Re: newidle balancing in NUMA domain?

[Ingo Molnar <mingo@elte.hu>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, 2009-11-23 at 12:22 +0100, Nick Piggin wrote:
> > Hi,
> > 
> > I wonder why it was decided to do newidle balancing in the NUMA
> > domain? And with newidle_idx == 0 at that.
> > 
> > This means that every time the CPU goes idle, every CPU in the
> > system gets a remote cacheline or two hit. Not very nice O(n^2)
> > behaviour on the interconnect. Not to mention trashing our
> > NUMA locality.
> > 
> > And then I see some proposal to do ratelimiting of newidle
> > balancing :( Seems like hack upon hack making behaviour much more
> > complex.
> > 
> > One "symptom" of bad mutex contention can be that increasing the
> > balancing rate can help a bit to reduce idle time (because it
> > can get the woken thread which is holding a semaphore to run ASAP
> > after we run out of runnable tasks in the system due to them 
> > hitting contention on that semaphore).
> > 
> > I really hope this change wasn't done in order to help -rt or
> > something sad like sysbench on MySQL.
> 
> IIRC this was kbuild and other spreading workloads that want this.
> 
> the newidle_idx=0 thing is because I frequently saw it make funny 
> balance decisions based on old load numbers, like f_b_g() selecting a 
> group that didn't even have tasks in anymore.
> 
> We went without newidle for a while, but then people started 
> complaining about that kbuild time, and there is a x264 encoder thing 
> that looses tons of throughput.

Yep, i too reacted in a similar way to Nick initially - but i think you 
are right, we really want good, precise metrics and want to be 
optional/fuzzy in our balancing _decisions_, not in our metrics.

	Ingo