markw@osdl.org wrote:

>Hello, I have some data with hyperthreading I wanted to share.
>
>I've seen about a 15% performance decrease in performance on a 4-way
>Xeon system when I enable hyperthreading for my DBT-2 workload.  I also
>gave Ingo's test11-C1 patch that someone pointed me to a try and only
>saw a 12% decrease. Has anyone found this to be common with any specific
>workloads?
>
>I'm not really sure what to look for, but I do see some changes in the
>readprofile data, which I've copied in part below.  It appears that the
>count of schedule, __make_request, and try_to_wake_up are the only
>functions at the top of the profile that are significantly different.
>The links I have posted also have pointers to oprofile data as well as
>annotated assembly source output, if that interests anyone.  If I can
>provide any other details, let me know.
>

Hi Mark,
It could be cache contention which I think is typically the reason
hyperthreading can hurt performance. Its basically impossible for
the scheduler to correct this automatically (access to performance
counters might make it slightly less impossible).

Probably the CPU hotplug interface would enable a tool to effectively
turn HT on or off and it would be up to an administrator to tune
performance.

You could try my scheduler patchset if you like. I have recently got
HT support working (its against test11, you need to turn CONFIG_SMT
on), although if Ingo's patch doesn't help much, mine probably won't
either.