From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4900A7C8.9020707@cosmosbay.com> Date: Thu, 23 Oct 2008 18:35:20 +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> <49009575.60004@cosmosbay.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, Eric Dumazet wrote: > >> 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. > > SLUB touches objects by default when allocating. And it does it > immediately in slab_alloc() in order to retrieve the pointer to the next > object. So there is no point of hinting there right now. > Please note SLUB touches by reading object. prefetchw() gives a hint to cpu saying this cache line is going to be *modified*, even if first access is a read. Some architectures can save some bus transactions, acquiring the cache line in an exclusive way instead of shared one. > If we go to the pointer arrays then the situation is similar to SLAB > where the object is not touched by the allocator. Then the hint would be > useful again. It is usefull right now for ((SLAB_DESTROY_BY_RCU | SLAB_POISON) or ctor caches. Probably not that important because many objects are very large anyway, and a prefetchw() of the begining of object is partial. -- 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