From: Christoph Lameter <cl@linux.com>
To: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
David Rientjes <rientjes@google.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [REPOST PATCH 3/4] slab: introduce byte sized index for the freelist of a slab
Date: Tue, 10 Sep 2013 21:25:05 +0000 [thread overview]
Message-ID: <0000014109c372c6-5f3c49d4-ce8b-4760-b80d-a32e042ec09b-000000@email.amazonses.com> (raw)
In-Reply-To: <20130910054342.GB24602@lge.com>
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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next prev parent reply other threads:[~2013-09-10 21:25 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-06 5:57 [REPOST PATCH 0/4] slab: implement byte sized indexes " Joonsoo Kim
2013-09-06 5:57 ` [REPOST PATCH 1/4] slab: factor out calculate nr objects in cache_estimate Joonsoo Kim
2013-09-06 15:48 ` Christoph Lameter
2013-09-09 4:32 ` Joonsoo Kim
2013-09-06 5:57 ` [REPOST PATCH 2/4] slab: introduce helper functions to get/set free object Joonsoo Kim
2013-09-06 15:49 ` Christoph Lameter
2013-09-06 5:57 ` [REPOST PATCH 3/4] slab: introduce byte sized index for the freelist of a slab Joonsoo Kim
2013-09-06 15:58 ` Christoph Lameter
2013-09-09 4:32 ` Joonsoo Kim
2013-09-09 14:44 ` Christoph Lameter
2013-09-10 5:43 ` Joonsoo Kim
2013-09-10 21:25 ` Christoph Lameter [this message]
2013-09-11 1:04 ` Joonsoo Kim
2013-09-11 14:22 ` Christoph Lameter
2013-09-12 6:52 ` Joonsoo Kim
2013-09-06 5:57 ` [REPOST PATCH 4/4] slab: make more slab management structure off the slab Joonsoo Kim
2013-09-06 15:59 ` Christoph Lameter
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