From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753425AbeDCVM5 (ORCPT ); Tue, 3 Apr 2018 17:12:57 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:43898 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752639AbeDCVM4 (ORCPT ); Tue, 3 Apr 2018 17:12:56 -0400 Date: Tue, 3 Apr 2018 14:12:53 -0700 From: Matthew Wilcox To: Buddy Lumpkin Cc: Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org, hannes@cmpxchg.org, riel@surriel.com, mgorman@suse.de, akpm@linux-foundation.org Subject: Re: [RFC PATCH 1/1] vmscan: Support multiple kswapd threads per node Message-ID: <20180403211253.GC30145@bombadil.infradead.org> References: <1522661062-39745-1-git-send-email-buddy.lumpkin@oracle.com> <1522661062-39745-2-git-send-email-buddy.lumpkin@oracle.com> <20180403133115.GA5501@dhcp22.suse.cz> <20180403190759.GB6779@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 03, 2018 at 01:49:25PM -0700, Buddy Lumpkin wrote: > > Yes, very much this. If you have a single-threaded workload which is > > using the entirety of memory and would like to use even more, then it > > makes sense to use as many CPUs as necessary getting memory out of its > > way. If you have N CPUs and N-1 threads happily occupying themselves in > > their own reasonably-sized working sets with one monster process trying > > to use as much RAM as possible, then I'd be pretty unimpressed to see > > the N-1 well-behaved threads preempted by kswapd. > > The default value provides one kswapd thread per NUMA node, the same > it was without the patch. Also, I would point out that just because you devote > more threads to kswapd, doesn’t mean they are busy. If multiple kswapd threads > are busy, they are almost certainly doing work that would have resulted in > direct reclaims, which are often substantially more expensive than a couple > extra context switches due to preemption. [...] > In my previous response to Michal Hocko, I described > how I think we could scale watermarks in response to direct reclaims, and > launch more kswapd threads when kswapd peaks at 100% CPU usage. I think you're missing my point about the workload ... kswapd isn't "nice", so it will compete with the N-1 threads which are chugging along at 100% CPU inside their working sets. In this scenario, we _don't_ want to kick off kswapd at all; we want the monster thread to clean up its own mess. If we have idle CPUs, then yes, absolutely, lets have them clean up for the monster, but otherwise, I want my N-1 threads doing their own thing. Maybe we should renice kswapd anyway ... thoughts? We don't seem to have had a nice'd kswapd since 2.6.12, but maybe we played with that earlier and discovered it was a bad idea?