Re: [rfc] balance-on-fork NUMA placement

Siddha, Suresh B
2007-08-03T20:10:13Z

On Fri, Aug 03, 2007 at 02:20:10AM +0200, Nick Piggin wrote:
> On Thu, Aug 02, 2007 at 11:33:39AM -0700, Martin Bligh wrote:
> > Nick Piggin wrote:
> > >On Wed, Aug 01, 2007 at 03:52:11PM -0700, Martin Bligh wrote:
> > >>>And so forth. Initial forks will balance. If the children refuse to
> > >>>die, forks will continue to balance. If the parent starts seeing short
> > >>>lived children, fork()s will eventually start to stay local.
> > >>Fork without exec is much more rare than without. Optimising for
> > >>the uncommon case is the Wrong Thing to Do (tm). What we decided
> > >
> > >It's only the wrong thing to do if it hurts the common case too
> > >much. Considering we _already_ balance on exec, then adding another
> > >balance on fork is not going to introduce some order of magnitude
> > >problem -- at worst it would be 2x but it really isn't too slow
> > >anyway (at least nobody complained when we added it).
> > >
> > >One place where we found it helps is clone for threads.
> > >
> > >If we didn't do such a bad job at keeping tasks together with their
> > >local memory, then we might indeed reduce some of the balance-on-crap
> > >and increase the aggressiveness of periodic balancing.
> > >
> > >Considering we _already_ balance on fork/clone, I don't know what
> > >your argument is against this patch is? Doing the balance earlier
> > >and allocating more stuff on the local node is surely not a bad
> > >idea.
> >
> > I don't know who turned that on ;-( I suspect nobody bothered
> > actually measuring it at the time though, or used some crap
> > benchmark like stream to do so. It should get reverted.
>
> So you have numbers to show it hurts? I tested some things where it
> is not supposed to help, and it didn't make any difference. Nobody
> else noticed either.
>
> If the cost of doing the double balance is _really_ that painful,
> then we ccould skip balance-on-exec for domains with balance-on-fork
> set.

Nick, Even if it is not painful, can we skip balance-on-exec if
balance-on-fork is set. There is no need for double balance, right?

Especially with the optimization you are trying to do with this patch,
balance-on-exec may lead to wrong decision making this optimization
not work as expected.

or perhaps do balance-on-fork based on clone_flags..

thanks,
suresh