From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-la0-f54.google.com (mail-la0-f54.google.com [209.85.215.54]) by kanga.kvack.org (Postfix) with ESMTP id C15416B0253 for ; Wed, 29 Jul 2015 10:45:59 -0400 (EDT) Received: by laah7 with SMTP id h7so7719144laa.0 for ; Wed, 29 Jul 2015 07:45:58 -0700 (PDT) Received: from relay.parallels.com (relay.parallels.com. [195.214.232.42]) by mx.google.com with ESMTPS id uo9si21690760lbb.12.2015.07.29.07.45.56 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 29 Jul 2015 07:45:57 -0700 (PDT) Date: Wed, 29 Jul 2015 17:45:39 +0300 From: Vladimir Davydov Subject: Re: [PATCH -mm v9 0/8] idle memory tracking Message-ID: <20150729144539.GU8100@esperanza> References: <20150729123629.GI15801@dhcp22.suse.cz> <20150729135907.GT8100@esperanza> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Michel Lespinasse Cc: Michal Hocko , 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 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. Thanks, Vladimir -- 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