From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2993588AbcBSUVE (ORCPT ); Fri, 19 Feb 2016 15:21:04 -0500 Received: from gum.cmpxchg.org ([85.214.110.215]:59080 "EHLO gum.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1946795AbcBSUVC (ORCPT ); Fri, 19 Feb 2016 15:21:02 -0500 Date: Fri, 19 Feb 2016 15:20:00 -0500 From: Johannes Weiner To: Mel Gorman Cc: Andrew Morton , Rik van Riel , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH] mm: scale kswapd watermarks in proportion to memory Message-ID: <20160219202000.GB17342@cmpxchg.org> References: <1455813719-2395-1-git-send-email-hannes@cmpxchg.org> <20160219112543.GJ4763@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160219112543.GJ4763@suse.de> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 19, 2016 at 11:25:43AM +0000, Mel Gorman wrote: > On Thu, Feb 18, 2016 at 11:41:59AM -0500, Johannes Weiner wrote: > > In machines with 140G of memory and enterprise flash storage, we have > > seen read and write bursts routinely exceed the kswapd watermarks and > > cause thundering herds in direct reclaim. Unfortunately, the only way > > to tune kswapd aggressiveness is through adjusting min_free_kbytes - > > the system's emergency reserves - which is entirely unrelated to the > > system's latency requirements. In order to get kswapd to maintain a > > 250M buffer of free memory, the emergency reserves need to be set to > > 1G. That is a lot of memory wasted for no good reason. > > > > On the other hand, it's reasonable to assume that allocation bursts > > and overall allocation concurrency scale with memory capacity, so it > > makes sense to make kswapd aggressiveness a function of that as well. > > > > Change the kswapd watermark scale factor from the currently fixed 25% > > of the tunable emergency reserve to a tunable 0.001% of memory. > > > > On a 140G machine, this raises the default watermark steps - the > > distance between min and low, and low and high - from 16M to 143M. > > > > Signed-off-by: Johannes Weiner > > Intuitively, the patch makes sense although Rik's comments should be > addressed. > > The caveat will be that there will be workloads that used to fit into > memory without reclaim that now have kswapd activity. It might manifest > as continual reclaim with some thrashing but it should only apply to > workloads that are exactly sized to fit in memory which in my experience > are relatively rare. It should be "obvious" when occurs at least. This is a problem only in theory, I think, because I doubt anybody is able to keep a workingset reliably at a margin of less than 0.001% of memory. I'd expect few users to even go within single digit margins without eventually thrashing anyway. It certainly becomes a real issue when users tune the scale factor, but then it will be a deliberate act with known consequences. That's what I choose to believe in. > Acked-by: Mel Gorman Thanks!