Comparison of Communication and I/O of the Cray T3E and IBM SP

1. Comparison of Communication and I/O of the Cray T3E and IBM SP Jonathan Carter NERSC User Services

2. Overview • Node Characteristics • Interconnect Characteristics • MPI Performance • I/O Configuration • I/O Performance

3. Interconnect Memory CPU T3E Architecture • Distributed memory, single CPU processing elements

4. T3E Communication Network • Processing Elements (PE) are connected by a 3D torus.

5. T3E Communication Network • The peak bandwidth of the torus is about 600 Mbyte/sec per link bidirectional • Sustainable bandwidth is about 480 Mbytes/sec bidirectional • Latency is  1 microsec. • shmem API gives latency of 1 microsec., bandwidth 350 Mbyte/sec bidirectional

6. SP Architecture • Cluster of SMP nodes Interconnect Memory CPU CPU

7. Nodes 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

8. SP Communication Network • The peak bandwidth per node is 300 Mbyte/sec bidirectional • Latency of the switch is about 2 microsec. • Sustainable bandwidth is about 185 Mbytes/sec bidirectional

9. MPI Performance Point-to-point, single transmitter, single receiver Intra-node is 1 MPI process per node, 2 MPI processes (typical) will halve bandwidth

10. MPI Performance

11. MPI Performance

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

13. Gigaring Disk Arrays I/O FCN T3E I/O Configuration

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 (Serial Storage Architecture) loop

15. SP I/O Configuration Disk Array GPFS Nodes Nodes Switch Switch

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)

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)

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

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.

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.

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 between T3E and SP • Different subsets implemented on T3E and SP

22. Summary of access routines for T3E

23. Summary of access routines for SP

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

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)

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

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, specify a large buffer with the bufalayer) 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

28. bufa FFIO layer Overview • bufa is an asynchronous buffering layer • performs read-ahead, write-behind • specify buffer size in assign call, with -F bufa:bs:nbufswhere bs is the buffer size in units of 4Kbyte blocks, and nbufs is the number of buffers • buffer space increases your application’s memory requirements

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 • This can sequentialize your reads and ruin your I/O performance

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.

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 assign-F global:bs:nbufswhere bs is the buffer size in units of 4Kbyte blocks, and nbufs is the number of buffers per PE

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

33. I/O Performance Comparison • Each process writes a 200 Mbyte file. 2 processes per node on SP.

34. Further Information • I/O on the T3E Tutorial by Richard Gerber at http://home.nersc.gov/training/tutorials • Cray Publication - Application Programmer’s I/O Guide • Cray Publication - Cray T3E Fortran Optimization Guide • man assign • XL Fortran User’s Guide