Messages in this thread | | | Date | Thu, 4 Oct 2001 16:56:52 -0700 (PDT) | From | Davide Libenzi <> | Subject | Re: Context switch times |
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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. > > I know that running LMbench with 2 active tasks on an 8 CPU system > results in those 2 tasks being 'round-robined' among all 8 CPUs. > Prior analysis leads me to believe the reason for this is due to > IPI latency. reschedule_idle() chooses the 'best/correct' CPU for > a task to run on, but before schedule() runs on that CPU another > CPU runs schedule() and the result is that the task runs on a > ?less desirable? CPU. The nature of the LMbench scheduler benchmark > makes this occur frequently. The real question is: how often > does this happen in real-life situations?
Well, if you remember the first time this issue was discussed on the mailing list was due a real life situation not due a bench run.
- Davide
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