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

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Date:	Thu, 02 Aug 2007 11:33:39 -0700
From:	Martin Bligh <mbligh@...igh.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	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

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.

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