt request back to the CPU  120 , allowing it to process data from the device  170  that is written to the memory  150  following the DMA operation. 
     Similarly to processes which run on the computing system  110 , DMA requests originating from the DMA device  170  may specify addresses from a virtual address space. In some examples, the DMA device  170  is arranged to perform address translation of addresses specified in DMA requests. For example, in some cases the DMA device  170  and/or computing system  110  include an input—output memory management unit (IOMMU) which performs address translation for I/O device in the computing system  110  in a manner similar to how the MMU  160  performs address translation for the CPU  120 . In other examples, the DMA device  170  tracks or queries page table entries using a remote procedure call to the operating system. 
     The computing system  110  further comprises a network interface controller (NIC)  180  that connects the computing system  110  to a network  190 . The NIC  180  may comprise a wired or wireless link to the network  190  e.g. Ethernet or a wireless transmitter and receiver. In some examples, the network  190  may be a local area network (LAN). In other examples, the network  190  is a wide area network. The network  190  facilitates communication between the computing system  110  and remote devices, such as other computing systems, web servers and remote storage and data processing facilities. 
     In  FIG.  1   , the computing system  110  is in communication with a remote computing device  195  across the network  190 . According to examples, the NIC  180  supports remote direct memory access (RDMA) requests from the remote device  195  to the memory  150 . Similarly to a DMA request from the DMA device  170 , a RDMA request originating for the remote device  195  is a request to perform an operation direct to a location in the memory  150 , which by-passes the operating system of the computing system  110 . RDMA permits high-throughput, low-latency networking between the computing system  110  and remote device  195  by enabling zero-copy of data to the memory  150  without the CPU  120  having to perform any copying of the data to further memory locations. 
     Similarly to processes which run on the computing system  110  and DMA requests originating from the DMA device  170 , RDMA requests that are received at the NIC  180  may include an address from a virtual address space. In some examples, the NIC  180  is arranged to perform address translation of addresses specified in RDMA requests. In other examples, the NIC  180  tracks or queries page table entries using, for example, an on board IOMMU. Once the physical address is determined the NIC  180  can write the data in the RDMA request directly in to the memory  150 . 
     The methods and systems described herein may be used to perform a DMA (or RDMA) to the storage device  140 , for a DMA request that is destined for an unmapped virtual address that does not have a corresponding physical address in the memory  150 . 
     One method of addressing the problem of a (R)DMA request targeting an unmapped virtual address is to pin a subset virtual addresses that may be used by the DMA device  170  or NIC  180  to physical addresses in the memory  150 . This ensures that (R)DMA operations never encounter unmapped memory. However, there is a considerable price to pay as the pinned memory is unavailable for use by other processes or devices in the computing system  120 . Furthermore, the memory consumption in this case can impair performance as other memory has to be swapped out from the memory  150  to the storage device  140  more frequently to accommodate the required space. 
     An alternative to permanently allocating memory to DMA operations using pinning is to provide temporarily pinned buffers in the memory  150  which serve as the destination for DMA operations. Each buffer may be re-used once the last DMA request has been completed. For example, incoming data from a RDMA request originating from the device  195  may first be placed in a pinned buffer in the memory  150  and then copied to a further buffer in the subspace of a virtual address space that is addressed by a process. Then the original pinned buffer is free for re-use in further DMA request. 
     Unfortunately, this method similarly suffers a number of disadvantages. Firstly, there is a considerable latency decrease because of the additional copy operation from the pinned buffer. In such embodiments, the buffer is not available for further DMA requests. This also requires the allocation of a dedicated pinned buffer pool which incurs a management overhead in the computing system  110 , similar to the previously described pinning method. Other methods similarly suffer latency penalties or incur large memory footprints due to excessive pinning of regions of the memory  150 . 
     In the methods and systems described herein (R)DMA operations to unmapped virtual addresses are not stalled, and are not transferred into a location in the memory  150 . Instead the the (R)DMA operations are served by the swap space in the storage device  140 . If a virtual address has a corresponding swap slot in the swap space of the storage device  140 , the data is written directly to the slot overwriting any previous data in the slot. Otherwise, the request is served by a pool of buffers with addresses in the swap space. The pool of addresses serve (R)DMA requests that are destined for unmapped memory without a corresponding swap slot. Serving the request does not require any operation to be performed by the CPU  120  and do not require the data to pass through the memory  150 . 
       FIG.  2 A  is a simplified diagram showing an example  200  of a RDMA request according to the methods described herein. The example  200  shown in  FIG.  2 A  is for a RDMA request processed by the NIC  180  shown in  FIG.  1   . A DMA request that originates at the DMA device  170  shown in  FIG.  1    is processed in a similar manner and the example shown in  FIG.  2 A  is not intended to limit the other methods and examples described herein to RDMA requests. 
     In the example  200  shown in  FIG.  2 A  the RDMA request  210  is received at the NIC  180  shown in  FIG.  1   . The RDMA request  210  may be received from e.g. the remote device  195  shown in  FIG.  1   . The RDMA request  210  comprises a destination (virtual) address  211  which is an address in a virtual address space of e.g. a targeted process running on the computing system  110 , and data  212  to be written to the computing system  110 . In some cases, the data  212  may be partitioned into one or more data packets where the virtual addresses of respective data packets are determined by an offset from the virtual address of the first packet, which is indicated as the virtual address  211  of the RDMA request  210 . When the RDMA request  210  is received by the NIC  180 , the NIC  180  performs address translation (either itself, or using e.g. an on board IOMMU, as previously described) to identify an address in the memory  150  for the destination virtual address  211 . 
