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From: Vlastimil Babka <vbabka@suse.cz>
To: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Johannes Weiner <hannes@cmpxchg.org>,
	Mel Gorman <mgorman@techsingularity.net>,
	David Rientjes <rientjes@google.com>,
	kernel-team@fb.com, kernel-team@lge.com
Subject: Re: [PATCH v3 2/8] mm, compaction: remove redundant watermark check in compact_finished()
Date: Wed, 29 Mar 2017 17:30:58 +0200	[thread overview]
Message-ID: <6af76744-260d-fc39-b6e0-fb47d7d6348b@suse.cz> (raw)
In-Reply-To: <20170316013018.GA14063@js1304-P5Q-DELUXE>

On 03/16/2017 02:30 AM, Joonsoo Kim wrote:
> Hello,

Hi, sorry for the late replies.

> On Tue, Mar 07, 2017 at 02:15:39PM +0100, Vlastimil Babka wrote:
>> When detecting whether compaction has succeeded in forming a high-order page,
>> __compact_finished() employs a watermark check, followed by an own search for
>> a suitable page in the freelists. This is not ideal for two reasons:
>> 
>> - The watermark check also searches high-order freelists, but has a less strict
>>   criteria wrt fallback. It's therefore redundant and waste of cycles. This was
>>   different in the past when high-order watermark check attempted to apply
>>   reserves to high-order pages.
> 
> Although it looks redundant now, I don't like removal of the watermark
> check here. Criteria in watermark check would be changed to more strict
> later and we would easily miss to apply it on compaction side if the
> watermark check is removed.

I see, but compaction is already full of various watermark(-like) checks that
have to be considered/updated if watermark checking changes significantly, or
things will go subtly wrong. I doubt this extra check can really help much in
such cases.

>> 
>> - The watermark check might actually fail due to lack of order-0 pages.
>>   Compaction can't help with that, so there's no point in continuing because of
>>   that. It's possible that high-order page still exists and it terminates.
> 
> If lack of order-0 pages is the reason for stopping compaction, we
> need to insert the watermark check for order-0 to break the compaction
> instead of removing it. Am I missing something?

You proposed that once IIRC, but didn't follow up? Currently we learn about
insufficient order-0 watermark in __isolate_free_page() from the free scanner.
We could potentially stop compacting earlier by checking it also in
compact_finished(), but maybe it doesn't happen that often and it's just extra
checking overhead.

So I wouldn't be terribly opposed by converting the current check to an order-0
fail-compaction check (instead of removing it), but I really wouldn't like to
insert the order-0 one and also keep the current one.

> Thanks.
> 

WARNING: multiple messages have this Message-ID
From: Vlastimil Babka <vbabka@suse.cz>
To: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Johannes Weiner <hannes@cmpxchg.org>,
	Mel Gorman <mgorman@techsingularity.net>,
	David Rientjes <rientjes@google.com>,
	kernel-team@fb.com, kernel-team@lge.com
Subject: Re: [PATCH v3 2/8] mm, compaction: remove redundant watermark check in compact_finished()
Date: Wed, 29 Mar 2017 17:30:58 +0200	[thread overview]
Message-ID: <6af76744-260d-fc39-b6e0-fb47d7d6348b@suse.cz> (raw)
In-Reply-To: <20170316013018.GA14063@js1304-P5Q-DELUXE>

On 03/16/2017 02:30 AM, Joonsoo Kim wrote:
> Hello,

Hi, sorry for the late replies.

> On Tue, Mar 07, 2017 at 02:15:39PM +0100, Vlastimil Babka wrote:
>> When detecting whether compaction has succeeded in forming a high-order page,
>> __compact_finished() employs a watermark check, followed by an own search for
>> a suitable page in the freelists. This is not ideal for two reasons:
>> 
>> - The watermark check also searches high-order freelists, but has a less strict
>>   criteria wrt fallback. It's therefore redundant and waste of cycles. This was
>>   different in the past when high-order watermark check attempted to apply
>>   reserves to high-order pages.
> 
> Although it looks redundant now, I don't like removal of the watermark
> check here. Criteria in watermark check would be changed to more strict
> later and we would easily miss to apply it on compaction side if the
> watermark check is removed.

I see, but compaction is already full of various watermark(-like) checks that
have to be considered/updated if watermark checking changes significantly, or
things will go subtly wrong. I doubt this extra check can really help much in
such cases.

