linux-kernel - Re: [patch 3/8] mm, mempolicy: remove per-process flag

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Date:	Wed, 4 Dec 2013 16:53:54 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Christoph Lameter <cl@...ux.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Pekka Enberg <penberg@...nel.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	cgroups@...r.kernel.org
Subject: Re: [patch 3/8] mm, mempolicy: remove per-process flag

On Wed, 4 Dec 2013, Christoph Lameter wrote:

> > PF_MEMPOLICY is an unnecessary optimization for CONFIG_SLAB users.
> > There's no significant performance degradation to checking
> > current->mempolicy rather than current->flags & PF_MEMPOLICY in the
> > allocation path, especially since this is considered unlikely().
> 
> The use of current->mempolicy increase the cache footprint since its in a
> rarely used cacheline. This performance issue would occur when memory
> policies are not used since that cacheline would then have to be touched
> regardless of memory policies be in effect or not. PF_MEMPOLICY was used
> to avoid touching the cacheline.
> 

Right, but it turns out not to matter in practice.  As one of the non-
default CONFIG_SLAB users, and PF_MEMPOLICY only does something for 
CONFIG_SLAB, this patch tested to not show any degradation for specjbb 
which stresses the allocator in terms of throughput:

	   with patch: 128761.54 SPECjbb2005 bops
	without patch: 127576.65 SPECjbb2005 bops

These per-process flags are a scarce resource so I don't think 
PF_MEMPOLICY warrants a bit when it's not shown to be advantageous in 
configurations without mempolicy usage where it's intended to optimize, 
especially for a non-default slab allocator.
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