并行计算机的内存架构

介绍现代计算机的内存架构,比如:共享内存,分布式内存以及混合共享与分布式内存等概念。对于CPU访问不同架构的内存带来的问题进行了深入剖析,
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1.Shared Memory. Distributed Memory. Hybrid Distributed-Shared Memory.

2.Shared Memory Shared memory parallel computers vary widely, but generally have in common the ability for all processors to access all memory as global address space. Multiple processors can operate independently but share the same memory resources. Changes in a memory location effected by one processor are visible to all other processors. Shared memory machines can be divided into two main classes based upon memory access times: UMA and NUMA . Uniform Memory Access (UMA): Non-Uniform Memory Access (NUMA )

3.Shared Memory : UMA vs. NUMA Uniform Memory Access (UMA): Most commonly represented today by Symmetric Multiprocessor (SMP) machines Identical processors Equal access and access times to memory Sometimes called CC-UMA - Cache Coherent UMA. Cache coherent means if one processor updates a location in shared memory, all the other processors know about the update. Cache coherency is accomplished at the hardware level . Non-Uniform Memory Access (NUMA): Often made by physically linking two or more SMPs One SMP can directly access memory of another SMP Not all processors have equal access time to all memories Memory access across link is slower If cache coherency is maintained, then may also be called CC-NUMA - Cache Coherent NUMA

4.MEMORY BUS Shared Memory MIMD machine Comm : Source PE writes data to GM & destination retrieves it Easy to build, conventional OSes of SISD can be easily be ported Limitation : reliability & expandibility . A memory component or any processor failure affects the whole system. Increase of processors leads to memory contention. Ex. : Silicon graphics supercomputers.... MEMORY BUS Global Memory System Processor A Processor B Processor C MEMORY BUS

5.Global Memory System Processor Cache Processor Cache Uniform Memory Access - UMA Symmetric Multiprocessor SMP Processor Cache Memory: centralized with uniform access time (“ UMA ”) and bus interconnect, I/O Examples: Dell Workstation 530, Sun Enterprise, SGI Challenge typical: 1 cycle to local cache 20 cycles to remote cache 100 cycles to memory

6.CPU x 4 Channel Cache Memory I/O CPU x 4 Channel Cache Memory I/O CPU x 4 Channel Cache Memory I/O CPU x 4 Channel Cache Memory I/O Non-Uniform Memory Access - NUMA Examples: DASH/Alewife/FLASH (academic), SGI Origin, Compaq GS320, Sequent (IBM) NUMA-Q

7.Shared Memory: Pros and Cons Advantages Global address space provides a user-friendly programming perspective to memory Data sharing between tasks is both fast and uniform due to the proximity of memory to CPUs Disadvantages : Primary disadvantage is the lack of scalability between memory and CPUs. Adding more CPUs can geometrically increases traffic on the shared memory-CPU path, and for cache coherent systems, geometrically increase traffic associated with cache/memory management. Programmer responsibility for synchronization constructs that insure "correct" access of global memory. Expense: it becomes increasingly difficult and expensive to design and produce shared memory machines with ever increasing numbers of processors.

8.Shared Memory: Pros and Cons Advantages Global address space provides a user-friendly programming perspective to memory Data sharing between tasks is both fast and uniform due to the proximity of memory to CPUs Disadvantages : Primary disadvantage is the lack of scalability between memory and CPUs. Adding more CPUs can geometrically increases traffic on the shared memory-CPU path, and for cache coherent systems, geometrically increase traffic associated with cache/memory management. Programmer responsibility for synchronization constructs that insure "correct" access of global memory. Expense: it becomes increasingly difficult and expensive to design and produce shared memory machines with ever increasing numbers of processors.

9.MEMORY BUS Distributed Memory MIMD Processor A Processor B Processor C MEMORY BUS MEMORY BUS Memory System A Memory System B Memory System C IPC channel

10.Distributed Memory: Pro and Con Advantages : Memory is scalable with number of processors. Increase the number of processors and the size of memory increases proportionately. Each processor can rapidly access its own memory without interference and without the overhead incurred with trying to maintain cache coherency. Cost effectiveness: can use commodity, off-the-shelf processors and networking. Disadvantages The programmer is responsible for many of the details associated with data communication between processors. It may be difficult to map existing data structures, based on global memory, to this memory organization. Non-uniform memory access (NUMA) times

