[Cbc] Parallel Cbc: adding threads fails to improve performance

[acw at ascent.com]

We have a Cbc application that has been doing quite well for us, but as we 
foray into more and more complicated problems we are always interested in 
improving performance.  So we were pleased to learn that parallelization 
had been added to Cbc, and we decided to test it out.

For this test we built Cbc  with the --enable-cbc-parallel-flag on a 
quad-core Linux box, and used the bin/cbc executable.  We downloaded a few 
test problems from MIPLIB2003 from the archive site miplib.zib.de.  We ran 
these problems with different values of the -threads parameter, and were 
surprised to discover that adding threads did not improve performance; 
instead, performance was almost unchanged for one problem (miu.mps), and 
degraded slightly with additional threads for some others (aflow30a.mps, 
aflow40b.mps, misc07.mps).

We used the following command structure:

cbc -threads <n> -import <mps file> -solve

We varied n from 0 (though we don't know what 0 means!) through 5 (even 
though we didn't have five cores!), and never saw a case where increasing 
n improved performance more than marginally, and many cases where it hurt.

We must be doing it wrong.  Is anyone else having success improving 
performance with parallelized Cbc?
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://list.coin-or.org/pipermail/cbc/attachments/20090605/0f0d2e2e/attachment.html