Re: [PATCH] 2.6.0 batch scheduling, HT aware

[Con Kolivas <kernel@kolivas.org>]

On Sat, 27 Dec 2003 10:42, Con Kolivas wrote:
> 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.

Or even if you have 1024x1Ghz cpus. One thread can still only run at 1Ghz. 
Sure there are other things that can run on other cpus which will allow the 
single thread to run unabated at 1Ghz, but no faster.

Con