linux-kernel - Re: [patch 09/10] Remove the SLOB allocator for 2.6.23

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Message-Id: <20070709110013.82d2273c.akpm@linux-foundation.org>
Date:	Mon, 9 Jul 2007 11:00:13 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, linux-mm@...r.kernel.org,
	suresh.b.siddha@...el.com, corey.d.gough@...el.com,
	Pekka Enberg <penberg@...helsinki.fi>,
	Matt Mackall <mpm@...enic.com>,
	Denis Vlasenko <vda.linux@...glemail.com>,
	Erik Andersen <andersen@...epoet.org>
Subject: Re: [patch 09/10] Remove the SLOB allocator for 2.6.23

On Mon, 9 Jul 2007 10:26:08 -0700 (PDT)
Christoph Lameter <clameter@....com> wrote:

> > I assume the tradeoff here is better packing versus having a ridiculous
> > number of caches.  Is there any other cost?
> > Because even having 1024 caches wouldn't consume a terrible amount of
> > memory and I bet it would result in aggregate savings.
> 
> I have tried any number of approaches without too much success. Even one 
> slab cache for every 8 bytes. This creates additional admin overhead 
> through more control structure (that is pretty minimal but nevertheless 
> exists)
> 
> The main issue is that kmallocs of different size must use different 
> pages. If one allocates one 64 byte item and one 256 byte item and both 64 
> byte and 256 byte are empty then SLAB/SLUB will have to allocate 2 pages. 
> SLUB can fit them into one. This is basically only relevant early after 
> boot. The advantage goes away as the system starts to work and as more 
> objects are allocated in the slabs but the power-of-two slab will always
> have to extend its size in page size chunks which leads to some overhead 
> that SLOB can avoid by placing entities of multiple size in one slab. 
> The tradeoff in SLOB is that is cannot be an O(1) allocator because it 
> has to manage these variable sized objects by traversing the lists.
> 
> I think the advantage that SLOB generates here is pretty minimal and is 
> easily offset by the problems of maintaining SLOB.

Sure.  But I wasn't proposing this as a way to make slub cover slob's advantage.
I was wondering what effect it would have on a more typical medium to large sized
system.

Not much, really: if any particular subsystem is using a "lot" of slab memory then
it should create its own cache rather than using kmalloc anyway, so forget it ;)
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