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

Nick Piggin
2007-08-03T00:20:10Z

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.