From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx169.postini.com [74.125.245.169]) by kanga.kvack.org (Postfix) with SMTP id 6DD1B6B0096 for ; Tue, 10 Sep 2013 17:25:07 -0400 (EDT) Date: Tue, 10 Sep 2013 21:25:05 +0000 From: Christoph Lameter Subject: Re: [REPOST PATCH 3/4] slab: introduce byte sized index for the freelist of a slab In-Reply-To: <20130910054342.GB24602@lge.com> Message-ID: <0000014109c372c6-5f3c49d4-ce8b-4760-b80d-a32e042ec09b-000000@email.amazonses.com> References: <1378447067-19832-1-git-send-email-iamjoonsoo.kim@lge.com> <1378447067-19832-4-git-send-email-iamjoonsoo.kim@lge.com> <00000140f3fed229-f49b95d4-7087-476f-b2c9-37846749aad6-000000@email.amazonses.com> <20130909043217.GB22390@lge.com> <00000141032dea11-c5aa9c77-b2f2-4cab-b7a0-d37665a6cec8-000000@email.amazonses.com> <20130910054342.GB24602@lge.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Joonsoo Kim Cc: Pekka Enberg , Andrew Morton , David Rientjes , linux-mm@kvack.org, linux-kernel@vger.kernel.org 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org