Re: Context switch times

Linus Torvalds
2001-10-04T23:50:32Z

On Thu, 4 Oct 2001, Mike Kravetz wrote:

> On Thu, Oct 04, 2001 at 10:42:37PM +0000, Linus Torvalds wrote:
> > Could we try to hit just two? Probably, but it doesn't really matter,
> > though: to make the lmbench scheduler benchmark go at full speed, you
> > want to limit it to _one_ CPU, which is not sensible in real-life
> > situations.
>
> Can you clarify? I agree that tuning the system for the best LMbench
> performance is not a good thing to do! However, in general on an
> 8 CPU system with only 2 'active' tasks I would think limiting the
> tasks to 2 CPUs would be desirable for cache effects.

Yes, limiting to 2 CPU's probably gets better cache behaviour, and it
might be worth looking into why it doesn't. The CPU affinity _should_
prioritize it down to two, but I haven't thought through your theory about
IPI latency.

However, the reason 2.2.x does so well is that in 2.2.x it will stay on
_once_ CPU if I remember correctly. We basically tuned the scheduler for
lmbench, and not much else.

Linus