From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f69.google.com (mail-ed1-f69.google.com [209.85.208.69]) by kanga.kvack.org (Postfix) with ESMTP id 45AA88E0002 for ; Fri, 18 Jan 2019 04:09:57 -0500 (EST) Received: by mail-ed1-f69.google.com with SMTP id e12so4714402edd.16 for ; Fri, 18 Jan 2019 01:09:57 -0800 (PST) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id o2-v6si6774775ejx.251.2019.01.18.01.09.55 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 18 Jan 2019 01:09:55 -0800 (PST) Subject: Re: [PATCH 20/25] mm, compaction: Reduce unnecessary skipping of migration target scanner References: <20190104125011.16071-1-mgorman@techsingularity.net> <20190104125011.16071-21-mgorman@techsingularity.net> <8e310c2a-5f2e-ee99-24c5-10a71972699a@suse.cz> <20190117193909.GO27437@techsingularity.net> From: Vlastimil Babka Message-ID: <7a4dcc5e-9c31-c3bb-e38a-5ef955ca409e@suse.cz> Date: Fri, 18 Jan 2019 10:09:54 +0100 MIME-Version: 1.0 In-Reply-To: <20190117193909.GO27437@techsingularity.net> Content-Type: text/plain; charset=iso-8859-15 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Mel Gorman Cc: Linux-MM , David Rientjes , Andrea Arcangeli , ying.huang@intel.com, kirill@shutemov.name, Andrew Morton , Linux List Kernel Mailing On 1/17/19 8:39 PM, Mel Gorman wrote: > On Thu, Jan 17, 2019 at 06:58:30PM +0100, Vlastimil Babka wrote: >> On 1/4/19 1:50 PM, Mel Gorman wrote: >> > The fast isolation of pages can move the scanner faster than is necessary >> > depending on the contents of the free list. This patch will only allow >> > the fast isolation to initialise the scanner and advance it slowly. The >> > primary means of moving the scanner forward is via the linear scanner >> > to reduce the likelihood the migration source/target scanners meet >> > prematurely triggering a rescan. >> >> Maybe I've seen enough code today and need to stop, but AFAICS the description >> here doesn't match the actual code changes? What I see are some cleanups, and a >> change in free scanner that will set pageblock skip bit after a pageblock has >> been scanned, even if there were pages isolated, while previously it would set >> the skip bit only if nothing was isolated. >> > > The first three hunks could have been split out but it wouldn't help > overall. Maybe a changelog rewrite will help; > > mm, compaction: Reduce premature advancement of the migration target scanner > > The fast isolation of free pages allows the cached PFN of the free > scanner to advance faster than necessary depending on the contents > of the free list. The key is that fast_isolate_freepages() can update > zone->compact_cached_free_pfn via isolate_freepages_block(). When the > fast search fails, the linear scan can start from a point that has skipped > valid migration targets, particularly pageblocks with just low-order > free pages. This can cause the migration source/target scanners to meet > prematurely causing a reset. > > This patch starts by avoiding an update of the pageblock skip information > and cached PFN from isolate_freepages_block() and puts the responsibility > of updating that information in the callers. The fast scanner will update > the cached PFN if and only if it finds a block that is higher than the > existing cached PFN and sets the skip if the pageblock is full or nearly > full. The linear scanner will update skipped information and the cached > PFN only when a block is completely scanned. The total impact is that > the free scanner advances more slowly as it is primarily driven by the > linear scanner instead of the fast search. > > Does that help? Yeah, now I get it, thanks! Acked-by: Vlastimil Babka