Re: [PATCH] 2.6.0 batch scheduling, HT aware

[Pavel Machek <pavel@ucw.cz>]

Hi!

> >>>BTW this is going to be an issue even on normal (non-HT)
> >>>systems. Imagine memory-bound scientific task on CPU0 and nice -20
> >>>memory-bound seti&home at CPU1. Even without hyperthreading, your
> >>>scientific task is going to run at 50% of speed and seti&home is going
> >>>to get second half. Oops.
> >>>
> >>>Something similar can happen with disk, but we are moving out of
> >>>cpu-scheduler arena with that.
> >>>
> >>>[I do not have SMP nearby to demonstrate it, anybody wanting to
> >>>benchmark a bit?]
> >>>
> >>This is definitely the case but there is one huge difference. If you have 
> >>2x1Ghz non HT processors then the fastest a single threaded task can run 
> >>is at 1Ghz. If you have 1x2Ghz HT processor the fastest a single threaded 
> >>task can run is 2Ghz. 
> >>
> >
> >Well, gigaherz is not the *only* important thing.
> >
> >On 2x1GHz, 2GB/sec RAM bandwidth, fastest a single threaded task can
> >run is 1GHz, 2GB/sec. If you run two of them, it is 1GHz,
> >*1*GB/sec. So you still have effect similar to hyperthreading. And
> >yes, it can be measured.
> >
> 
> Hi Pavel,
> Sure this might be a real problem sometimes, but I don't see the
> CPU scheduler ever handling it unless we want to add a few kitchen
> sinks to its nice lean code as well.

Why is it a problem? If you are handling HT case, anyway, it should be
fairly easy to say "imagine it is HT system, not SMP one", and poof,
problem magically goes away.
								Pavel

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[Ready-made kitchen-sink for scheduler :-)))]
-- 
When do you have a heart between your knees?
[Johanka's followup: and *two* hearts?]