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SHMEM Programming Model

SHMEM Programming Model. Hung-Hsun Su UPC Group, HCS lab 1/23/2004. Outline. Background Nuts and Bolts GPSHMEM Performance Conclusion Reference. Background What is SHMEM?. SHard MEMory library Based on SPMD model Available for C / Fortran

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SHMEM Programming Model

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  1. SHMEM Programming Model Hung-Hsun Su UPC Group, HCS lab 1/23/2004

  2. Outline • Background • Nuts and Bolts • GPSHMEM • Performance • Conclusion • Reference

  3. BackgroundWhat is SHMEM? • SHard MEMory library • Based on SPMD model • Available for C / Fortran • Hybrid Message Passing / Shared Memory Programming Model • Message Passing Like • Explicit communication, replication and synchronization • Specification of remote data location (processor id) is required • Shard Memory like • Provides logically shared memory system view • Communication require processor on one side only • Allows any processor element (PE) to access memory in a remote PE without involving the microprocessor on the remote PE (put / get) • Non-blocking data transfer

  4. BackgroundWhat is SHMEM? • Must know the address of a variable on the remote processor for transfer • same on all PEs • Remotely accessible data objects (Symmetric Vars.) • Global variables • Local static variables • Variables in common blocks • Fortran variables modified by a !DIR$ SYMMETRIC directive • C variables modified by a #pragma symmetric directive

  5. BackgroundWhy program in SHMEM? • Easier to program in than MPI / PVM • Low latency, high bandwidth data transfer • Puts • Gets • Provide efficient collective communication • Gather / Scatter • All-to-all • Broadcast • Reductions • Provide mechanisms to implement mutual exclusion • Atomic swap • Locking • Provide synchronization mechanisms • Barrier • Fence, Quiet

  6. BackgroundSupported Platforms • SHMEM • Cray T3D, T3E, PVP • SGI Irix, Origin • Compaq SC • IBM SP • Quadrics Linux Cluster • SCI (?) • GPSHMEM (Version 1.0) • IBM SP • SGI Origin • Cray J90, T3E • Unix/Linux • Windows NT • Myrinet (?)

  7. Nuts & BoltsInitialization • Include header shmem.h / shmem.fh to access the library • shmem_init() – Initializes SHMEM • my_pe() – Get the PE ID of local processor • num_pes() – Get the total number of PE in the system #include <stdio.h> #include <stdlib.h> #include "shmem.h“ int main(int argc, char **argv) {int my_pe, num_pe;shmem_init();my_pe = my_pe();num_pe = num_pes();printf("Hello World from process %d of %d\n", my_pe, num_pes);exit(0); }

  8. Nuts & BoltsData Transfer • Put • Specific Variable • void shmem_TYPE_p(TYPE *addr, TYPE value, int pe) • TYPE = double, float, int, long, short • Contiguous Object • void shmem_put(void *target, const void *source, size_t len, int pe) • void shmem_TYPE_put(TYPE *target, const TYPE*source, size_t len, int pe) • TYPE = double, float, int, long, longdouble, longlong, short • void shmem_putSS(void *target, const void *source, size_t len, int pe) • Storage Size (SS) = 32, 64 (default), 128, mem (any size)

  9. Nuts & BoltsData Transfer • Get • Specific Variable • void shmem_TYPE_g(TYPE *addr, TYPE value, int pe) • TYPE = double, float, int, long, short • Contiguous Object • void shmem_get(void *target, const void *source, size_t len, int pe) • void shmem_TYPE_get(TYPE *target, const TYPE*source, size_t len, int pe) • TYPE = double, float, int, long, longdouble, longlong, short • void shmem_getSS(void *target, const void *source, size_t len, int pe) • Storage Size (SS) = 32, 64 (default), 128, mem (any size)

  10. Nuts & BoltsCollective Communication • Broadcast • void shmem_broadcast(void *target, void *source, int nlong, int PE_root, int PE_start, int PE_group, int PE_size, long *pSync) • One-to-all communication • Collection • void shmem_collect(void *target, void *source, int nlong, int PE_start, int PE_group, int PE_size, long *pSync) • void shmem_fcollect(void *target, void *source, int nlong, int PE_start, int PE_group, int PE_size, long *pSync) • Concatenates data items from the source array into the target array over the defined set of PEs. The resultant target array consists of the contribution from the 1st PE, followed by 1st PE + 2nd PE, etc. pSync - symmetric work array. Every element of this array must be initialized with the value _SHMEM_SYNC_VALUE before any of the PEs in the active set enter the routine. Use to prevent overlapping collective communication

  11. Nuts & BoltsSynchronization • Barrier • void shmem_barrier_all(void) • Suspend all operations until all PE calls this function • void shmem_barrier(int PE_start, int PE_group, int PE_size, long *pSync) • Barrier operation on subset of PEs • Wait • Suspend until a remote PE writes a value NOT equal to the one specified • void shmem_wait(long *var, long value) • void shmem_TYPE_wait(TYPE *var, TYPE value) • TYPE = int, long, longlong, short • Conditional Wait • Same as wait except the comparison can now be >=, >, =, !=, <, <= • void shmem_wait_until(long *var, int cond, long value)

