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/
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