Patent Publication Number: US-2020278804-A1

Title: Managing memory system quality of service (qos)

Description:
TECHNICAL FIELD 
     Examples described herein are generally related to managing access to memory systems in a computing system. 
     BACKGROUND 
     Managing memory system Quality of Service (QoS) is typically implemented via controlling central processing unit (CPU) credits based on a process address space identifier (PASID)—where each PASID is allocated some credits and, based on these credits, some memory bandwidth is guaranteed by using the availability of queue entries that correspond to the credits. Memory controller QoS can also be implemented by controlling allocation of queue slots in the memory controller based on PASIDs. 
     There are several recent computing trends that may impact how QoS is implemented in the memory system: 1) the emergence of alternate means to attach to memory systems, such as via Computer Express Link (CXL) connections (a new high speed interconnect that enables high speed, efficient connections between the CPU and data center accelerators), via CXL switches, and other such interconnects; 2) the emergence of various additional compute engines, including various accelerators and field programmable gate arrays (FPGAs) that can be attached to the CPU via CXL; and 3) the emergence of multitenancy usages in the data center arising from the growth of cloud computing. With CXL and these emerging trends, a current mechanism of “throttling at the source” or limiting queue entries at the source (e.g., at the CPU) no longer applies, because arbitrary new devices can be added, and memory traffic from these new devices no longer go through the processor queues. This calls for a destination-based throttling scheme in the memory subsystem itself, as opposed to a source based throttling scheme in the CPU for ensuring QoS. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of a system according to some embodiments. 
         FIG. 2  illustrates an example of a memory request instruction according to some embodiments. 
         FIG. 3  illustrates an example memory request and QoS manager according to some embodiments. 
         FIG. 4  illustrates an example computing platform. 
         FIG. 5  illustrates an example of a storage medium. 
         FIG. 6  illustrates another example computing platform. 
     
    
    
     DETAILED DESCRIPTION 
     As contemplated in the present disclosure, new QoS mechanisms are included in a computing system and a memory system to achieve one or more service level agreement (SLA) requirements per tenant for implementing memory requests received from tenants being executed on multiple computing devices (such as CPUs, graphic processing units (GPUs), FPGAs, accelerators, storage devices, etc.). In an embodiment, one or more SLA requirements include one or more of latency requirements, bandwidth requirements, or other performance metrics requirements. 
     As used herein, a tenant comprises one or more applications, processes, tasks, FPGA bitstreams, applications within an AI accelerator, or other processing entity being executed by a computing device that requests access to memory resources (such as reading data from a memory device and writing data to a memory device). Tenants can include sub-tenants. 
     There is a need to be able to implement memory QoS through alternate means as compared to the conventional processor/memory controller path, and closer to the computing devices themselves. For example, a computer express link (CXL) attached accelerator accesses the same memory as processor cores of a CPU and consumes device memory bandwidth, however there is currently no means to limit the memory bandwidth used by the CXL-attached accelerator. 
     There is a need to have a globally unique ID for each tenant throughout individual computing systems, and throughout a cloud computing environment (e.g., a plurality of data centers operated by a cloud service provider for the benefit of cloud computing customers), for use in implementing memory system QoS, and the means to track memory resource usage based on this tenant ID throughout the cloud computing environment. For example, tenant A may comprise 10 processes accessing a memory from one computing device, each process having a different process ID, and tenant B may comprise 20 processes across four computing devices (including one or more accelerators), all of which are accessing the same memory as tenant A. It may be desirable, or even required according to an SLA, to guarantee that 50% of the bandwidth to that memory is always available and/or allocated to tenant B. To implement managing the memory system QoS, new mechanisms in the data center infrastructure (e.g., the cloud computing environment) are needed to be able to map and track memory requests based on tenant ID. 
