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 14:43:42 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Christoph Lameter <cl@...ux.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 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.

js1304@...304-P5Q-DELUXE:~/Projects/remote_git/linux$ sudo cat /proc/slabinfo | awk '{if($4 < 32) print $0}'
slabinfo - version: 2.1
ecryptfs_file_cache      0      0     16  240    1 : tunables  120   60    8 : slabdata      0      0      0
jbd2_revoke_table_s      2    240     16  240    1 : tunables  120   60    8 : slabdata      1      1      0
journal_handle         0      0     24  163    1 : tunables  120   60    8 : slabdata      0      0      0
revoke_table           0      0     16  240    1 : tunables  120   60    8 : slabdata      0      0      0
scsi_data_buffer       0      0     24  163    1 : tunables  120   60    8 : slabdata      0      0      0
fsnotify_event_holder      0      0     24  163    1 : tunables  120   60    8 : slabdata      0      0      0
numa_policy            3    163     24  163    1 : tunables  120   60    8 : slabdata      1      1      0

> 
> > Moreover, we can configure slab_max_order in boot time so that we can't know
> > how many object are in a certain slab in compile time. Therefore we can't
> > decide the size of the index in compile time.
> 
> You can ignore the slab_max_order if necessary.
> 
> > I think that byte and short int sized index support would be enough, but
> > it should be determined at runtime.
> 
> 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.

Thanks.
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