Michael Lesniak wrote:
Hello,
getTime? I wonder if that number might be causing the problem; can you
replicate it with lower sys times?
That was it! Thanks Neil!
When I'm using some number crunching without getTime it works (with
more or less the expected speedup and usage of two cores) on my Ubuntu
9.10, too.
Out of curiosity, the question is still open: Why does the old example
(using getTime) work so much better on an older version of
Ubuntu/RedHat and not on the new ones?
Your kernels were:
Setup:
Machine A: Quadcore, Ubuntu 9.04, Kernel 2.6.28-13 SMP
Machine B: AMD Opteron 875, 8 cores, 2.6.18-164 SMP- (some redhat)
Machine C: Dual-Core, Ubuntu 9.10, Kernel 2.6.31-14 SMP
Looking at the implementation of getTime ThreadCPUTime in the clock
package, it calls clock_gettime(CLOCK_THREAD_CPUTIME_ID,..). According
to this page
(http://www.h-online.com/open/news/item/Kernel-Log-What-s-new-in-2-6-29-Part-8-Faster-start-up-and-other-behind-the-scenes-changes-740591.html),
the changes in 2.6.29 (changes which only your Ubuntu 9.10 machine has)
included a patch
(http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=c742b31c03f37c5c499178f09f57381aa6c70131)
which altered the implementation of that function. Perhaps on some
multi-processor machines the new implementation effectively serialises
the code? I know there used to be issues of whether some of the timers
were synchronised across processors/cores (to stop them appearing to go
backwards), so maybe something with the timers and their
synchronisations effectively stops your program running in parallel. If
it helps, my machine is: "Intel(R) Core(TM)2 Duo CPU E8400 @
3.00GHz" according to /proc/cpuinfo.
Thanks,
Neil.
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