From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: Re: SLUB defrag pull request? Date: Thu, 23 Oct 2008 17:17:09 +0200 Message-ID: <49009575.60004@cosmosbay.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE 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 To: Christoph Lameter Return-path: Received: from smtp2e.orange.fr ([80.12.242.113]:40016 "EHLO smtp2e.orange.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751144AbYJWPRZ convert rfc822-to-8bit (ORCPT ); Thu, 23 Oct 2008 11:17:25 -0400 In-Reply-To: Sender: linux-fsdevel-owner@vger.kernel.org List-ID: Christoph Lameter a =E9crit : > On Thu, 23 Oct 2008, Pekka Enberg wrote: >=20 >>> 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= =20 >>> 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? >=20 > Exactly. Slab adds a special management structure to each slab page t= hat=20 > contains the freelist and other stuff. Freeing first occurs to a per = cpu=20 > queue that contains an array of pointers. Then later the objects are=20 > moved from the pointer array into the management structure for the sl= ab. >=20 > What we could do for SLUB is to generate a linked list of pointer arr= ays=20 > in the free objects of a slab page. If all objects are allocated then= no=20 > pointer array is needed. The first object freed would become the firs= t=20 > pointer array. If that is found to be exhausted then the object=20 > currently being freed is becoming the next pointer array and we put a= =20 > link to the old one into the object as well. >=20 This idea is very nice, especially considering that many objects are fr= eed 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 SL= AB gives a hint to processor to warm its cache. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel= " in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html