       FIG. 1  illustrates an example of a system  100  according to some embodiments. System  100  includes a computing system  103  comprising a plurality of computing devices to request access to memory, such as one or more processors  102  (each processor having one or more processing cores), zero or more GPUs  104 , zero or more network interface cards (NICs)  106 , zero or more FPGAs  108 , and zero or more artificial intelligence (AI) accelerators  110 . Other circuits, components, and devices (not shown in  FIG. 1 ) in system  100  may also initiate memory requests. Tenants  101  execute on computing devices and initiate memory requests. Memory request  112  is sent by one of the computing devices to memory system  114  over an interconnect (such as CXL, for example, but the interconnect is not shown in  FIG. 1 ) to read data from one or more memory devices  120  of memory system  114  or write data to one or more memory devices of the memory system. Memory request  112  is received via interface  116  of memory system  114  and forwarded to memory request and QoS manager  118 . In an embodiment, memory request and QoS manager  118  manages the QoS (e.g., as specified by the SLA) requirements for access to memory system  114  for tenants  101  of system  100  and implements received memory requests to access one or more memory devices  120  of memory system  114  according to the QoS requirements. Configuration information  122  is input to memory request and QoS manager  118  via interface  116  for use in determining memory bandwidth allocation for memory devices  120 . In an embodiment, configuration  122  defines parameters for managing memory system  114  (such as a prefetch policy, a power limit for the overall memory system, and other general memory configuration information not specific to a tenant). In an embodiment, memory configuration  122  is defined for a default setting for system  100  but may be overwritten or modified by a hypervisor (not shown in  FIG. 1 ) or by actions of a system administrator for system  100 . 
     A tenant  101  (having a globally unique tenant ID) sends one or more commands  123  to memory system  114  to register a tenant for QoS management and to specify one or more SLA requirements for the tenant. In an embodiment, a register SLA and tenant command includes one or more of a tenant ID, tenant tag, a priority level, and one or more SLA requirements (such as a bandwidth allocation (such as a percentage or other indicator) or a latency requirement). In an embodiment, a tenant tag is used for performing fast comparisons of tenants using content addressable memory (not shown in  FIG. 1 ) according to well-known mechanisms. In an embodiment, an SLA comprises one or more requirements for allocation of memory resources (e.g., memory bandwidth, latency, other performance metrics) for system  100  for a tenant. 
       FIG. 2  illustrates an example of a memory request instruction  112  according to some embodiments. Memory request  112  includes memory operation  202 , such as read or write. Memory request  112  includes payload  206 , such as the data to be written to the memory. Memory request  112  includes address  204 . Address  204  includes a physical address  210  in the memory (e.g., in memory devices  120 ) to be read from or written to. In an embodiment, address  204  also includes a globally unique tenant ID  208  storing the ID of the tenant requesting the memory access. In one embodiment, tenant ID  208  is stored in a first set of upper bits of address  204 , and physical address  210  is stored in a second set of lower bits of address  204 . In another embodiment, the locations of tenant ID  208  and physical address  210  are exchanged (e.g., the tenant ID is in the second set and the physical address is in the first set). In an embodiment, the number of bits in the first set and the number of bits in the second set is implementation dependent, depending on the computing devices requesting access to memory and memory subsystem  114 . 
       FIG. 3  illustrates an example memory request and QoS manager  118  according to some embodiments. In an embodiment, memory request and QoS manager  118  comprises three separate components—tenant registrar  302 , tenant resource monitor  306 , and QoS enforcer  310 . In other embodiments, any two or more of these components may be combined. In various embodiments, memory request and QoS manager  118  is implemented in software, firmware, hardware, or any combination. 
     Tenant registrar  302  receives commands to register a tenant and/or the tenant&#39;s SLA requirements. In one embodiment, a command includes both the tenant information and the one or more SLA requirements. In another embodiment, a register tenant command includes the information for registering a tenant identified by a tenant ID. In another embodiment, a register SLA command includes the one or more SLA requirements for a tenant identified by a tenant ID. Tenant registrar  302  stores received tenant registration information in tenant registrations  304 . In an embodiment, tenant registrations  304  is implemented by any suitable data structure, such as a table or an array, for example, indexed by tenant ID or tenant tag. In an embodiment, tenant registrar  302  causes QoS enforcer  310  to store one or more SLA requirements for a tenant in SLAs  312 . In an embodiment, SLAs  312  is implemented by any suitable data structure, such as a table or array, for example, indexed by tenant ID or tenant tag. In another embodiment, SLAs  312  is stored within or accessible by tenant registrar  302 . 
     In an embodiment, a command  123  to register a tenant and/or an SLA is defined as an instruction in the instruction set architecture (ISA) of the computing device (e.g., one of processor  102 , GPU  104 , NIC  106 , etc.). 
