From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <49009575.60004@cosmosbay.com> Date: Thu, 23 Oct 2008 17:17:09 +0200 From: Eric Dumazet MIME-Version: 1.0 Subject: Re: SLUB defrag pull request? References: <1223883004.31587.15.camel@penberg-laptop> <84144f020810221348j536f0d84vca039ff32676e2cc@mail.gmail.com> <1224745831.25814.21.camel@penberg-laptop> <84144f020810230658o7c6b3651k2d671aab09aa71fb@mail.gmail.com> <84144f020810230714g7f5d36bas812ad691140ee453@mail.gmail.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8BIT Sender: owner-linux-mm@kvack.org Return-Path: To: Christoph Lameter Cc: Pekka Enberg , Miklos Szeredi , nickpiggin@yahoo.com.au, hugh@veritas.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org List-ID: Christoph Lameter a ecrit : > On Thu, 23 Oct 2008, Pekka Enberg wrote: > >>> The problem looks like its freeing objects on a different processor that >>> where it was used last. With the pointer array it is only necessary >>> to touch >>> the objects that contain the arrays. >> >> Interesting. SLAB gets away with this because of per-cpu caches or >> because it uses the bufctls instead of a freelist? > > Exactly. Slab adds a special management structure to each slab page that > contains the freelist and other stuff. Freeing first occurs to a per cpu > queue that contains an array of pointers. Then later the objects are > moved from the pointer array into the management structure for the slab. > > What we could do for SLUB is to generate a linked list of pointer arrays > in the free objects of a slab page. If all objects are allocated then no > pointer array is needed. The first object freed would become the first > pointer array. If that is found to be exhausted then the object > currently being freed is becoming the next pointer array and we put a > link to the old one into the object as well. > This idea is very nice, especially considering that many objects are freed by RCU, and their rcu_head (which is hot at kfree() time), might be far away the linked list anchor actually used in SLUB. At alloc time, I remember I added a prefetchw() call in SLAB in __cache_alloc(), this could explain some differences between SLUB and SLAB too, since SLAB gives a hint to processor to warm its cache. -- 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