Re: Execution times running multiple instances of an application

On Mon, 26 Nov 2007 12:55:56 -0500, deltaseq0 <deltaseq0@xxxxxxxxxx>
wrote in <JMD2j.18$gL1.13@xxxxxxxxxxxx>:
I'm doing some testing of an application using a Pentium 4 HT cpu. The
application runs under Cygwin using the XWin X Server and an xterm window.
It takes 1.9 hrs to complete using gcc 4.2.1 configured with thread model:
single. When I look at Windows Task Manager, it shows a cpu usage of 50%
with no other apps running at the time. From a previous optimization thread,
I tried running 2 instances of the application at the same time by opening
up another window under XWin. Task manager shows 100% cpu usage but instead
of completing the 2 applications in 2 hours, it takes 3.9 hours; slightly
more than twice the time if I had run the applications sequentially. Was
this to be expected?

I have seen instances where a programme has completely slowed down
due to its "hopping" from one CPU to another and (presumably) losing its
cache in the process -- although this may have been on a proper dual-core
rather than a P4. You can check if it makes a difference in Task Manager --
in the Processes tab, right-click on the programme and select "Set Affinity"
from the drop-down menu. Set one copy of the programme to run on CPU 0
and the other to run on CPU 1 and see if it runs any faster.

--
Ivan Reid, School of Engineering & Design, _____________ CMS Collaboration,
Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
KotPT -- "for stupidity above and beyond the call of duty".
.

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