Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761969AbcLUSkw (ORCPT ); Wed, 21 Dec 2016 13:40:52 -0500 Received: from mail-pf0-f194.google.com ([209.85.192.194]:36387 "EHLO mail-pf0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1761922AbcLUSku (ORCPT ); Wed, 21 Dec 2016 13:40:50 -0500 Date: Thu, 22 Dec 2016 04:40:35 +1000 From: Nicholas Piggin To: Linus Torvalds Cc: Dave Hansen , Linux Kernel Mailing List , linux-mm , Andreas Gruenbacher , Bob Peterson , Mel Gorman , Peter Zijlstra , Andrew Lutomirski , Steven Whitehouse Subject: Re: [RFC][PATCH] make global bitlock waitqueues per-node Message-ID: <20161222044035.164ae188@roar.ozlabs.ibm.com> In-Reply-To: References: <20161219225826.F8CB356F@viggo.jf.intel.com> <20161221223056.17c37dd6@roar.ozlabs.ibm.com> Organization: IBM X-Mailer: Claws Mail 3.14.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1380 Lines: 34 On Wed, 21 Dec 2016 10:12:36 -0800 Linus Torvalds wrote: > On Wed, Dec 21, 2016 at 4:30 AM, Nicholas Piggin wrote: > > > > I've been doing a bit of testing, and I don't know why you're seeing > > this. > > > > I don't think I've been able to trigger any actual page lock contention > > so nothing gets put on the waitqueue to really bounce cache lines around > > that I can see. > > The "test is the waitqueue is empty" is going to cause cache misses > even if there is no contention. > > In fact, that's why I want the contention bit in the struct page - not > because of any NUMA issues, but simply due to cache misses. > > And yes, with no contention the bit waiting should hopefully be able > to cache things shared - which should make the bouncing much less - > but there's going to be a shitload of false sharing with any actual > IO, so you will get bouncing due to that. Well that's what I'm actually interested in, but I could not get it to do much bouncing at all. There was a significant amount of writes going through when having the backing store files on writeback filesystem, but even that was not really triggering a lot of actual waiters. Not that I don't believe it could happen, and Dave's system is a lot bigger and faster and more NUMA than the one I was testing on. I'm just curious. Thanks, Nick