Starting an OpenMP parallel section is extremely slow on a hyper-threaded Nehalem

[Edwin Bennink]

Thanks Tim, I thought that the gcc list was the most appropriate one 
regarding the gomp implementation, but I'll post this question on the 
gcc-help list.

By the way, Ubuntu 9.10 is the latest version (dd Oct. 2009). HTT works 
fine for daily use, but massive parallel applications show some odd 
behaviour:
Depending on the structure of the algorithm some pieces of code run 
significantly faster (about 10%) with HTT enabled, while other pieces of 
code run slower (some more than 50%). This slowdown happens due to 
parallel sections inside loops...

Edwin


Tim Prince wrote:
> On 2/11/2010 2:00 AM, Edwin Bennink wrote:
>> Dear gcc list,
>>
>>
>> I noticed that starting an OpenMP parallel section takes a 
>> significant amount of time on Nehalem cpu's with hyper-threading 
>> enabled.
>>
> If you think a question might be related to gcc, but don't know which 
> forum to use, gcc-help is more appropriate.  As your question is 
> whether there is a way to avoid anomalous behaviors when an old Ubuntu 
> is run on a CPU released after that version of Ubuntu, an Ubuntu forum 
> might be more appropriate.  A usual way is to shut off HyperThreading 
> in the BIOS when running on a distro which has trouble with it.  I do 
> find your observation interesting.
> As far as I know, the oldest distro which works well on Core I7 is 
> RHEL5.2 x86_64, which I run, with updated gcc and binutils, and HT 
> disabled, as I never run applications which could benefit from HT.
>