Re: [patch] sched: introduce SD_BALANCE_FORK for ht/mc/smp domains

[Siddha, Suresh B]

On Fri, Jul 27, 2007 at 03:22:14AM +0200, Nick Piggin wrote:
> On Thu, Jul 26, 2007 at 03:34:56PM -0700, Suresh B wrote:
> > On Fri, Jul 27, 2007 at 12:18:30AM +0200, Ingo Molnar wrote:
> > > 
> > > * Siddha, Suresh B <suresh.b.siddha@xxxxxxxxx> wrote:
> > > 
> > > > Introduce SD_BALANCE_FORK for HT/MC/SMP domains.
> > > > 
> > > > For HT/MC, as caches are shared, SD_BALANCE_FORK is the right thing to 
> > > > do. Given that NUMA domain already has this flag and the scheduler 
> > > > currently doesn't have the concept of running threads belonging to a 
> > > > process as close as possible(i.e., forking may keep close, but 
> > > > periodic balance later will likely take them far away), introduce 
> > > > SD_BALANCE_FORK for SMP domain too.
> > > > 
> > > > Signed-off-by: Suresh Siddha <suresh.b.siddha@xxxxxxxxx>
> > > 
> > > i'm not opposed to this fundamentally, but it would be nice to better 
> > > map the effects of this change: do you have any particular workload 
> > > under which you've tested this and under which you've seen it makes a 
> > > difference? I'd expect this to improve fork-intense half-idle workloads 
> > > perhaps - things like a make -j3 on a 4-core CPU.
> > 
> > They might be doing more exec's and probably covered by exec balance.
> > 
> > There was a small pthread test case which was calculating the time to create
> > all the threads and how much time each thread took to start running. It
> > appeared as if the threads ran sequentially one after another on a DP system
> > with four cores leading to this SD_BALANCE_FORK observation.
> 
> If it helps throughput in a non-trivial microbenchmark it would be
> helpful.

We are planning to collect some data with this.

> I'm not against it either really, but keep in mind that it
> can make fork more expensive and less scalable; the reason we do it
> for the NUMA domain is because today we're basically screwed WRT
> existing working set if we have to migrage processes over nodes. It
> is really important to try to minimise that any way we possibly can.
> 
> When (if) we get NUMA page replication and automatic migration going,
> I will be looking at whether we can make NUMA migration more
> aggressive (and potentially remove SD_BALANCE_FORK). Not that either
> replication or migration help with kernel allocations, nor are they
> cheap, so NUMA placement will always be worth spending more cycles
> on to get right.

I agree. Perhaps we can set it only for ht/mc domains.

thanks,
suresh
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