     Tenant resource monitor  306  receives memory requests  112  and, based at least in part on the requests to read memory and/or write memory, modifies current usages  308 . Current usages  308  comprises statistical information regarding memory usage of memory devices  120  over time by one or more tenants. In an embodiment, current usages  308  is implemented by any suitable data structure, such as a table or array, for example, indexed by tenant ID or tenant tag. In an embodiment, successful completion of a memory request is included in updated current usages  308 . 
     When a memory request is received, QoS enforcer  310  determines how and to what extent the memory request is to be implemented. QoS enforcer  310  accesses tenant registrations  304 , SLAs  312 , and current usages  308  to determine if the requesting tenant is valid and the memory request should be allowed (e.g., implemented or performed) based on the tenant&#39;s one or more SLA requirements  312  and the current usages  308  by the tenant (or in one embodiment, by all tenants). For example, if the tenant&#39;s one or more SLA requirements specify a maximum read/write limit per unit time of 5 GB and the current usage is only 1 GB, then QoS enforcer  310  allows memory request and QoS manager  118  to implement the request. However, if for example the tenant&#39;s one or more SLA requirements specify a maximum read/write limit per unit time of 5 GB and the current usage is 5 GB, then QoS enforcer does not allow memory request and QoS manager  118  to implement the request. Instead, an error message may be returned to the requesting tenant. In another example, if the requesting tenant is not currently using a selected bandwidth as a percentage of the overall bandwidth for a memory device  120 , then QoS enforcer  310  allows memory request and QoS manager  118  to implement the request. However, if the requesting tenant already is using all bandwidth allocated to the tenant as a percentage of the overall bandwidth of a memory device  120 , then QoS enforcer denies the request. In an embodiment, QoS enforcer  310  controls memory bandwidth and prioritization of memory requests for tenants by managing queue slots (not shown in  FIG. 3 ) used to access a memory device  120 . In an embodiment, QoS enforcer  310  operates periodically to check usages against SLA requirements per tenant and does not check usages for every received memory request. 
     In an embodiment a register SLA and tenant command  123  specifies latency and/or bandwidth values for a tenant  101 . In an embodiment, tenant registrations  304  and/or SLAs  312  can only be accessed by a ring  0  component (e.g., in privileged mode) within compute devices  102 - 110 . An entry SLAs  312  for a tenant  101  may include a tenant ID and SLA requirements assigned to the tenant. In an embodiment, the one or more SLA requirements may include one or more of a latency maximum value, a memory bandwidth maximum value over a period of time, and an indication as to whether the SLA requirements have a best-effort requirement or a guaranteed requirement. Memory request and QoS manager  118  uses these configuration settings per tenant to implement QoS policies. 
     In an embodiment, a tenant  101  may comprise a plurality of sub-tenants, and this structure may be applied recursively. In an embodiment, the relationship between a tenant and sub-tenants of the tenant comprises a hierarchical tree structure. Tenant registrations  304  and/or SLAs  312  may include information about the hierarchy of the tenant and sub-tenants. In an embodiment, the tenant comprises a plurality of sub-tenants and the memory request manager  118  allows a memory request to access a selected one of memory devices when usage of the plurality of memory devices meets the one or more SLA requirements for the tenant and all sub-tenants of the tenant. 
     To meet the one or more SLA requirements, memory request and QoS manager  118  checks that there is no oversubscription of the bandwidth to memory devices  120 . The bandwidth to memory devices  120  may be partitioned into a best effort bandwidth partition where all tenants mapped to the best effort bandwidth partition will have a best effort to achieve the specified SLA requirements. The bandwidth to memory devices  120  may be partitioned into a guaranteed bandwidth partition where all tenants mapped to the guaranteed partition will have guaranteed access to the bandwidth, but if there is no bandwidth available on the guaranteed bandwidth partition an error may be returned in response to the memory request. Using tenant registrations  304  and/or SLAs  312 , memory request and QoS manager  118  implements load balancing among all memory requests  112  coming from the compute devices  102 - 110  based on the two type of SLAs. In the case that the tenant  101  associated with a memory request  112  has a best effort SLA, memory request and QoS manager  118  will apply a best effort QoS to try to meet the SLA requirements for the tenant. In the case that the tenant  101  associated with the memory request  112  has a guaranteed SLA, memory request and QoS manager  118  will schedule the pending memory requests for that tenant up to the specified SLA requirements. Once the maximum value of the one or more SLA requirements is reached, the incoming memory requests for that tenant will be throttled. 
