[GE users] Implications of Hyperthread enabled CPUs and jobs

reuti reuti at staff.uni-marburg.de
Thu Nov 27 13:37:42 GMT 2008

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Am 27.11.2008 um 10:27 schrieb Neil Baker:

> I?ve been reading up on Hyper-thread enabled CPUs and am wondering  
> if it is best to disable or enable Hyper-threading in the BIOS?

I switched it always off in the past. My experience was also around  
up to 50% only. Depending of you applications, you might want to try  
a setup with 3 slots per node (with 2 physical CPUs). Maybe one of  
them in a different queue with a nice value (i.e. priority in the  
queue definition) of 19 as some kind of background-queue.

> Most of our grid machines have 2x physical 3.0 GHz Pentium 4 CPUs  
> with Hyper-threading enabled.  We are running openSuse10.0 on them  
> (BIGSMP 2.6 kernel) so they appear to be 4x logical processors.
> If 2 jobs are assigned to run on the same machine, is the kernel or  
> grid engine smart enough to run them on two physically separate CPUs,

Yes, the Linux kernel scheduler should take care of it.

> or is there a random chance that they may both run on the same  
> physical CPU (i.e. the main and hyper-threaded logical CPU on the  
> same physical CPU) and therefore leave the other physical CPU  
> totally unused?

As the Linux kernel scheduler will change the assigment over time:  
yes, it might happen and you might see a system task on the other CPU.

-- Reuti

> I?ve read that hyper-threading only gives up to 30% more  
> performance, so by accidentally running 2 jobs on the same physical  
> CPU it is going to drastically slow these jobs down compared with  
> running them on two separate physical CPUs.
> Regards
> Neil


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