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

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Date:	Wed, 18 Mar 2009 21:37:35 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Pekka Enberg <penberg@...helsinki.fi>
CC:	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3]: xvmalloc memory allocator

Pekka Enberg wrote:
> 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.

Compressed page lengths sometimes do tend to cluster within somewhat
small range. However, the range of where majority of pages will lie depends
highly of workload - sometimes range is not clear and sometime there is no
preferred range at all. Please refer this data:
http://code.google.com/p/compcache/wiki/CompressedLengthDistribution

It shows compressed page size distribution for various workloads.

> Using kmemtrace to analyze the exact reason for the bad
> fragmentation would probably be helpful.
> 

That was purely internal fragmentation.
Wastage per obj = ksize(obj) - actual_size.

Code used for testing:
http://code.google.com/p/compcache/source/browse/trunk/sub-projects/testing/kmalloc_test/kmalloc_test.c
This is a "SwapReplay client". Please see:
http://code.google.com/p/compcache/wiki/SwapReplayDesign

Thanks,
Nitin

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