linux-kernel - Re: [PATCH 2/3]: xvmalloc memory allocator

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Message-ID: <84144f020903171134q2283d01aq21a2faaa77ab07c6@mail.gmail.com>
Date:	Tue, 17 Mar 2009 11:34:00 -0700
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Nitin Gupta <ngupta@...are.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3]: xvmalloc memory allocator

On Tue, 17 Mar 2009, Nitin Gupta wrote:
>> Creating slabs for sizes in range, say, [32, 3/4*PAGE_SIZE] separated by
>> 64bytes
>> will require 48 slabs! Then for slab of each size class will have wastage
>> due to
>> unused slab objects in each class.
>> Larger difference in slab sizes (and thus small no. of them), will surely
>> cause too much
>> wastage due to internal fragmentation.

On Tue, Mar 17, 2009 at 10:58 AM, Christoph Lameter
<cl@...ux-foundation.org> wrote:
> The slabs that match existing other slabs of similar sizes will be aliased
> and not created. Create the 48 slabs and you likely will only use 10 real
> additional ones. The rest will just be pointing to existing ones.

Yup. One thing I don't quite understand is why you need all the 48
caches in the first place. Allocation sizes tend to be clustered and I
would have imagined you'd see that when compressing page sized chunks
as well. Using kmemtrace to analyze the exact reason for the bad
fragmentation would probably be helpful.
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