[CentOS] Intel Xeon and hyperthreading

[Sam Drinkard]

Jim Perrin wrote:

> On 6/7/06, Sam Drinkard <sam at wa4phy.net> wrote:
>
>> Another question some of you may help me make decisions on.  I've been
>> doing some reading that indicates that having HT turned on in this dual
>> xeon machine might actually slow down the computing process rather than
>> speeding it up.  I rebooted this a.m., and turned HT off, just prior to
>> my main application run.  One thing that might be of note, this
>> application is using OMP for utilizing both cpu's,  and prior to turning
>> off HT, I had been running the software using 2 cpu's of the 4 that the
>> OS sees.  I'm waiting on a model run to finish to see if there is any
>> appreciable difference, but the one thing I do notice right off is cpu
>> utilization is running close to 100% on both, where before, it averaged
>> maybe 50% or thereabouts.  Again, sar is showing at last count, 83.56%
>> utilization for user, 10.27% system and only 0.02% nice.  Idle was 
>> 5.74%.
>
>
> It's entirely possible that your system is lying about load when
> moving back and forth between HT and real SMP. Logical CPUS (HT) are
> nearly identical to physical CPUs as far as the operating system is
> concerned. Since you have half the number of CPUS with HT turned off,
> but you're still running the same amount of jobs, the load should be
> higher. Hopefully this page will explain that a little bit better.
> http://www.cognitive-dissonance.org/wiki/Load+Average
>
> Additionally as far as HT performance is concerned, I've only really
> found two pages that help, although the IBM load is a bit older and
> may not be accurate anymore.
>
> http://perfcap.blogspot.com/2005/05/performance-monitoring-with.html
> http://www-128.ibm.com/developerworks/linux/library/l-htl/
>
>
>
>> I'm attempting to squeeze every last bit of processing power out of this
>> machine, and would entertain suggestions on tuning if there happen to be
>> any types of tuning that would help.
>
>
> For performance tuning, I usually start with the filesystem tweaks:
> mount ext3 noatime, and changing the commit time from 5 to 30
>
> After that I move to /etc/sysctl.conf and tweak the kernel.shmmax,
> shmmin, shmall, and vdso values depending on the application I'm most
> concerned about, as well as fs.file-max.
>
>> From there I move to the I/O scheduler/elevator for the system. RH
>
> magazine had a decent article about this.
> http://www.redhat.com/magazine/008jun05/features/schedulers/
>
>
>
Thanks Jim.  Those articles were enlightening to say the least.  Time 
for more study on things!

-- 
Sam W.Drinkard -- sam at wa4phy.net
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