From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932473AbdCIPJL (ORCPT ); Thu, 9 Mar 2017 10:09:11 -0500 Received: from mx2.suse.de ([195.135.220.15]:54460 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932401AbdCIPJJ (ORCPT ); Thu, 9 Mar 2017 10:09:09 -0500 Date: Thu, 9 Mar 2017 15:09:04 +0000 From: Mel Gorman To: Anshuman Khandual Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, mhocko@suse.com, vbabka@suse.cz, minchan@kernel.org, aneesh.kumar@linux.vnet.ibm.com, bsingharora@gmail.com, srikar@linux.vnet.ibm.com, haren@linux.vnet.ibm.com, jglisse@redhat.com, dave.hansen@intel.com, dan.j.williams@intel.com, zi.yan@cs.rutgers.edu, Naoya Horiguchi Subject: Re: [PATCH 0/6] Enable parallel page migration Message-ID: <20170309150904.pnk6ejeug4mktxjv@suse.de> References: <20170217112453.307-1-khandual@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.6.2 (2016-07-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 08, 2017 at 09:34:27PM +0530, Anshuman Khandual wrote: > > Any comments, suggestions are welcome. > > Hello Vlastimil/Michal/Minchan/Mel/Dave, > > Apart from the comments from Naoya on a different thread posted by Zi > Yan, I did not get any more review comments on this series. Could you > please kindly have a look on the over all design and its benefits from > page migration performance point of view and let me know your views. > Thank you. > I didn't look into the patches in detail except to get a general feel for how it works and I'm not convinced that it's a good idea at all. I accept that memory bandwidth utilisation may be higher as a result but consider the impact. THP migrations are relatively rare and when they occur, it's in the context of a single thread. To parallelise the copy, an allocation, kmap and workqueue invocation are required. There may be a long delay before the workqueue item can start which may exceed the time to do a single copy if the CPUs on a node are saturated. Furthermore, a single thread can preempt operations of other unrelated threads and incur CPU cache pollution and future misses on unrelated CPUs. It's compounded by the fact that a high priority system workqueue is used to do the operation, one that is used for CPU hotplug operations and rolling back when a netdevice fails to be registered. It treats a hugepage copy as an essential operation that can preempt all other work which is very questionable. The series leader has no details on a workload that is bottlenecked by THP migrations and even if it is, the primary question should be *why* THP migrations are so frequent and alleviating that instead of preempting multiple CPUs to do the work. -- Mel Gorman SUSE Labs