N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 1 Comparison of Communication and I/O of the Cray T3E and IBM SP Jonathan Carter NERSC User Services

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 2 Overview Node Characteristics Interconnect Characteristics MPI Performance I/O Configuration I/O Performance

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 3 T3E Architecture Distributed memory, single CPU processing elements Interconnect CPU Memory

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 4 T3E Communication Network Processing Elements (PE) are connected by a 3D torus.

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 5 T3E Communication Network The peak bandwidth of the torus is about 600 Mbyte/sec bidirectional Sustainable bandwidth is about 480 Mbytes/sec bidirectional Latency is  1μs Shmem API gives latency of 1μs, bandwidth 350 Mbyte/sec bidirectional

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 6 SP Architecture Cluster of SMP nodes Interconnect Memory CPU

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 7 SP Communication Network Nodes are connected via adapters to the SP Switch. Switch is composed of boards which link 16 nodes. Boards are linked to form larger network. Switch Board Nodes

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 8 SP Communication Network The peak bandwidth of adapter and switch is 300 Mbyte/sec bidirectional Latency of the switch is about 2μs Sustainable bandwidth is about 185 Mbytes/sec bidirectional

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 9 MPI Performance Intra-node is 1 MPI process per node, 2 MPI processes (typical) will halve bandwidth

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 10 MPI Performance

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 11 MPI Performance

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 12 T3E I/O Configuration PEs do not have local disk All PEs access all filesystems equivalently Path for (optimum) I/O generally looks like: –PE to I/O node via torus –I/O node to Fibre Channel Node (FCN) via Gigaring –FCN to Disk Array via Fibre loop In some cases data on APP PE must be transferred to a system buffer on an OS PE then out to an FCN

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 13 T3E I/O Configuration I/O FCN Gigaring Disk Arrays

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 14 SP I/O Configuration Nodes have local disk. One SCSI disk for all local filesystems. Non-optimal. All nodes access Global Parallel File System (GPFS) filesystems equivalently Path for GPFS I/O looks like: –Node to GPFS Node via IP over the switch –GPFS Node to Disk Array via SSA loop

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 15 SP I/O Configuration Nodes Switch GPFS Nodes Disk Array

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 16 T3E Filesystems /usr/tmp –fast –subject to 14 day purge, not backed up –check quota with quota -s /usr/tmp (usually 75Gb and 6000 inodes) $TMPDIR –fast –purged at end of job or session –shares quota with /usr/tmp $HOME –slower –permanent, backed up –check quota with quota (usually 2Gb and 3500 inodes)

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 17 SP Filesystems /scratch and $SCRATCH –global –fast (GPFS) –subject to 14 day purge (or at session end for $SCRATCH), not backed up –check quota with myquota (usually 100Gb and 6000 inodes) $TMPDIR –local (created in /scr) - only 2 Gbyte total –slower –purged at end of job or session $HOME –global –slower (GPFS) –permanent, not backed up yet –check quota with myquota (usually 4Gb and 5000 inodes)

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 18 Types of I/O Bewildering number of choices on both machines: –Standard Language I/O: Fortran or C (ANSI or POSIX) –Vendor extensions to language I/O –MPI I/O –Cray FFIO library (can be used from Fortran or C) –IBM MIO library, requires code changes

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 19 Standard Language I/O Fortran direct access is slightly more efficient then sequential access both on the T3E (see comments on FFIO later) and the SP. It also allows file transferability. C language I/O (fopen, fwrite, etc.) is inefficient on both machines. POSIX standard I/O (open, read, etc.) can be efficient on the T3E, but requires care (see comments on FFIO later). Works well on the SP.

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 20 Vendor Extensions to Language I/O Cray has a number of I/O routines (aqopen, etc.) which are legacies from the PVP systems. Non-portable. IBM has extended Fortran syntax to provide asynchronous I/O. Non-portable.

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 21 MPI I/O Part of MPI-2 Interface for High Performance Parallel I/O –data partitioning –collective I/O –asynchronous I/O –portability and interoperability bwteen T3E and SP Different subset implemented on T3E and SP

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 22 Summary of access routines for T3E

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 23 Summary of access routines for SP

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 24 Cray FFIO library FFIO is a set of I/O layers tuned for different I/O characteristics Buffering of data (configurable size) Caching of data (configurable size) Available to regular Fortran I/O without reprogramming Available for C through POSIX-like calls, e.g. ffopen, ffwrite

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 25 FFIO - The assign command controls program behavior at runtime the assign command controls –controls which FFIO layer is active –striping across multiple partitions –lots more scope of assign –File name –Fortran unit number –File type (e.g. all sequential unformatted files)

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 26 IBM MIO library User interface based on POSIX I/O routines, so requires program modification Useful trace module to collect statistics Not much experience with using on GPFS filesystem Coming soon

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 27 I/O Strategies - Exclusive access files Each process reads and writes to a separate file –Language I/O Increase language I/O performance with FFIO library (for example, sepcify a large buffer with the bufa layer) on T3E. For Fortran direct access default buffer is only the maximum of the record length or 32 Kbytes read/write large amounts of data per request on the SP –MPI I/O read/write large amounts of data per request

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 28 bufa FFIO layer Overview bufa is an asynchronous buffering layer performs read-ahead, write-behind specify buffer size with -F bufa:bs:nbufs where bs is the buffer size in units of 4Kbyte blocks, and nbufs is the number of buffers buffer space increases your applications memory requirements

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 29 I/O Strategies - Shared files All PEs read and write the same file simultaneously –Language I/O (requires FFIO library global layer for T3E) –MPI I/O –On T3E, language I/O with FFIO library global layer and Cray extensions for additional flexibility

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 30 Positioning with a shared file Positioning of a read or write is your responsibility File pointers are private Fortran –Use a direct access file, and read/write(rec=num) –Use Cray T3E extensions setpos and getpos to position file pointer (not portable) C –Use ffseek MPI I/O –MPI I/O fileview generally takes care of this. Positioning routines also available.

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 31 global FFIO layer Overview global is a caching and buffering layer which enables multiple PEs to read and write to the same file if one PE has already read the data, an additional read request from another PE will result in a remote memory copy file open is a synchronizing event By default, all PEs must open a global file, this can be changed by calling GLIO_GROUP_MPI(comm) specify buffer size with -F global:bs:nbufs where bs is the buffer size in units of 4Kbyte blocks, and nbufs is the number of buffers per PE

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 32 GPFS and shared files On the T3E the global FFIO layer takes care of updates to a file from multiple PEs by tracking the state of the file across all PEs. On the SP, GPFS implements a safe update scheme via tokens and a token manager. –If two processes access the same block of a GPFS file (256 Kbytes), a negotiation is conducted between the nodes and the token manager to determine the order of updates. This can slow down I/O considerably. –MPI I/O merges requests from different processes to alleviate this problem

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 33 I/O Performance Comparison Each process writes a 200 Mbyte file. 2 processes per node on SP.

N ATIONAL E NERGY R ESEARCH S CIENTIFIC C OMPUTING C ENTER 34 Further Information I/O on the T3E Tutorial by Richard Gerber at als Cray Publication - Application Programmer’s I/O Guide Cray Publication - Cray T3E Fortran Optimization Guide man assign XL Fortran User’s Guide