     In an embodiment, a system may include multiple memory systems/subsystems, for example, a first CXL based memory system (comprising a set of memory devices (e.g., DRAM)) and a second CXL based memory system (comprising a different set of memory devices (e.g., 3DXPoint based devices)) within the same server, each of which independently accepts tenant registrations and SLA registrations, and where the same tenant X may have registered one set of SLA/rules in the first CXL based memory system and a different set of SLA/rules in the second CXL based memory system. 
     In another embodiment, the capability to register inter-tenant SLAs is provided, where the SLA of a first tenant A is conditioned on the presence of another tenant B, wherein when tenant B is present, a priority of tenant A is increased so that tenant A and receives 50% of the bandwidth; otherwise tenant A receives only 10% of the bandwidth. This is an example of a SLA specification that cannot be done at the source (e.g., CPU) efficiently and needs to be done at the target (e.g., memory subsystem). 
       FIG. 4  illustrates an example computing system  400 . In an embodiment, computing system  400  is an example of at least portions of computing system  103  and/or memory system  114 . As shown in  FIG. 4 , computing system  400  includes a computing platform  401  coupled to a network  470 . In some examples, as shown in  FIG. 4 , computing platform  401  may couple to network  470  via a network communication channel  475  and through a network I/O device  410  (e.g., a network interface controller (NIC)) having one or more ports connected or coupled to network communication channel  475 . 
     According to some examples, computing platform  401 , as shown in  FIG. 4 , may include circuitry  420 , primary memory  430 , a network (NW) I/O device driver  440 , an operating system (OS)  450 , one or more application(s)  460 , and storage devices  465  (such as memory system  114 ). In at least one embodiment, storage devices  465  may comprise one or more of hard disk drives (HDDs) and/or solid-state drives (SSDs). In an embodiment, storage devices  465  may be non-volatile memories (NVMs). In some examples, as shown in  FIG. 4 , circuitry  420  may communicatively couple to primary memory  430  and network I/O device  410  via communications link  455 . Although not shown in  FIG. 4 , in some examples, operating system  450 , NW I/O device driver  440  or application(s)  460  may be implemented, at least in part, via cooperation between one or more memory devices included in primary memory  430  (e.g., volatile or non-volatile memory devices) and elements of circuitry  420  such as processing cores  422 - 1  to  422 - m , where “m” is any positive whole integer greater than 2. 
     In some examples, computing platform  401 , may include, but is not limited to, a server, a server array or server farm, a web server, a network server, an Internet server, a work station, a mini-computer, a main frame computer, a supercomputer, a network appliance, a web appliance, a distributed computing system, multiprocessor systems, processor-based systems, a laptop computer, a tablet computer, a smartphone, or a combination thereof. Also, circuitry  420  having processing cores  422 - 1  to  422 - m  may include various commercially available processors, including without limitation Intel® Atom®, Celeron®, Core (2) Duo®, Core i3, Core i5, Core i7, Itanium®, Pentium®, Xeon® or Xeon Phi® processors; ARM processors, AMD processors, and similar processors. Circuitry  420  may include at least one cache  435  to store data. 
     According to some examples, primary memory  430  may be composed of one or more memory devices or dies which may include various types of volatile and/or non-volatile memory. Volatile types of memory may include, but are not limited to, dynamic random-access memory (DRAM), static random-access memory (SRAM), thyristor RAM (TRAM) or zero-capacitor RAM (ZRAM). Non-volatile types of memory may include byte or block addressable types of non-volatile memory having a 3-dimensional (3-D) cross-point memory structure that includes chalcogenide phase change material (e.g., chalcogenide glass) hereinafter referred to as “3-D cross-point memory”. Non-volatile types of memory may also include other types of byte or block addressable non-volatile memory such as, but not limited to, multi-threshold level NAND flash memory, NOR flash memory, single or multi-level phase change memory (PCM), resistive memory, nanowire memory, ferroelectric transistor random access memory (FeTRAM), magneto-resistive random-access memory (MRAM) that incorporates memristor technology, spin transfer torque MRAM (STT-MRAM), or a combination of any of the above. In another embodiment, primary memory  430  may include one or more hard disk drives within and/or accessible by computing platform  401 . 
