netdev - Re: [PATCH 08/40] mm: kmem_cache

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Message-ID: <463B812D.5090009@cs.helsinki.fi>
Date:	Fri, 04 May 2007 21:53:33 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Christoph Lameter <clameter@....com>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	netdev@...r.kernel.org,
	Trond Myklebust <trond.myklebust@....uio.no>,
	Thomas Graf <tgraf@...g.ch>,
	David Miller <davem@...emloft.net>,
	James Bottomley <James.Bottomley@...elEye.com>,
	Mike Christie <michaelc@...wisc.edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Phillips <phillips@...gle.com>
Subject: Re: [PATCH 08/40] mm: kmem_cache_objsize

Christoph Lameter wrote:
> SLAB can calculate exactly how many pages are needed. The per 
> cpu and per node stuff is setup at boot and does not change. We are 
> talking about the worst case scenario here. True in case of an off slab
> we have additional overhead that would also have to go into worst case 
> scenario.

Fair enough. But there's no way it can take into account any slab 
management structures it needs to allocate. The slab simply doesn't know 
how many pages are needed to _allocate n amount of objects_.

Peter is interested in a _rough estimate_ so I don't see the point of 
adding that kind of logic in the slab. It's an API that simply cannot 
satisfy all its callers which is why I suggested exposing buffer size in 
the first place (the slab certainly knows how many bytes it needs for 
one object).

			Pekka
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