  12. Nuts & BoltsSynchronization • Fence • All put operations issued to a particular PE prior to call are guaranteed to be delivered before any subsequent remote write operation to the same PE which follows the call • Ensures ordering of remote write (put) operations • Quiet • Waits for completion of all outstanding remote writes initiated from the calling PE

  13. Nuts & BoltsAtomic Operations • Atomic Swap • Unconditional • long shmem_swap(long *target, long value, int pe) • Conditional • int shmem_int_cswap(int *target, int cond, int value, int pe) • Arithmetic • add, increment • int shmem_int_fadd(int *target, int value, int pe)

  14. Nuts & BoltsCollective Reduction • Collective logical operations • and, or, xor • void shmem_int_and_to_all(int *target, int *source, int nreduce, int PE_start, int PE_group, int PE_size, int *pWrk, long *pSync) • Collective comparison operations • max, min • void shmem_double_max_to_all(double *target, double *source, int nreduce, int PE_start, int PE_group, int PE_size, double *pWrk, long *pSync) • Collective arithmetic operations • product, sum • void shmem_double_prod_to_all(double *target, double *source, int nreduce, int PE_start, int PE_group, int PE_size, double *pWrk, long *pSync)

  15. Nuts & BoltsOther • Address Manipulation • shmem_ptr - Returns a pointer to a data object on a remote PE • Cache Control • shmem_clear_cache_inv - Disables automatic cache coherency mode • shmem_set_cache_inv - Enables automatic cache coherency mode • shmem_set_cache_line_inv - Enables automatic line cache coherency mode • shmem_udcflush - Makes the entire user data cache coherent • shmem_udcflush_line - Makes coherent a cache line

  16. Nuts & BoltsExample (Array copy) 14. /* Initialize and send on PE 1 */ 15. if(me == 1) { 16. for(i=0; i<8; i++) 17. source[i] = i+1; 18. shmem_put64(dest, source, 8*sizeof(dest[0])/8, 0); 19. } 20. 21. /* Make sure the transfer is complete */ 22. shmem_barrier_all(); 23. 24. /* Print from the receiving PE */ 25. if(me == 0) { 26. _shmem_udcflush(); 27. printf(" DEST ON PE 0:"); 28. for(i=0; i<8; i++) 29. printf(" %d%c", dest[i], (i<7) ? ',' : '\n'); 30. }} 1. #include <stdio.h> 2. #include <mpp/shmem.h> 3. #include <intrinsics.h> 4. 6. int me, npes, i; 7. int source[8], dest[8]; 8. main() 9. { 10. /* Get PE information */ 11. me = _my_pe(); 12. npes = _num_pes(); 13.

  17. GPSHMEM • AMES Lab / Pacific Northwest National Lab collaborative project • Communication library like SHMEM library, but tries to achieve full portability • Mostly the T3D components with some “extensions” of functionality • Research Quality at this point ARMCI = A Portable Remote Memory Copy Library for Distributed Array Libraries and Compiler Run-time Systems

  18. Performance – Latency (Origin 2000)

  19. Performance – Latency (T3E 600)

  20. Performance – Bandwidth Taken from http://infm.cineca.it/documenti/incontro_infm/comunicazio/sld015.htm

  21. Performance – Bandwidth

  22. Performance - Broadcast

  23. Performance – All to all

  24. Performance – Ocean On SGI Origin 2000

  25. Performance – Radix On SGI Origin 2000

  26. Conclusion • Hybrid MP/Shard Memory programming model • Compare to MP • Pro. • Easier to use • Lower latency, higher bandwidth communication • More scalable (within limit) • Remote CPU not interrupted during transfer • Con. • Limited platform support (as of now)

  27. Reference • Ricky A. Kendall et. al., GPSHMEM and other Parallel Programming Models Powerpoint presentation • Hongzhang Shan and Jaswinder Pal Singh, A Comparison of MPI, SHMEM and Cache-coherent Shared Address Space Programming Models on the SGI Origin2000 http://citeseer.nj.nec.com/rd/48418321%2C296348%2C1%2C0.25%2CDownload/http://citeseer.nj.nec.com/cache/papers/cs/14068/http:zSzzSzwww.cs.princeton.eduzSz%7EshzzSzpaperszSzics99.pdf/a-comparison-of-mpi.pdf • Quadrics SHMEM Programming Manual http://www.psc.edu/~oneal/compaq/ShmemMan.pdf • Karl Feind, Shared Memory Access (SHMEM) Routines • Glenn Leucke et. al., The Performance and Scalability of SHMEM and MPI-2 One-Sided Routines on a SCI Origin 2000 and a Cray T3E-600 http://dsg.port.ac.uk/Journals/PEMCS/papers/paper19.pdf • Patrick H. Worley,CCSM Component Performance Benchmarking and Status of the CRAY X1 at ORNL http://www.csm.ornl.gov/~worley/talks/index.html

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