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

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Date:	Fri, 3 Aug 2007 13:10:13 -0700
From:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Martin Bligh <mbligh@...igh.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Andi Kleen <ak@...e.de>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Eric Whitney <eric.whitney@...com>
Subject: Re: [rfc] balance-on-fork NUMA placement

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
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