linux-kernel - Re: + fix-spellings-of-slab-allocator-section-in-init-kconfig.patch added to -mm tree

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Date:	Tue, 8 May 2007 20:24:41 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Matt Mackall <mpm@...enic.com>
cc:	akpm@...ux-foundation.org, Nick Piggin <nickpiggin@...oo.com.au>,
	David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org
Subject: Re: + fix-spellings-of-slab-allocator-section-in-init-kconfig.patch
 added to -mm tree

On Tue, 8 May 2007, Matt Mackall wrote:

> > Exactly. That overhead does not exist in SLUB. Thus SLOB is less efficient 
> > than SLUB.
> 
> What size object does kmalloc(80) return? In SLAB, the answer is 128
> bytes with 48 bytes of slack space. In SLOB, the answer is 88 for 8
> bytes of slack space. Looks like SLUB is in the same camp as SLAB
> here:

There is a 96 sized general slab. So it would go up to that size. But you 
can create a 80 byte slab of course. And that may cost minimal overhead 
since 80 byte slabs may be merged. If one already exist then you get it 
for free.

> +/*
> + * We keep the general caches in an array of slab caches that are used for
> + * 2^x bytes of allocations.
> + */
> +extern struct kmem_cache kmalloc_caches[KMALLOC_SHIFT_HIGH + 1];
> ..
> +	if (size <=        128) return 7;

96 please. You skipped the first part.

> SLOB's kmalloc overhead is 8 bytes, always. That's 1/8th the average
> SLAB kmalloc overhead.

SLUB can generate an 80 byte slab with minimal overhead if you wanted. But 
yes I agree the flexbility there is an advantage if you have objects of 
various sizes.
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