[Bug c++/53292] multi-threaded (OpenMP) is slower than single-threaded
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53292 --- Comment #9 from FH fh_p at hotmail dot com 2012-05-12 21:27:31 UTC --- Well... I tested an OpenMP benchmarch (design to demonstrate OpenMP performances) found on the web : multi-threaded (OpenMP) is again slower than single-threaded. I looked at coding with pthreads : same thing. So, I have a dual-core hyper-threaded PC : I end up with multi-threaded applications slower than single-threaded and this is supposed to be a normal behavior ?!... Anyway this is still illogical to me ?!?!
[Bug c++/53292] New: multi-threaded (OpenMP) is slower than single-threaded
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53292 Bug #: 53292 Summary: multi-threaded (OpenMP) is slower than single-threaded Classification: Unclassified Product: gcc Version: 4.6.2 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ AssignedTo: unassig...@gcc.gnu.org ReportedBy: f...@hotmail.com Created attachment 27352 -- http://gcc.gnu.org/bugzilla/attachment.cgi?id=27352 cpp file to compile Hello, The problem is : multi-threaded (OpenMP) is slower than single-threaded Test has been ran several times successively. ChunkSize imposed to avoid false sharing. Test ran on Ubuntu 12.04 64 bits / 4Gb RAM / CPU i3 (dual core hyper-threaded : 2 physical cores + 2 logical cores) / gcc version : 4.6.2. Is the problem related to CPU ? i3 hyper-threaded would not be handled correctly by g++ or Ubuntu ? Can somebody help or give me clue ? Thanks, FH PS : I am not used to post bug, if this bug has not been posted in the right place feel free to tell me where to post it
[Bug c++/53292] multi-threaded (OpenMP) is slower than single-threaded
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53292 --- Comment #1 from FH fh_p at hotmail dot com 2012-05-09 09:17:29 UTC --- I am not sure to know if this problem is related rather to gcc or rather to Ubuntu. I started with the assumption that is should rather to related to gcc.
[Bug c++/53292] multi-threaded (OpenMP) is slower than single-threaded
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53292 --- Comment #2 from FH fh_p at hotmail dot com 2012-05-09 10:16:52 UTC --- I have just tested on another computer (CPU : Xeon5650 12 cores + OS : Scientific Linux) = I reproduce the unexpected behavior (OpenMP slower than single-threaded). So, I believe the problem is rather related to gcc (than to the OS) When I use more threads (export OMP_NUM_THREADS=2, then 6, then 12), OpenMP is more slower than single-threaded. (behavior related to thread initialisation ?)
[Bug c++/53292] multi-threaded (OpenMP) is slower than single-threaded
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53292 --- Comment #4 from FH fh_p at hotmail dot com 2012-05-09 12:53:46 UTC --- I don't understand your answer. Timing just times the for loop. Checking array content is single threaded : this is added to make sure the for loop has done the job correctly and this check is not timed. The array to initialize is shared by threads (shared by default) ans not private to each thread. To me, the test seems relevant. If it's not, why ? And how to modify it ?
[Bug c++/53292] multi-threaded (OpenMP) is slower than single-threaded
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53292 FH fh_p at hotmail dot com changed: What|Removed |Added Status|RESOLVED|UNCONFIRMED Resolution|INVALID | --- Comment #5 from FH fh_p at hotmail dot com 2012-05-09 12:55:56 UTC --- To me, the test seems relevant. If it's not, why ? And how to modify it ?
[Bug c++/53292] multi-threaded (OpenMP) is slower than single-threaded
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53292 --- Comment #7 from FH fh_p at hotmail dot com 2012-05-09 14:36:00 UTC --- Well... Still don't really get why it is not possible to improve performance for such basic things. I tried with allocations up to 7 Gb or more (RAM full + SWAP full) : I still get the same result that looks unexpected to me ?! Anyway, I guess I won't be able to get the logic of this...
