linux-kernel - Re: [patch 0/7] [RFC] SLUB: Improve allocpercpu to reduce per cpu access overhead

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Message-ID: <Pine.LNX.4.64.0711020734070.26635@schroedinger.engr.sgi.com>
Date:	Fri, 2 Nov 2007 07:35:35 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	David Miller <davem@...emloft.net>, akpm@...ux-foundation.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	mathieu.desnoyers@...ymtl.ca, penberg@...helsinki.fi
Subject: Re: [patch 0/7] [RFC] SLUB: Improve allocpercpu to reduce per cpu
 access overhead

On Fri, 2 Nov 2007, Peter Zijlstra wrote:

> On Thu, 2007-11-01 at 15:58 -0700, David Miller wrote:
> 
> > Since you're the one who wants to change the semantics and guarentees
> > of this interface, perhaps it might help if you did some greps around
> > the tree to see how alloc_percpu() is actually used.  That's what
> > I did when I started running into trouble with your patches.
> 
> This fancy new BDI stuff also lives off percpu_counter/alloc_percpu().

Yes there are numerous uses. I even can increase page allocator 
performance and reduce its memory footprint by using it here.

> That means that for example each NFS mount also consumes a number of
> words - not quite sure from the top of my head how many, might be in the
> order of 24 bytes or something.
> 
> I once before started looking at this, because the current
> alloc_percpu() can have some false sharing - not that I have machines
> that are overly bothered by that. I like the idea of a strict percpu
> region, however do be aware of the users.

Well I wonder if I should introduce it not as a replacement but as an 
alternative to allocpercpu? We can then gradually switch over. The 
existing API does not allow the specification of gfp_masks or alignements.

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