From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx201.postini.com [74.125.245.201]) by kanga.kvack.org (Postfix) with SMTP id 056E56B0044 for ; Wed, 7 Nov 2012 02:13:21 -0500 (EST) Message-ID: <509A0A04.2030503@parallels.com> Date: Wed, 7 Nov 2012 08:13:08 +0100 From: Glauber Costa MIME-Version: 1.0 Subject: Re: [PATCH v6 25/29] memcg/sl[au]b: shrink dead caches References: <1351771665-11076-1-git-send-email-glommer@parallels.com> <1351771665-11076-26-git-send-email-glommer@parallels.com> <20121105164813.2eba5ecb.akpm@linux-foundation.org> In-Reply-To: <20121105164813.2eba5ecb.akpm@linux-foundation.org> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kamezawa.hiroyu@jp.fujitsu.com, Johannes Weiner , Tejun Heo , Michal Hocko , Christoph Lameter , Pekka Enberg , David Rientjes , Pekka Enberg , Suleiman Souhlal On 11/06/2012 01:48 AM, Andrew Morton wrote: > On Thu, 1 Nov 2012 16:07:41 +0400 > Glauber Costa wrote: > >> This means that when we destroy a memcg cache that happened to be empty, >> those caches may take a lot of time to go away: removing the memcg >> reference won't destroy them - because there are pending references, and >> the empty pages will stay there, until a shrinker is called upon for any >> reason. >> >> In this patch, we will call kmem_cache_shrink for all dead caches that >> cannot be destroyed because of remaining pages. After shrinking, it is >> possible that it could be freed. If this is not the case, we'll schedule >> a lazy worker to keep trying. > > This patch is really quite nasty. We poll the cache once per minute > trying to shrink then free it? a) it gives rise to concerns that there > will be scenarios where the system could suffer unlimited memory windup > but mainly b) it's just lame. > > The kernel doesn't do this sort of thing. The kernel tries to be > precise: in a situation like this we keep track of the number of > outstanding objects and when that falls to zero, we free their > container synchronously. If those objects are normally left floating > around in an allocated but reclaimable state then we can address that > by synchronously freeing them if their container has been destroyed. > > Or something like that. If it's something else then fine, but not this. > > What do we need to do to fix this? > The original patch had a unlikely() test in the free path, conditional on whether or not the cache is dead, that would then call this is the cache would now be empty. I got several requests to remove it and change it to something like this, because that is a fast path (I myself think an unlikely branch is not that bad) If you think such a test is acceptable, I can bring it back and argue in the basis of "akpm made me do it!". But meanwhile I will give this extra though to see if there is any alternative way I can do it... -- 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