Execution times running multiple instances of an application

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?
Mike


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