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

[Nick Piggin]

On Fri, Aug 03, 2007 at 01:10:13PM -0700, Suresh B wrote:
> 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?

I guess we could. There is no need for the double balance if the exec
happens immediately after the fork which is surely the common case. I
think there can be some other weird cases (eg multi-threaded code) that
does funny things though...


> 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.

That's true.

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