HPF: Programming Linux Clusters the Easy Way

In Statement 1, the identical distribution of a and b ensures that for all i, a(i) and b(i) are on the same processor; thus, the compiler does not generate any message passing.

In statement 2, there is again no need for message passing. If the ALIGN statement had lined up x(i) with y(i) rather than y(i+1), communication would have been needed for some values of i.

Statement 3 looks very much like Statement 1; but the communication requirements are very different because of the different distribution of a and c. The array elements a(i) and c(i) are on the same processor for only 10 of the possible values of i, and hence for nearly all of the elements; communication of data between processors is needed. This is an unwise choice of distribution for c, if indeed this statement represents the bulk of the work.

A good choice of distribution and alignment can greatly help efficiency, and that is the point of having the directives. It is much easier to write FORTRAN90 code and embellish it with HPF directives than to write the equivalent message-passing code.

In practice, the steps taken in writing an HPF program are:

This process is illustrated in the code shown in Listing 1, which represents a subroutine to solve a set of linear equations. The subroutine is in standard FORTRAN90 and will run happily through any FORTRAN90 compiler, which will treat the HPF directives as comments. The code makes good use of the FORTRAN90 array facilities and has been parallelized by adding just four HPF directives. The resulting HPF code runs well on a Linux PC cluster, provided the size of the problem being solved is large enough to warrant the use of parallelism.

HPF makes life easy for the programmer, by leaving nearly everything to the compiler. So, can the compilers cope? Can you really get parallel efficiency by using HPF? And, can you get useful speedups on networked PCs with relatively high latency communications?

Of course, no compiler can find parallelism where none exists; you need to give it the parallelism in the beginning. Given this, then the answer is yes, current HPF compilers are surprisingly efficient. On a PC cluster connected by Ethernets, the message-passing latency using PVM or MPI is typically around 0.6ms; this translates to “use fairly coarse-grain parallelism if you can and don't expect to use too many PCs.”

Table 1 shows some timings to illustrate what can be achieved. They were taken on a four-PC Linux P100 cluster with 100Mb Ethernet. “Serial” times are those given using the N. A. Software (NASL) FORTRANPlus F90 compiler, release 1.3.57. These times are absent where the code uses HPF extensions (FORALL, EXTRINSIC(HPFSERIAL)) not supported in FORTRAN90 (for some, we timed equivalent FORTRAN90 versions). HPF times used the NASL HPFPlus compiler, release 2.0. Optimization was set “on” for both FORTRAN and HPF. Times are in seconds.

The overheads intrinsic to using HPF rather than FORTRAN are shown by comparing the Serial and P = 1 times. These overheads are quite low—often negligible and, for Gauss, even negative (we see this on other platforms too). The gain in using HPF is shown by comparing the Serial and P = 4 times. Speedups achieved relative to the serial times range from 2.1 to 4.5.

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