     Examples of translations of the destination address  211  are shown in the box  220 . Three separate examples are shown. When the virtual address  211  of the RDMA request is already mapped in memory  150  the NIC  180  simply performs the address translation and writes the data  212  to the corresponding location in memory  150 . For example, in  FIG.  2 A , if the virtual address  211  maps to the physical address  221 , the NIC  180  writes the data  212  to the location  221  in the memory  150 . If the NIC  180  determines that the page of the destination address  211  is not present in the memory  150  but is already allocated a swap slot at an address  222  in the swap space on the storage device  140 , then the NIC  180  writes the data  212  to the slot  222 . If the NIC  180  determines that the address  211  is not mapped to any address either in the physical memory  150  or in the swap space in the storage device  140 , then the NIC  180  writes the data  212  to a temporary swap space buffer  230 , which points to an address  223 , selected from a pool of available addresses, in the swap space of the storage device  140  and interrupts the CPU  120  to re-map to the swap space buffer  230 . 
       FIG.  2 B  shows the same example  200  as shown in  FIG.  2 A . In  FIG.  2 B  the virtual address space  240  and a process  241  which addresses a range of address in the virtual address space  240  is shown. In the event that the virtual address targeted by the RDMA request is unmapped, once the NIC  180  has determined the address  223  of the swap slot to write the data  212  of the RDMA request  210  to, the NIC  180  notifies the CPU  120  to update the address translation table  250  for the process  241 , such that the entry  251  of the address table points to the swap buffer  223 . The CPU  120  (re)maps the virtual address in the address translation table  250  of the process  241  to point to the buffer  230 . 
       FIG.  3    is a block diagram showing a method  300  according to an example. The method  300  shown in  FIG.  3    may be used in conjunction with other methods and systems described herein. In particular the method  300  may be implemented on the computing system  110  shown in  FIG.  1    to process DMA and RDMA requests to the memory  150 . 
     At block  310  a data packet comprising data to be written to the computing system and address data comprising an address in a set of addresses of a first address space of the computing system is received. According to examples, the data packet may be a data packet of a DMA request or an RDMA request. The computing system may be the computing system  110  shown in  FIG.  1   . According to examples the first address space is a virtual address space of the computing system. 
     At block  320 , a determination is made of an address in a second address space of the computing system identified with the address of the data packet. In some examples described herein the determination is made on the basis of an address translation table that identifies a first subset of the set with a first subset of addresses of the second address space and a second subset of the set with a second subset of addresses of the second address space. The first subset of addresses of the second address space is associated with a region of memory of the computing system and the second subset of addresses of the second address space is associated with a region of a data storage area. In examples, the memory and data storage area may be the memory  150  and storage device  140  shown in  FIG.  1   . 
     At block  330 , the data is written to the computing system on the basis of the determination. In examples, when the address of the data packet is not identified with an address in the first or second subset of the second address space, the method  300  comprises determining an address from a third subset of addresses in the second address space associated with a region of the data storage area writing the data to the region of the data storage area associated to the determined address. In examples, the method  300  comprises updating the address translation table to identify the address in the data packet with the determined address. 
     Determining an address from the third subset of addresses may comprise accessing stored address data for one or more addresses in the third subset and determining an address in the third subset from the stored address data. For example, when the method  300  is implemented on the computing system  110  shown in  FIG.  1   , the NIC  180  may store a pool of addresses which are used to as “swap buffers” for data of an RDMA request to in the event that the request comprises an unmapped virtual address. 
     In some cases, determining an address from the third subset comprises communicating a query comprising a request to identify an address from the third subset receiving a response comprising address data for an address in the third susbet from the computing system. For example, the NIC  180  may determine the swap buffer address from an IOMMU rather than storing the addresses locally. 
     In some examples, the method  300  further comprises removing a determined address from the third subset of addresses. The method  300  may also comprise. determining a number of addresses of the second address space in the third subset and replenishing the third subset when the number of addresses falls below a threshold number. Replenishing the third subset may comprise determining the availability of an address in the second address space on the basis of a criterion and including the address in the further subset on the basis of the determination. According to examples, the criterion may comprise a criterion of usage of the address by the computing system. These examples allow the computing system to maintain a supply of swap buffers to write data from DMA requests into the system on demand in a stall-free manner without copying data to a further buffer. 
     The methods and examples described herein serve DMA operations to unmapped memory without incurring delays due to paging requests. Such DMA operations result in data being stored either directly to their swap slot in the storage device  140 , or to a location in a swap buffer. DMA to the storage device, whether a swap slot is mapped to the targeted virutal address or not, is done by the (R)DMA device and does not require any action to be taken by the host computing system. This further eliminates redundant swap operations from the storage device. DMA operations instigated by an independent device do not need to be stalled or halted. The instigating device thus does not need to support data stalling or halting due to the receiver side&#39;s constraints. Control packets on RDMA capable networks are reduced due to the redundancy of halt and throttle commands that used to result from receive-side stalls. The methods and system herein also eliminate the need to allocate new pages for DMA operations, since the DMA operation does not require physical memory. Since no new page allocations are needed, the operating system does not need to perform page frame reclaim operations since the number of free pages in the system has not decreased. 
     It should be appreciated that one or more steps of the embodiment methods provided herein may be performed by corresponding units or modules. The respective units or modules may be hardware, software, or a combination thereof. For instance, one or more of the units or modules may be an integrated circuit, such as field programmable gate arrays (FPGAs) or application-specific integrated circuits (ASICs). 
     Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. 
     The present inventions can be embodied in other specific apparatus and/or methods. The described embodiments are to be considered in all respects as illustrative and not restrictive. In particular, the scope of the invention is indicated by the appended claims rather than by the description and figures herein. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.