[Haskell-cafe] Strange parallel behaviour with Ubuntu Karmic / GHC 6.10.4

[Neil Brown]

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.