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From: Vlastimil Babka <vbabka@suse.cz>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Linux-MM <linux-mm@kvack.org>,
	David Rientjes <rientjes@google.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	ying.huang@intel.com, kirill@shutemov.name,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux List Kernel Mailing <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 12/25] mm, compaction: Keep migration source private to a single compaction instance
Date: Thu, 17 Jan 2019 10:29:00 +0100	[thread overview]
Message-ID: <ff6953a1-6cb4-8f5d-4bfa-f1e8f5dd4f7a@suse.cz> (raw)
In-Reply-To: <20190116161537.GG27437@techsingularity.net>

On 1/16/19 5:15 PM, Mel Gorman wrote:
> On Wed, Jan 16, 2019 at 04:45:59PM +0100, Vlastimil Babka wrote:
>> On 1/4/19 1:49 PM, Mel Gorman wrote:
>> > Due to either a fast search of the free list or a linear scan, it is
>> > possible for multiple compaction instances to pick the same pageblock
>> > for migration.  This is lucky for one scanner and increased scanning for
>> > all the others. It also allows a race between requests on which first
>> > allocates the resulting free block.
>> > 
>> > This patch tests and updates the pageblock skip for the migration scanner
>> > carefully. When isolating a block, it will check and skip if the block is
>> > already in use. Once the zone lock is acquired, it will be rechecked so
>> > that only one scanner can set the pageblock skip for exclusive use. Any
>> > scanner contending will continue with a linear scan. The skip bit is
>> > still set if no pages can be isolated in a range.
>> 
>> Also the skip bit will remain set even if pages *could* be isolated,
> 
> That's the point -- the pageblock is scanned by one compaction instance
> and skipped by others.

OK, I understood wrongly that this is meant just to avoid races.

>> AFAICS there's no clearing after a block was finished with
>> nr_isolated>0. Is it intended?
> 
> Yes, defer to a full reset later when the compaction scanners meet.
> Tracing really indicated we spent a stupid amount of time scanning,
> rescanning and competing for pageblocks within short interval.

Right.

>> > Migration scan rates are reduced by 52%.
>> 
>> Wonder how much of that is due to not clearing as pointed out above.
>> Also interesting how free scanned was reduced so disproportionally.
>> 
> 
> The amount of free scanning is related to the amount of migration
> scanning. If migration sources are scanning, rescanning and competing
> for the same pageblocks, it can result in unnecessary free scanning too.
> It doesn't fully explain the drop but I didn't specifically try to quantify
> it either as the free scanner is altered further in later patches.

Perhaps lots of skipping in migration scanners mean that they progress faster
into the parts of zone that would otherwise be scanned by the free scanner, so
the free scanner has less work to do. But agree that it's moot to investigate
too much if there are further changes later.


  reply	other threads:[~2019-01-17  9:29 UTC|newest]

