linux-kernel - Re: [REPOST PATCH 3/4] slab: introduce byte sized index for the freelist of a slab

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Date:	Tue, 10 Sep 2013 21:25:05 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
cc:	Pekka Enberg <penberg@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [REPOST PATCH 3/4] slab: introduce byte sized index for the
 freelist of a slab

On Tue, 10 Sep 2013, Joonsoo Kim wrote:

> On Mon, Sep 09, 2013 at 02:44:03PM +0000, Christoph Lameter wrote:
> > On Mon, 9 Sep 2013, Joonsoo Kim wrote:
> >
> > > 32 byte is not minimum object size, minimum *kmalloc* object size
> > > in default configuration. There are some slabs that their object size is
> > > less than 32 byte. If we have a 8 byte sized kmem_cache, it has 512 objects
> > > in 4K page.
> >
> > As far as I can recall only SLUB supports 8 byte objects. SLABs mininum
> > has always been 32 bytes.
>
> No.
> There are many slabs that their object size are less than 32 byte.
> And I can also create a 8 byte sized slab in my kernel with SLAB.

Well the minimum size for the kmalloc array is 32 bytes. These are custom
slabs. KMALLOC_SHIFT_LOW is set to 5 in include/linux/slab.h.

Ok so there are some slabs like that. Hmmm.. We have sizes 16 and 24 in
your list. 16*256 is still 4096. So this would still work fine if we would
forbid a size of 8 or increase that by default to 16.

> > On x86 f.e. it would add useless branching. The branches are never taken.
> > You only need these if you do bad things to the system like requiring
> > large contiguous allocs.
>
> As I said before, since there is a possibility that some runtime loaded modules
> use a 8 byte sized slab, we can't determine index size in compile time. Otherwise
> we should always use short int sized index and I think that it is worse than
> adding a branch.

We can enforce a mininum slab size and an order limit so that it fits. And
then there would be no additional branching.


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