Date: Wed, 01 Feb 2012 16:29:59 -0800 From: Dennis Glatting <freebsd@pki2.com> To: Anton Shterenlikht <mexas@bristol.ac.uk> Cc: freebsd-questions@freebsd.org Subject: Re: OpenMP on FreeBSD Message-ID: <1328142599.53835.6.camel@btw.pki2.com> In-Reply-To: <20120202000947.GA71405@mech-cluster241.men.bris.ac.uk> References: <20120202000947.GA71405@mech-cluster241.men.bris.ac.uk>
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On Thu, 2012-02-02 at 00:09 +0000, Anton Shterenlikht wrote:
> I'm new to OpenMP. I wonder
> if there are any special considerations
> when running OpenMP on FreeBSD?
>
I run OpenMP. No special consideration. Here's a chunk from my Makefile:
TARG=ecc.enc ecc.dec
$TARG: *.cc *.h Makefile
g++ -Wall -fopenmp -g -O ${INCL} ${LIBS} -o ecc.enc *.cc
g++ -Wall -fopenmp -g -O ${INCL} ${LIBS} -o ecc.dec *.cc
> For example, I have this OMP parallelised
> fortran program, nested do loops, compiled
> with gfortran46. When I run it with 2 threads
> on a 2-cpu box, I see in top(1):
>
> PID UID PRI NICE SIZE RES STATE C TIME CPU COMMAND
> 63995 1001 89 0 57048K 34272K CPU1 1 1:06 55.08% dummy.sx{dummy.sx}
> 63995 1001 87 0 57048K 34272K RUN 1 1:02 52.39% dummy.sx{dummy.sx}
> 11 0 155 ki31 0K 32K RUN 0 376:58 51.46% idle{idle: cpu0}
> 11 0 155 ki31 0K 32K RUN 1 368:18 45.36% idle{idle: cpu1}
>
> I wonder why, even after a minute of run time,
> I still have nearly a whole cpu idle?
>
What is the program doing? I/O can significantly limit OMP value. Also,
you need to make sure you code your loops properly or else you have a
single-threaded application, without warning.
> As a result the run time with 2 threads
> is nearly identical to run time with 1 thread.
>
> It's likely that I'm not using OMP correctly,
> but I wanted to check if there are any
> special FreeBSD related issues to bear
> in mind when coding with OMP.
>
As an example, this is one of my key sections of code in C++. It works.
I can't say for Fortran.
#pragma omp parallel
{
#pragma omp for
for( size_t i = 0; i < bq.size(); ++i ) {
Block& b = bq[ i ];
// Adjust any padding.
//
if( b.size() != b.szSYMS ) {
eofPad = ( b.szSYMS - b.size());
for( ssize_t j = b.size(); j < b.szSYMS; ++j )
b.syms()[ j ] = eofPad;
if( verbose )
fprintf( stderr,
"Padding: read=%ld, pad=%d\n",
b.size(), eofPad );
b.size( b.szSYMS );
}
// Encode the buffer.
//
encode_rs_8( b.syms(), b.parity(), 0 );
// Set it to its new size.
// (the encoder is an outside routine.)
//
b.size( b.szBLOCK );
// Interleave the buffer.
//
add_interleave( b.buf());
} /* for */
} /* pragma */
> Thanks
>
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