       FIG. 5  illustrates an example of a storage medium  500 . Storage medium  500  may comprise an article of manufacture. In some examples, storage medium  500  may include any non-transitory tangible computer readable medium or machine readable medium, such as an optical, magnetic or semiconductor storage. Storage medium  500  may store various types of computer executable instructions, such as instructions  502  for apparatus  118 . Examples of a computer readable or machine-readable storage medium may include any tangible media capable of storing electronic data, including volatile memory or non-volatile memory, removable or non-removable memory, erasable or non-erasable memory, writeable or re-writeable memory, and so forth. Examples of computer executable instructions may include any suitable type of code, such as source code, compiled code, interpreted code, executable code, static code, dynamic code, object-oriented code, visual code, and the like. The examples are not limited in this context. 
       FIG. 6  illustrates an example computing platform  600 . In some examples, as shown in  FIG. 6 , computing platform  600  may include a processing component  602 , other platform components  604  and/or a communications interface  606 . 
     According to some examples, processing component  602  may execute processing operations or logic for apparatus  118  and/or storage medium  500 . Processing component  602  may include various hardware elements, software elements, or a combination of both. Examples of hardware elements may include devices, logic devices, components, processors, microprocessors, circuits, processor circuits, circuit elements (e.g., transistors, resistors, capacitors, inductors, and so forth), integrated circuits, application specific integrated circuits (ASIC), programmable logic devices (PLD), digital signal processors (DSP), field programmable gate array (FPGA), memory units, AI cores, logic gates, registers, semiconductor device, chips, microchips, chip sets, and so forth. Examples of software elements may include software components, programs, applications, computer programs, application programs, device drivers, system programs, software development programs, machine programs, operating system software, middleware, firmware, software modules, routines, subroutines, functions, methods, procedures, software interfaces, application program interfaces (API), instruction sets, computing code, computer code, code segments, computer code segments, words, values, symbols, or any combination thereof. Determining whether an example is implemented using hardware elements and/or software elements may vary in accordance with any number of factors, such as desired computational rate, power levels, heat tolerances, processing cycle budget, input data rates, output data rates, memory resources, data bus speeds and other design or performance constraints, as desired for a given example. 
     In some examples, other platform components  604  may include common computing elements, such as one or more processors, multi-core processors, co-processors, memory units, chipsets, controllers, peripherals, interfaces, oscillators, timing devices, video cards, audio cards, multimedia input/output (I/O) components (e.g., digital displays), power supplies, and so forth. Examples of memory units may include without limitation various types of computer readable and machine readable storage media in the form of one or more higher speed memory units, such as read-only memory (ROM), random-access memory (RAM), dynamic RAM (DRAM), Double-Data-Rate DRAM (DDRAM), synchronous DRAM (SDRAM), static RAM (SRAM), programmable ROM (PROM), erasable programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), types of non-volatile memory such as 3-D cross-point memory that may be byte or block addressable. Non-volatile types of memory may also include other types of byte or block addressable non-volatile memory such as, but not limited to, multi-threshold level NAND flash memory, NOR flash memory, single or multi-level PCM, resistive memory, nanowire memory, FeTRAM, MRAM that incorporates memristor technology, STT-MRAM, or a combination of any of the above. Other types of computer readable and machine-readable storage media may also include magnetic or optical cards, an array of devices such as Redundant Array of Independent Disks (RAID) drives, solid state memory devices (e.g., USB memory), solid state drives (SSD) and any other type of storage media suitable for storing information. 
     In some examples, communications interface  606  may include logic and/or features to support a communication interface. For these examples, communications interface  606  may include one or more communication interfaces that operate according to various communication protocols or standards to communicate over direct or network communication links or channels. Direct communications may occur via use of communication protocols or standards described in one or more industry standards (including progenies and variants) such as those associated with the PCIe specification. Network communications may occur via use of communication protocols or standards such those described in one or more Ethernet standards promulgated by IEEE. For example, one such Ethernet standard may include IEEE 802.3. Network communication may also occur according to one or more OpenFlow specifications such as the OpenFlow Switch Specification. 
     The components and features of computing platform  600  may be implemented using any combination of discrete circuitry, ASICs, logic gates and/or single chip architectures. Further, the features of computing platform  600  may be implemented using microcontrollers, programmable logic arrays and/or microprocessors or any combination of the foregoing where suitably appropriate. It is noted that hardware, firmware and/or software elements may be collectively or individually referred to herein as “logic” or “circuit.” 
