Re: [PATCH] 2.6.0 batch scheduling, HT aware

[Mika Penttilä]

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?] 
> 									Pavel
heh...and the situation gets even worse when you add cpus, with 16way 
you get only 1/16 of the speed ;)

--Mika


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