Re: HT scheduler: is it really correct? or is it feature of HT?

[Con Kolivas <kernel@kolivas.org>]

On Thu, 19 May 2005 04:42, Tetsuji "Maverick" Rai wrote:
> I'm wondering linux kernel's HT support is correct or not, or whether
> it's a feature of P4 HT.
>
> I'm running boinc/seti in the background with nice=19 on my P4 2.8G HT
> enabled linux box, kernel 2.6.11.9, where SMT/HT is enabled.
>
> I often watch system monitor applet on gnome desktop or top command in
> a termianl window and see when no other applications than boinc is
> running, boinc takes full power of both virtual cpus.   It is designed
> to run to "fill" the idle power of the cpu(s).   However any
> application is running, there is always some "idle" part appears on
> virtual cpus, hence it looks like it wastes up to half of cpu power as
> "idle."
>
> For ex, see this "top" result while a vmware is running.   (HT is
> enabled)  setiathome-4.7(blah--) are the background boinc applications
> with nice=19.

Hyperthread sibling cpus share cpu power. If you let a nice 19 task run full 
power on the sibling cpu of a nice 0 task it will drain performance from the 
nice 0 task and make it run approximately 40% slower. The only way around 
this is to temporarily make the sibling run idle so that a nice 0 task gets 
the appropriate proportion of cpu resources compared to a nice 19 task. It is 
intentional and quite unique to the linux cpu scheduler as far as I can tell. 
On any other scheduler or OS a nice 19 "background" task will make your 
machine run much slower.

Cheers,
Con

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