Computer Architecture

1. Computer Architecture MIMD Parallel Processors Iolanthe II racing in Waitemata Harbour

2. Classification of Parallel Processors • Flynn’s Taxonomy • Classifies according to instruction and data stream • Single Instruction Single Data • Sequential processors • Single Instruction Multiple Data • CM-2 – multiple small processors • Vector processors • Parts of commercial processors - MMX, Altivec • Multiple Instruction Single Data • ? • Multiple Instruction Multiple Data • General Parallel Processors

3. MIMD Systems • Recipe • Buy a few high performance commercial PEs • DEC Alpha • MIPS R10000 • UltraSPARC • Pentium? • Put them together with some memory and peripherals on a common bus • Instant parallel processor! • How to program it?

4. Programming Model • Problem not unique to MIMD • Even sequential machines need one • von Neuman (stored program) model • Parallel - Splitting the work load • Data • Distribute data to PEs • Instructions • Distribute tasks to PEs • Synchronization • Having divided the data & tasks,how do we synchronize tasks?

5. Programming Model • Shared Memory Model • Flavour of the year • Generally thought to be simplest to manage • All PEs see a common(virtual) address space • PEs communicateby writing into the common address space

6. Data Distribution • Trivial • All the data sits in the common address space • Any PE can access it! • Uniform Memory Access(UMA) systems • All PEs access all data with same tacc • Non-UMA (NUMA) systems • Memory is physically distributed • Some PEs are “closer” to some addresses • More later!

7. Synchronisation • Read static shared data • No problem! • Update problem • PE0 writes x • PE1 reads x • How to ensure thatPE1 reads the lastvalue written by PE0? • Semaphores • Lock resources(memory areas or ...)while being updatedby one PE

8. Synchronisation • Semaphore • Data structure in memory • Count of waiters • -1 = resource free • >= 0 resource in use • Pointer to list of waiters • Two operations • Wait • Proceed immediately if resource free(waiter count = -1) • Notify • Advise semaphore that you have finished with resource • Decrement waiter count • First waiter will be given control

9. Semaphores - Implementation • Scenario • Semaphore free (-1) • PE0: wait .. • Resource free, so PE0 uses it (sets 0) • PE1: wait .. • Reads count (0) • Starts to increment it .. • PE0notify .. • Gets bus and writes -1 • PE1:(finishing wait) • Adds 1 to 0, writes 1 to count, adds PE1 TCB to list • Stalemate! • Who issues notify to free the resource?

10. Atomic Operations • Problem • PE0 wrote a new value (-1) after PE1 had read the counter • PE1 increments the value it read (0) and writes it back • Solution • PE1’s read and update must be atomic • No other PE must gain access to counterwhile PE1 is updating • Usually an architecture will provide • Test and set instruction • Read a memory location, test it,if it’s 0, write a new value,else do nothing • Atomic or indivisible .. No other PE can access the value until the operation is complete

11. Atomic Operations • Test & Set • Read a memory location, test it,if it’s 0, write a new value,else do nothing • Can be used to guard a resource • When the location contains 0 -access to the resource is allowed • Non-zero value means the resource is locked • Semaphore: • Simple semaphore (no wait list) • Implement directly • Waiter “backs off” and tries again (rather than being queued) • Complex semaphore (with wait list) • Guards the wait counter

12. Atomic Operations • Processor must provide an atomic operation for • Multi-tasking or multi-threading on a single PE • Multiple processes • Interrupts occur at arbitrary points in time • including timer interrupts signaling end of time-slice • Any process can be interrupted in the middle of a read-modify-write sequence • Shared memory multi-processors • One PE can lose control of the bus after the read of a read-modify-write • Cache? • Later!

13. Atomic Operations • Variations • Provide equivalent capability • Sometimes appear in strange guises! • Read-modify-write bus transactions • Memory location is read, modified and written back as a single, indivisible operation • Test and exchange • Check register’s value, if 0, exchange with memory • Reservation Register (PowerPC) • lwarx - load word and reserve indexed • stwcx - store word conditional indexed • Reservation register stores address of reserved word • Reservation and use can be separated by sequence of instructions

14. Barriers • In shared memoryenvironment • PEs must know whenanother PE hasproduced a result • Simplest case:barrier for all PEs • Must be inserted byprogrammer • Potentially expensive • All PEs stall and waste time in the barrier