Thread overview: 75+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-04 12:49 [PATCH 00/25] Increase success rates and reduce latency of compaction v2 Mel Gorman
2019-01-04 12:49 ` [PATCH 01/25] mm, compaction: Shrink compact_control Mel Gorman
2019-01-04 12:49 ` [PATCH 02/25] mm, compaction: Rearrange compact_control Mel Gorman
2019-01-04 12:49 ` [PATCH 03/25] mm, compaction: Remove last_migrated_pfn from compact_control Mel Gorman
2019-01-04 12:49 ` [PATCH 04/25] mm, compaction: Remove unnecessary zone parameter in some instances Mel Gorman
2019-01-15 11:43   ` Vlastimil Babka
2019-01-04 12:49 ` [PATCH 05/25] mm, compaction: Rename map_pages to split_map_pages Mel Gorman
2019-01-15 11:59   ` Vlastimil Babka
2019-01-04 12:49 ` [PATCH 06/25] mm, compaction: Skip pageblocks with reserved pages Mel Gorman
2019-01-15 12:10   ` Vlastimil Babka
2019-01-15 12:50     ` Mel Gorman
2019-01-16  9:42       ` Mel Gorman
2019-01-04 12:49 ` [PATCH 07/25] mm, migrate: Immediately fail migration of a page with no migration handler Mel Gorman
2019-01-04 12:49 ` [PATCH 08/25] mm, compaction: Always finish scanning of a full pageblock Mel Gorman
2019-01-04 12:49 ` [PATCH 09/25] mm, compaction: Use the page allocator bulk-free helper for lists of pages Mel Gorman
2019-01-15 12:39   ` Vlastimil Babka
2019-01-16  9:46     ` Mel Gorman
2019-01-04 12:49 ` [PATCH 10/25] mm, compaction: Ignore the fragmentation avoidance boost for isolation and compaction Mel Gorman
2019-01-15 13:18   ` Vlastimil Babka
2019-01-04 12:49 ` [PATCH 11/25] mm, compaction: Use free lists to quickly locate a migration source Mel Gorman
2019-01-16 13:15   ` Vlastimil Babka
2019-01-16 14:33     ` Mel Gorman
2019-01-16 15:00       ` Vlastimil Babka
2019-01-16 15:43         ` Mel Gorman
2019-01-04 12:49 ` [PATCH 12/25] mm, compaction: Keep migration source private to a single compaction instance Mel Gorman
2019-01-16 15:45   ` Vlastimil Babka
2019-01-16 16:15     ` Mel Gorman
2019-01-17  9:29       ` Vlastimil Babka [this message]
2019-01-17  9:40   ` Vlastimil Babka
2019-01-04 12:49 ` [PATCH 13/25] mm, compaction: Use free lists to quickly locate a migration target Mel Gorman
2019-01-17 14:36   ` Vlastimil Babka
2019-01-17 15:51     ` Mel Gorman
2019-01-04 12:50 ` [PATCH 14/25] mm, compaction: Avoid rescanning the same pageblock multiple times Mel Gorman
2019-01-17 15:16   ` Vlastimil Babka
2019-01-17 16:00     ` Mel Gorman
2019-01-04 12:50 ` [PATCH 15/25] mm, compaction: Finish pageblock scanning on contention Mel Gorman
2019-01-17 16:38   ` Vlastimil Babka
2019-01-17 17:11     ` Mel Gorman
2019-01-18  8:57       ` Vlastimil Babka
2019-01-04 12:50 ` [PATCH 16/25] mm, compaction: Check early for huge pages encountered by the migration scanner Mel Gorman
2019-01-17 17:01   ` Vlastimil Babka
2019-01-17 17:35     ` Mel Gorman
2019-01-04 12:50 ` [PATCH 17/25] mm, compaction: Keep cached migration PFNs synced for unusable pageblocks Mel Gorman
2019-01-17 17:17   ` Vlastimil Babka
2019-01-17 17:37     ` Mel Gorman
2019-01-04 12:50 ` [PATCH 18/25] mm, compaction: Rework compact_should_abort as compact_check_resched Mel Gorman
2019-01-17 17:27   ` Vlastimil Babka
2019-01-04 12:50 ` [PATCH 19/25] mm, compaction: Do not consider a need to reschedule as contention Mel Gorman
2019-01-17 17:33   ` Vlastimil Babka
2019-01-17 18:05     ` Mel Gorman
2019-01-04 12:50 ` [PATCH 20/25] mm, compaction: Reduce unnecessary skipping of migration target scanner Mel Gorman
2019-01-17 17:58   ` Vlastimil Babka
2019-01-17 19:39     ` Mel Gorman
2019-01-18  9:09       ` Vlastimil Babka
2019-01-04 12:50 ` [PATCH 21/25] mm, compaction: Round-robin the order while searching the free lists for a target Mel Gorman
2019-01-18  9:17   ` Vlastimil Babka
2019-01-04 12:50 ` [PATCH 22/25] mm, compaction: Sample pageblocks for free pages Mel Gorman
2019-01-18 10:38   ` Vlastimil Babka
2019-01-18 13:44     ` Mel Gorman
2019-01-04 12:50 ` [PATCH 23/25] mm, compaction: Be selective about what pageblocks to clear skip hints Mel Gorman
2019-01-18 12:55   ` Vlastimil Babka
2019-01-18 14:10     ` Mel Gorman
2019-01-04 12:50 ` [PATCH 24/25] mm, compaction: Capture a page under direct compaction Mel Gorman
2019-01-18 13:40   ` Vlastimil Babka
2019-01-18 14:39     ` Mel Gorman
2019-01-04 12:50 ` [PATCH 25/25] mm, compaction: Do not direct compact remote memory Mel Gorman
2019-01-18 13:51   ` Vlastimil Babka
2019-01-18 14:46     ` Mel Gorman
2019-01-07 23:43 ` [PATCH 00/25] Increase success rates and reduce latency of compaction v2 Andrew Morton
2019-01-08  9:12   ` Mel Gorman
2019-01-09 11:13 ` [PATCH] mm, compaction: Use free lists to quickly locate a migration target -fix Mel Gorman
2019-01-09 19:27   ` Andrew Morton
2019-01-09 21:26     ` Mel Gorman
2019-01-09 11:15 ` [PATCH] mm, compaction: Finish pageblock scanning on contention -fix Mel Gorman
2019-01-09 11:16 ` [PATCH] mm, compaction: Round-robin the order while searching the free lists for a target -fix Mel Gorman

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