Re: nice and hyperthreading on atom

[Peter Zijlstra]

On Sat, 2008-09-06 at 11:30 -0700, Ulrich Drepper wrote:
> On Sat, Sep 6, 2008 at 9:42 AM, Arjan van de Ven <arjan@xxxxxxxxxxxxx> wrote:
> > As an OS one COULD decide to just not schedule the nice task at all,
> > but then, especially on atom where HT has a high efficiency, your cpu
> > is mostly idle ...
> 
> One thread being idle is even on Atom the right thing to do in some
> situations.  If you have processes which, when HT is used, experience
> high pressure on the common cache(s) then you should not schedule them
> together.  We can theoretically find out whether this is the case
> using the PMCs.  With perfmon2 hopefully on the horizon soon it might
> actually be possible to automatically make these measurements.
> 
> There is another aspect I talked to Peter about already.  We really
> want concurrent threat scheduling in some cases.  For the
> implementation of helper threads you don't want two threads to be
> scheduled independently, you want them to be scheduled on a HT pair.
> Currently this isn't possible except by pinning them to fixed threads.
>  We really want to have a new way to express this type of scheduling
> (Peter, how did you call it?)

Really helps if you make sure I'm on the CC ;-)

I think I called it something like affinity grouping - but I'm still a
bit scared of the bin-packing issues involved - those will really mess
up the already complex balancing rules.

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