Re: [PATCH] 2.6.0 batch scheduling, HT aware

[Nick Piggin]

Pavel Machek wrote:

> 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.

If the need really arises, then probably a userspace daemon could
do it.


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