linux-kernel - Re: kmem_cache_create loop for find the proper gfporder

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Message-ID: <84144f020703250830k5fb3e2eo6145a88cbe5a859d@mail.gmail.com>
Date:	Sun, 25 Mar 2007 17:30:04 +0200
From:	"Pekka Enberg" <penberg@...helsinki.fi>
To:	"Bin Chen" <binary.chen@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: kmem_cache_create loop for find the proper gfporder

On 3/25/07, Bin Chen <binary.chen@...il.com> wrote:
> It is done by increase gfporder for low number to high(possibly 0 to
> MAX_GFP_ORDER). But why increase the gfporder(or slab size) can
> decrease the internal fragmentation?)
>
> A simple example, suppose the slab management stuff is kept off-slab,
> if the gfporder is zero, and the object size in slab is 1000, the
> wasted space is 4096 mod 1000 = 96, but with 4096 * 2(increase
> gfporder by 1), the space is 8192 mod 1000 = 192, 192 > 96.

You didn't simulate the algorithm long enough. If you had, you'd hit
order five which wastes only 72 bytes in your example.

                                 Pekka
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