From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wj0-f199.google.com (mail-wj0-f199.google.com [209.85.210.199]) by kanga.kvack.org (Postfix) with ESMTP id 517B96B0033 for ; Fri, 27 Jan 2017 07:01:06 -0500 (EST) Received: by mail-wj0-f199.google.com with SMTP id kq3so46326807wjc.1 for ; Fri, 27 Jan 2017 04:01:06 -0800 (PST) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id 13si5687914wrz.282.2017.01.27.04.01.04 for (version=TLS1 cipher=AES128-SHA bits=128/128); Fri, 27 Jan 2017 04:01:04 -0800 (PST) Date: Fri, 27 Jan 2017 13:01:01 +0100 From: Michal Hocko Subject: Re: [PATCH 3/5] mm: vmscan: remove old flusher wakeup from direct reclaim path Message-ID: <20170127120101.GA4148@dhcp22.suse.cz> References: <20170123181641.23938-1-hannes@cmpxchg.org> <20170123181641.23938-4-hannes@cmpxchg.org> <20170126100509.gbf6rxao6gsmqyq3@suse.de> <20170126185027.GB30636@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170126185027.GB30636@cmpxchg.org> Sender: owner-linux-mm@kvack.org List-ID: To: Johannes Weiner Cc: Mel Gorman , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@fb.com On Thu 26-01-17 13:50:27, Johannes Weiner wrote: > On Thu, Jan 26, 2017 at 10:05:09AM +0000, Mel Gorman wrote: > > On Mon, Jan 23, 2017 at 01:16:39PM -0500, Johannes Weiner wrote: > > > Direct reclaim has been replaced by kswapd reclaim in pretty much all > > > common memory pressure situations, so this code most likely doesn't > > > accomplish the described effect anymore. The previous patch wakes up > > > flushers for all reclaimers when we encounter dirty pages at the tail > > > end of the LRU. Remove the crufty old direct reclaim invocation. > > > > > > Signed-off-by: Johannes Weiner > > > > In general I like this. I worried first that if kswapd is blocked > > writing pages that it won't reach the wakeup_flusher_threads but the > > previous patch handles it. > > > > Now though, it occurs to me with the last patch that we always writeout > > the world when flushing threads. This may not be a great idea. Consider > > for example if there is a heavy writer of short-lived tmp files. In such a > > case, it is possible for the files to be truncated before they even hit the > > disk. However, if there are multiple "writeout the world" calls, these may > > now be hitting the disk. Furthermore, multiplle kswapd and direct reclaimers > > could all be requested to writeout the world and each request unplugs. > > > > Is it possible to maintain the property of writing back pages relative > > to the numbers of pages scanned or have you determined already that it's > > not necessary? > > That's what I started out with - waking the flushers for nr_taken. I > was using a silly test case that wrote < dirty background limit and > then allocated a burst of anon memory. When the dirty data is linear, > the bigger IO requests are beneficial. They don't exhaust struct > request (like kswapd 4k IO routinely does, and SWAP_CLUSTER_MAX is > only 32), and they require less frequent plugging. > > Force-flushing temporary files under memory pressure is a concern - > although the most recently dirtied files would get queued last, giving > them still some time to get truncated - but I'm wary about splitting > the flush requests too aggressively when we DO sustain throngs of > dirty pages hitting the reclaim scanners. I think the above would be helpful in the changelog for future reference. -- Michal Hocko SUSE Labs -- 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