     It should be appreciated that the exemplary computing platform  600  shown in the block diagram of  FIG. 6  may represent one functionally descriptive example of many potential implementations. Accordingly, division, omission or inclusion of block functions depicted in the accompanying figures does not infer that the hardware components, circuits, software and/or elements for implementing these functions would necessarily be divided, omitted, or included in embodiments. 
     One or more aspects of at least one example may be implemented by representative instructions stored on at least one tangible, non-transitory machine-readable medium which represents various logic within the processor, which when read by a machine, computing device or system causes the machine, computing device or system to fabricate logic to perform the techniques described herein. Such representations, known as “IP cores” may be stored on a tangible, machine readable medium and supplied to various customers or manufacturing facilities to load into the fabrication machines that actually make the logic or processor. 
     Various examples may be implemented using hardware elements, software elements, or a combination of both. In some examples, hardware elements may include devices, components, processors, microprocessors, circuits, circuit elements (e.g., transistors, resistors, capacitors, inductors, and so forth), integrated circuits, ASIC, programmable logic devices (PLD), digital signal processors (DSP), FPGAs, AI accelerators/cores, memory units, logic gates, registers, semiconductor device, chips, microchips, chip sets, and so forth. In some examples, software elements may include software components, programs, applications, computer programs, application programs, system programs, machine programs, operating system software, middleware, firmware, software modules, routines, subroutines, functions, methods, procedures, software interfaces, application program interfaces (API), instruction sets, computing code, computer code, code segments, computer code segments, words, values, symbols, or any combination thereof. Determining whether an example is implemented using hardware elements and/or software elements may vary in accordance with any number of factors, such as desired computational rate, power levels, heat tolerances, processing cycle budget, input data rates, output data rates, memory resources, data bus speeds and other design or performance constraints, as desired for a given implementation. 
     Some examples may include an article of manufacture or at least one computer-readable medium. A computer-readable medium may include a non-transitory storage medium to store logic. In some examples, the non-transitory storage medium may include one or more types of computer-readable storage media capable of storing electronic data, including volatile memory or non-volatile memory, removable or non-removable memory, erasable or non-erasable memory, writeable or re-writeable memory, and so forth. In some examples, the logic may include various software elements, such as software components, programs, applications, computer programs, application programs, system programs, machine programs, operating system software, middleware, firmware, software modules, routines, subroutines, functions, methods, procedures, software interfaces, API, instruction sets, computing code, computer code, code segments, computer code segments, words, values, symbols, or any combination thereof. 
     Some examples may be described using the expression “in one example” or “an example” along with their derivatives. These terms mean that a particular feature, structure, or characteristic described in connection with the example is included in at least one example. The appearances of the phrase “in one example” in various places in the specification are not necessarily all referring to the same example. 
     Included herein are logic flows or schemes representative of example methodologies for performing novel aspects of the disclosed architecture. While, for purposes of simplicity of explanation, the one or more methodologies shown herein are shown and described as a series of acts, those skilled in the art will understand and appreciate that the methodologies are not limited by the order of acts. Some acts may, in accordance therewith, occur in a different order and/or concurrently with other acts from that shown and described herein. For example, those skilled in the art will understand and appreciate that a methodology could alternatively be represented as a series of interrelated states or events, such as in a state diagram. Moreover, not all acts illustrated in a methodology may be required for a novel implementation. 
     A logic flow or scheme may be implemented in software, firmware, and/or hardware. In software and firmware embodiments, a logic flow or scheme may be implemented by computer executable instructions stored on at least one non-transitory computer readable medium or machine readable medium, such as an optical, magnetic or semiconductor storage. The embodiments are not limited in this context. 
     Some examples are described using the expression “coupled” and “connected” along with their derivatives. These terms are not necessarily intended as synonyms for each other. For example, descriptions using the terms “connected” and/or “coupled” may indicate that two or more elements are in direct physical or electrical contact with each other. The term “coupled,” however, may also mean that two or more elements are not in direct contact with each other, but yet still co-operate or interact with each other. 
     It is emphasized that the Abstract of the Disclosure is provided to comply with 37 C.F.R. Section 1.72(b), requiring an abstract that will allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in a single example for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed examples require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed example. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate example. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein,” respectively. Moreover, the terms “first,” “second,” “third,” and so forth, are used merely as labels, and are not intended to impose numerical requirements on their objects. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.