Messages in this thread | | | Date | Mon, 29 Dec 2003 13:49:14 +0100 | From | Pavel Machek <> | Subject | Re: [PATCH] 2.6.0 batch scheduling, HT aware |
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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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