15. Cache? • What happens to cachedlocations?

16. Multiple Caches • Coherence • PEA reads location xfrom memory • Copy in cache A • PEB reads location x from memory • Copy in cache B • PEA adds 1

17. Multiple Caches - Inconsistent states • Coherence • PEA reads location xfrom memory • Copy in cache A • PEB reads location x from memory • Copy in cache B • PEA adds 1 • A’s copy now 201 • PEB reads location x • reads 200 from cache B

18. Multiple Caches - Inconsistent states • Coherence • PEA reads location xfrom memory • Copy in cache A • PEB reads location x from memory • Copy in cache B • PEA adds 1 • A’s copy now 201 • PEB reads location x • reads 200 from cache B • Caches and memory are now inconsistent ornotcoherent

19. Cache - Maintaining Coherence • Invalidate on write • PEA reads location xfrom memory • Copy in cache A • PEB reads location x from memory • Copy in cache B • PEA adds 1 • A’s copy now 201 • Issues invalidate x • Cache B marks x invalid • Invalidate is address transaction only

20. Cache - Maintaining Coherence • Reading the new value • PEB reads location x • Main memoryis wrong also • PEAsnoops read • Realises it hasvalid copy • PEA issues retry

21. Cache - Maintaining Coherence • Reading the new value • PEB reads location x • Main memoryis wrong also • PEAsnoops read • Realises it hasvalid copy • PEA issues retry • PEA writes x back • Memory now correct • PEB reads location x again • Reads latest version

22. Coherent Cache - Snooping • SIU “snoops” bus for transactions • Addresses compared with local cache • Matches • Initiate retries • Local copy is modified • Local copy is written to bus • Invalidate local copies • Another PE is writing • Mark local copies shared • second PE is readingsame value

23. Coherent Cache - MESI protocol • Cache line has 4 states • Invalid • Modified • Only valid copy • Memory copy is invalid • Exclusive • Only cached copy • Memory copy is valid • Shared • Multiple cached copies • Memory copy is valid

24. MESI State Diagram • Note the number of bus transactions needed! WH Write Hit WM Write Miss RH Read Hit RMS Read Miss Shared RME Read Miss Exclusive SHW Snoop Hit Write

25. Coherent Cache - The Cost • Cache coherency transactions • Additional transactions needed • Shared • Write Hit • Other caches must be notified • Modified • Other PE read • Push-out needed • Other PE write • Push-out needed - writing one word of n-word line • Invalid - modified in other cache • Read or write • Wait for push-out

26. Clusters • A bus which is too long becomes slow! • eg PCI is limited to 10 TTL loads • Lots of processors? • On the same bus • Bus speed must be limited • Low communication rate • Better to use a single PE! • Clusters • ~8 processors on a bus

27. Clusters £8 cache coherent (CC) processors on a bus Interconnect network ~100? clusters

28. Clusters Network Interface Unit Detects requests for “remote” memory

29. Clusters Message despatched to remote cluster’s NIU Memory Request Message

30. Clusters - Shared Memory • Non Uniform Memory Access • Access time to memory depends on location! From PEs in this cluster This memory is much closer than this one!

31. Clusters - Shared Memory • Non Uniform Memory Access • Access time to memory depends on location! Worse! NIU needs to maintain cache coherence across the entire machine

32. Clusters - Maintaining Cache Coherence • NIU (or equivalent) maintains directory • Directory Entries • All lines from local memory cached elsewhere • NIU software (firmware) • Checks memory requests against directory • Update directory • Send invalidate messages to other clusters • Fetch modified (dirty) lines from other clusters • Remote memory access cost • 100s of cycles! Directory (Cluster 2) Address Status Clusters 4340 S 1, 3, 8 5260 E 9

33. Clusters - “Off the shelf” • Commercial clusters • Provide page migration • Make copy of a remote page on the local PE • Programmer remains responsible for coherence • Don’t provide hardware support for cache coherence (across network) • Fully CC machines may never be available! • Software Systems • .... è

34. Shared Memory Systems • Software Systems • eg Treadmarks • Provide shared memory on page basis • Software • detects references to remote pages • moves copy to local memory • Reduces shared memory overhead • Provides some of the shared memory model convenience • Without swamping interconnection network with messages • Message overhead is too high for a single word! • Word basis is too expensive!!

