Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752615AbbG2PJB (ORCPT ); Wed, 29 Jul 2015 11:09:01 -0400 Received: from mail-wi0-f181.google.com ([209.85.212.181]:34474 "EHLO mail-wi0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750814AbbG2PI7 (ORCPT ); Wed, 29 Jul 2015 11:08:59 -0400 Date: Wed, 29 Jul 2015 17:08:55 +0200 From: Michal Hocko To: Vladimir Davydov Cc: Michel Lespinasse , Andrew Morton , Andres Lagar-Cavilla , Minchan Kim , Raghavendra K T , Johannes Weiner , Greg Thelen , David Rientjes , Pavel Emelyanov , Cyrill Gorcunov , Jonathan Corbet , linux-api@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH -mm v9 0/8] idle memory tracking Message-ID: <20150729150855.GM15801@dhcp22.suse.cz> References: <20150729123629.GI15801@dhcp22.suse.cz> <20150729135907.GT8100@esperanza> <20150729144539.GU8100@esperanza> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150729144539.GU8100@esperanza> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2253 Lines: 44 On Wed 29-07-15 17:45:39, Vladimir Davydov wrote: > On Wed, Jul 29, 2015 at 07:12:13AM -0700, Michel Lespinasse wrote: > > On Wed, Jul 29, 2015 at 6:59 AM, Vladimir Davydov > > wrote: > > >> I guess the primary reason to rely on the pfn rather than the LRU walk, > > >> which would be more targeted (especially for memcg cases), is that we > > >> cannot hold lru lock for the whole LRU walk and we cannot continue > > >> walking after the lock is dropped. Maybe we can try to address that > > >> instead? I do not think this is easy to achieve but have you considered > > >> that as an option? > > > > > > Yes, I have, and I've come to a conclusion it's not doable, because LRU > > > lists can be constantly rotating at an arbitrary rate. If you have an > > > idea in mind how this could be done, please share. > > > > > > Speaking of LRU-vs-PFN walk, iterating over PFNs has its own advantages: > > > - You can distribute a walk in time to avoid CPU bursts. > > > - You are free to parallelize the scanner as you wish to decrease the > > > scan time. > > > > There is a third way: one could go through every MM in the system and scan > > their page tables. Doing things that way turns out to be generally faster > > than scanning by physical address, because you don't have to go through > > RMAP for every page. But, you end up needing to take the mmap_sem lock of > > every MM (in turn) while scanning them, and that degrades quickly under > > memory load, which is exactly when you most need this feature. So, scan by > > address is still what we use here. > > Page table scan approach has the inherent problem - it ignores unmapped > page cache. If a workload does a lot of read/write or map-access-unmap > operations, we won't be able to even roughly estimate its wss. That page cache is trivially reclaimable if it is clean. If it needs writeback then it is non-idle only until the next writeback. So why does it matter for the estimation? -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/