11.Distributing memory among processing nodes has 2 pluses: It’s a great way to save some bandwidth With memory distributed at nodes, most accesses are to local memory within a particular node No need for bus communication Reduces latency for accesses to local memory It also has 1 big minus! Have to communicate among various processors Leads to a higher latency for intra-node communication Also need bandwidth to actually handle communication

12.Distributing memory among processing nodes has 2 pluses: It’s a great way to save some bandwidth With memory distributed at nodes, most accesses are to local memory within a particular node No need for bus communication Reduces latency for accesses to local memory It also has 1 big minus! Have to communicate among various processors Leads to a higher latency for intra-node communication Also need bandwidth to actually handle communication

13.Hybrid Distributed-Shared Memory Comparison of Shared and Distributed Memory Architectures Architecture CC-UMA CC-NUMA Distributed Examples SMPs Sun Vexx DEC/Compaq SGI Challenge IBM POWER3 Bull NovaScale SGI Origin Sequent HP Exemplar DEC/Compaq IBM POWER4 (MCM) Cray T3E Maspar IBM SP2 IBM BlueGene Communications MPI Threads OpenMP shmem MPI Threads OpenMP shmem MPI Scalability to 10s of processors to 100s of processors to 1000s of processors Draw Backs Memory-CPU bandwidth Memory-CPU bandwidth Non-uniform access times System administration Programming is hard to develop and maintain Software Availability many 1000s ISVs many 1000s ISVs 100s ISVs Summarizing a few of the key characteristics of shared and distributed memory machines

14.Taxonomy of Parallel Processor Architectures

15.More detail on cache coherency protocols with some examples…

16.More on centralized shared memory Its worth studying the various ramifications of a centralized shared memory machine (and there are lots of them) Later we’ll look at distributed shared memory… When studying memory hierarchies we saw… …cache structures can substantially reduce memory bandwidth demands of a processor Multiple processors may be able to share the same memory

17.More on centralized shared memory Centralized shared memory supports private/shared data If 1 processor in a multiprocessor network operates on private data, caching, etc. are handled just as in uniprocessors But if shared data is cached there can be multiple copies and multiple updates Good b/c it reduces required memory bandwidth; bad because we now must worry about cache coherence

18.Cache coherence – why it’s a problem Assumes that neither cache had value/location X in it 1 st Both a write-through cache and a write-back cache will encounter this problem If B reads the value of X after Time 3, it will get 1 which is the wrong value! Time Event Cache contents for CPU A Cache contents for CPU B Memory contents for location X 0 1 1 CPU A reads X 1 1 2 CPU B reads X 1 1 1 3 CPU A stores 0 into X 0 1 0

19.Coherence in shared memory programs Must have coherence and consistency Memory system coherent if: Program order preserved (always true in uniprocessor) Say we have a read by processor P of location X Before the read processor P wrote something to location X In the interim, no other processor has written to X A read to X should always return the value written by P A coherent view of memory is provided 1 st , processor A writes something to memory location X Then, processor B tries to read from memory location X Processor B should get the value written by processor A assuming… Enough time has past b/t the two events No other writes to X have occurred in the interim

20.Coherence in shared memory programs (continued) Memory system coherent if: (continued) Writes to same location are serialized Two writes to the same location by any two processors are seen in the same order by all processors Ex. Values of A and B are written to memory location X Processors can’t read the value of B and then later as A If writes not serialized… One processor might see the write of processor P2 to location X 1 st Then, it might later see a write to location X by processor P1 (P1 actually wrote X before P2) Value of P1 could be maintained indefinitely even though it was overwritten

21.Coherence/consistency Coherence and consistency are complementary Coherence defines actions of reads and writes to same memory location Consistency defines actions of reads and writes with regard to accesses of other memory locations Assumption for the following discussion: Write does not complete until all processors have seen effect of write Processor does not change order of any write with any other memory accesses Not exactly the case for either one really…but more later…

22.Caches in coherent multiprocessors In multiprocessors, caches at individual nodes help w/ performance Usually by providing properties of “migration” and “replication” Migration: Instead of going to centralized memory for each reference, data word will “migrate” to a cache at a node Reduces latency Replication: If data simultaneously read by two different nodes, copy is made at each node Reduces access latency and contention for shared item Supporting these require cache coherence protocols Really, we need to keep track of shared blocks…