35. Shared Memory Systems - Granularity • Granularity • Word basis is too expensive!! • Sharing data at low granularity • Fine grain sharing • Access / sharing for individual words • Overheads too high • Number of messages • Message overhead is high for one word • Compare • Burst access to memory • Don’t fetch a single word - • Overhead (bus protocol) is too high • Amortize cost of access over multiple words

36. Shared Memory Systems - Granularity • Coarse Grain Systems • Transferring data from cluster to cluster • Overhead • Messages • Updating directory • Amortise the overhead over a whole page • Lower relative overhead • Applies to thread size also • Split program into small threads of control • Parallel Overhead • cost of setting up & starting each thread • cost of synchronising at the end of a set of threads • Can be more efficient to run a single sequential thread!

37. Coarse Grain Systems • So far ... • Most experiments suggest that fine grain systems are impractical • Larger, coarser grain • Blocks of data • Threads of computation • needed to reduce overall computation time by using multiple processors • Too Fine grain parallel systems • can run slower than a single processor!

38. Parallel Overhead • Ideal • Time = 1/n • Add Overhead • Time > optimal • No point to usemore than4 PEs!!

39. Parallel Overhead • Ideal • Time = 1/n • Add Overhead • Time > optimal • No point to usemore than4 PEs!!

40. Parallel Overhead • Shared memory systems • Best results if you • Share on large block basis eg page • Split program into coarse grain(long running) threads • Give away some parallelismto achieve any parallel speedup! • Coarse grain • Data • Computation There’s parallelism at the instruction level too!The instruction issue unit in a sequential processor is trying to exploit it!

41. Clusters - Improving multiple PE performance • Bandwidth to memory • Cache reduces dependency on the memory-CPU interface • 95% cache hits • 5% of memory accesses crossing the interface • but add • a few PEs and • a few CC transactions • even if the interface was coping before,it won’t in a multiprocessor system! A major bottleneck!

42. Clusters - Improving multiple PE performance • Bus protocols add to access time • Request / Grant / Release phases needed • “Point-to-point” is faster! • Cross-bar switch interface to memory • No PE contends with any other for the common bus Cross-bar? Name taken from old telephone exchanges!

43. Clusters - Memory Bandwidth • Modern Clusters • Use “Point-to-point” X-bar interfaces to memory to get bandwidth! • Cache coherence? • Now really hard!! • How does each cachesnoop all transactions?

44. Programming Model • Distributed Memory • Message passing • Alternative to shared memory • Each PE has own address space • PEs communicate with messages • Messages providesynchronisation • PE can block orwait for a message

45. Programming Model - Distributed Memory • Distributed Memory Systems • Hardware is simple! • Network can be as simple as ethernet • Networks of Workstations model • Commodity (cheap!) PEs • Commodity Network • Standard • Ethernet • ATM • Proprietary • Myrinet • Achilles (UWA!)

46. Programming Model - Distributed Memory • Distributed Memory Systems • Software is considered harder • Programmer responsible for • Distributing data to individual PEs • Explicit Thread control • Starting, stopping & synchronising • At least two commonly available systems • Parallel Virtual Machine (PVM) • Message Passing Interface (MPI) • Built on two operations • Send data, destPE, block | don’t block • Receive data, srcPE, block | don’t block • Blocking ensures synchronisation

47. Programming Model - Distributed Memory • Distributed Memory Systems • Performance generally better (versus shared memory) • Shared memory has hidden overheads • Grain size poorly chosen • eg data doesn’t fit into pages • Unnecessary coherencetransactions • Updating a shared region (each page)before end of computation • MP system waits and updates page when computation is complete

48. Programming Model - Distributed Memory • Distributed Memory Systems • Performance generally better (versus shared memory) • False sharing • Severely degrades performance • May not be apparent on superficial analysis Memory page PEa accesses this data This whole page ping-pongs between PEa and PEb PEb accesses this data

49. Distributed Memory - Summary • Simpler (almost trivial) hardware • Software • More programmer effort • Explicit data distribution • Explicit synchronisation • Performance generally better • Programmer knows more about the problem • Communicates only when necessary • Communication grain size can be optimum • Lower overheads

50. Data Flow • Conventional programming models are control driven • Instruction sequence is precisely specified • Sequence specifies control • which instruction the CPU will execute next • Execution rule: • Execute an instruction when its predecessor has completed • s1: r = a*b;s2: s = c*d;s3: y = r + s; s2 executes when s1 is complete s3 executes when s2 is complete