>> 
>> - The watermark check might actually fail due to lack of order-0 pages.
>>   Compaction can't help with that, so there's no point in continuing because of
>>   that. It's possible that high-order page still exists and it terminates.
> 
> If lack of order-0 pages is the reason for stopping compaction, we
> need to insert the watermark check for order-0 to break the compaction
> instead of removing it. Am I missing something?

You proposed that once IIRC, but didn't follow up? Currently we learn about
insufficient order-0 watermark in __isolate_free_page() from the free scanner.
We could potentially stop compacting earlier by checking it also in
compact_finished(), but maybe it doesn't happen that often and it's just extra
checking overhead.

So I wouldn't be terribly opposed by converting the current check to an order-0
fail-compaction check (instead of removing it), but I really wouldn't like to
insert the order-0 one and also keep the current one.

> Thanks.
> 

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  reply	other threads:[~2017-03-29 15:31 UTC|newest]

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-03-07 13:15 [PATCH v3 0/8] try to reduce fragmenting fallbacks Vlastimil Babka
2017-03-07 13:15 ` Vlastimil Babka
2017-03-07 13:15 ` [PATCH v3 1/8] mm, compaction: reorder fields in struct compact_control Vlastimil Babka
2017-03-07 13:15   ` Vlastimil Babka
2017-03-07 13:15 ` [PATCH v3 2/8] mm, compaction: remove redundant watermark check in compact_finished() Vlastimil Babka
2017-03-07 13:15   ` Vlastimil Babka
2017-03-16  1:30   ` Joonsoo Kim
2017-03-16  1:30     ` Joonsoo Kim
2017-03-29 15:30     ` Vlastimil Babka [this message]
2017-03-29 15:30       ` Vlastimil Babka
2017-03-07 13:15 ` [PATCH v3 3/8] mm, page_alloc: split smallest stolen page in fallback Vlastimil Babka
2017-03-07 13:15   ` Vlastimil Babka
2017-03-07 13:15 ` [PATCH v3 4/8] mm, page_alloc: count movable pages when stealing from pageblock Vlastimil Babka
2017-03-07 13:15   ` Vlastimil Babka
2017-03-16  1:53   ` Joonsoo Kim
2017-03-16  1:53     ` Joonsoo Kim
2017-03-29 15:49     ` Vlastimil Babka
2017-03-29 15:49       ` Vlastimil Babka
2017-03-07 13:15 ` [PATCH v3 5/8] mm, compaction: change migrate_async_suitable() to suitable_migration_source() Vlastimil Babka
2017-03-07 13:15   ` Vlastimil Babka
2017-03-07 13:15 ` [PATCH v3 6/8] mm, compaction: add migratetype to compact_control Vlastimil Babka
2017-03-07 13:15   ` Vlastimil Babka
2017-03-07 13:15 ` [PATCH v3 7/8] mm, compaction: restrict async compaction to pageblocks of same migratetype Vlastimil Babka
2017-03-07 13:15   ` Vlastimil Babka
2017-03-16  2:14   ` Joonsoo Kim
2017-03-16  2:14     ` Joonsoo Kim
2017-03-29 16:06     ` Vlastimil Babka
2017-03-29 16:06       ` Vlastimil Babka
2017-04-07  0:38       ` Joonsoo Kim
2017-04-07  0:38         ` Joonsoo Kim
2017-05-04  6:12         ` Vlastimil Babka
2017-05-04  6:12           ` Vlastimil Babka
2017-03-07 13:15 ` [PATCH v3 8/8] mm, compaction: finish whole pageblock to reduce fragmentation Vlastimil Babka
2017-03-07 13:15   ` Vlastimil Babka
2017-03-16  2:18   ` Joonsoo Kim
2017-03-16  2:18     ` Joonsoo Kim
2017-03-29 16:13     ` Vlastimil Babka
2017-03-29 16:13       ` Vlastimil Babka
2017-03-08 16:46 ` [PATCH v3 0/8] try to reduce fragmenting fallbacks Johannes Weiner
2017-03-08 16:46   ` Johannes Weiner
2017-03-08 19:17   ` Vlastimil Babka
2017-03-08 19:17     ` Vlastimil Babka
2017-03-16 18:34     ` Johannes Weiner
2017-03-16 18:34       ` Johannes Weiner
2017-03-17 18:29       ` Vlastimil Babka
2017-03-17 18:29         ` Vlastimil Babka
2017-03-19 21:23         ` Johannes Weiner
2017-03-19 21:23           ` Johannes Weiner

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