Re: [PATCH] 2.6.0 batch scheduling, HT aware

[Con Kolivas <kernel@kolivas.org>]

On Sat, 27 Dec 2003 09:56, Pavel Machek wrote:
> Hi!
>
> > I've done a resync and update of my batch scheduling that is also
> > hyper-thread aware.
> >
> > What is batch scheduling? Specifying a task as batch allows it to only
> > use cpu time if there is idle time available, rather than having a
> > proportion of the cpu time based on niceness.
> >
> > Why do I need hyper-thread aware batch scheduling?
> >
> > If you have a hyperthread (P4HT) processor and run it as two logical cpus
> > you can have a very low priority task running that can consume 50% of
> > your physical cpu's capacity no matter how high priority tasks you are
> > running. For example if you use the distributed computing client
> > setiathome you will be effectively be running at half your cpu's speed
> > even if you run setiathome at nice 20. Batch scheduling for normal cpus
> > allows only idle time to be used for batch tasks, and for HT cpus only
> > allows idle time when both logical cpus are idle.
>
> 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. 

Con