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Re: Starting an OpenMP parallel section is extremely slow on a hyper-threaded Nehalem
- From: Tim Prince <n8tm at aol dot com>
- To: gcc-help at gcc dot gnu dot org
- Date: Thu, 11 Feb 2010 09:41:01 -0800
- Subject: Re: Starting an OpenMP parallel section is extremely slow on a hyper-threaded Nehalem
- References: <4B73D530.4010401@svi.nl> <4B74103A.6030301@aol.com> <4B7415C5.1090208@svi.nl> <4B741AF1.70407@aol.com>
- Reply-to: tprince at computer dot org
On 2/11/2010 6:57 AM, Tim Prince wrote:
On 2/11/2010 6:35 AM, Edwin Bennink wrote:
Thanks Tim, I thought that the gcc list was the most appropriate one
regarding the gomp implementation, but I'll post this question on the
gcc-help list.
In order to assist rapid replacement of parallel regions, such as your
example, most OpenMP libraries have a latency setting before a parallel
region is wound down automatically. Then, when the regions are
identical, some of the setup is skipped, and private data may be
inherited. It still doesn't perform as well as retaining a single
parallel region.
For the intel library, the default latency KMP_BLOCKTIME is 0.2 second,
which some people consider far too small, and others too large. I don't
immediately see an equivalent for this setting in libgomp manual.
--
Tim Prince