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From: Vlastimil Babka <vbabka@suse.cz>
To: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Mel Gorman <mgorman@techsingularity.net>,
	David Rientjes <rientjes@google.com>,
	kernel-team@fb.com, kernel-team@lge.com
Subject: Re: [PATCH v3 7/8] mm, compaction: restrict async compaction to pageblocks of same migratetype
Date: Thu, 4 May 2017 08:12:56 +0200	[thread overview]
Message-ID: <45323114-97d7-f34a-8336-51efff26bc8b@suse.cz> (raw)
In-Reply-To: <20170407003851.GA17231@js1304-P5Q-DELUXE>

On 04/07/2017 02:38 AM, Joonsoo Kim wrote:
> On Wed, Mar 29, 2017 at 06:06:41PM +0200, Vlastimil Babka wrote:
>> On 03/16/2017 03:14 AM, Joonsoo Kim wrote:
>>> On Tue, Mar 07, 2017 at 02:15:44PM +0100, Vlastimil Babka wrote:
>>>> The migrate scanner in async compaction is currently limited to MIGRATE_MOVABLE
>>>> pageblocks. This is a heuristic intended to reduce latency, based on the
>>>> assumption that non-MOVABLE pageblocks are unlikely to contain movable pages.
>>>>
>>>> However, with the exception of THP's, most high-order allocations are not
>>>> movable. Should the async compaction succeed, this increases the chance that
>>>> the non-MOVABLE allocations will fallback to a MOVABLE pageblock, making the
>>>> long-term fragmentation worse.
>>>
>>> I agree with this idea but have some concerns on this change.
>>>
>>> *ASYNC* compaction is designed for reducing latency and this change
>>> doesn't fit it. If everything works fine, there is a few movable pages
>>> in non-MOVABLE pageblocks as you noted above. Moreover, there is quite
>>> less the number of non-MOVABLE pageblock than MOVABLE one so finding
>>> non-MOVABLE pageblock takes long time. These two factors will increase
>>> the latency of *ASYNC* compaction.
>>
>> Right. I lately started to doubt the whole idea of async compaction (for
>> non-movable allocations). Seems it's one of the compaction heuristics tuned
>> towards the THP usecase. But for non-movable allocations, we just can't have
>> both the low latency and long-term fragmentation avoidance. I see now even my
>> own skip_on_failure mode in isolate_migratepages_block() as a mistake for
>> non-movable allocations.
> 
> Why do you think that skip_on_failure mode is a mistake? I think that
> it would lead to reduce the latency and it fits the goal of async
> compaction.

Yes, but the downside is that compaction will create just the single
high-order page that is requested, while previously it would also
migrate away some more lower-order pages. When compacting for
MIGRATE_UNMOVABLE allocation, we then can't steal extra pages, so next
allocation might pollute a different pageblock. It's not a good tradeoff.

>>
>> Ideally I'd like to make async compaction redundant by kcompactd, and direct
>> compaction would mean a serious situation which should warrant sync compaction.
>> Meanwhile I see several options to modify this patch
>> - async compaction for non-movable allocations will stop doing the
>> skip_on_failure mode, and won't restrict the pageblock at all. patch 8/8 will
>> make sure that also this kind of compaction finishes the whole pageblock
>> - non-movable allocations will skip async compaction completely and go for sync
>> compaction immediately
> 
> IMO, concept of async compaction is also important for non-movable allocation.
> Non-movable allocation is essential for some workload and they hope
> the low latency.

The low latency should not be at the expense of making long-term
fragmentation worse.

> Thanks.
> 

WARNING: multiple messages have this Message-ID
From: Vlastimil Babka <vbabka@suse.cz>
To: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Mel Gorman <mgorman@techsingularity.net>,
	David Rientjes <rientjes@google.com>,
	kernel-team@fb.com, kernel-team@lge.com
Subject: Re: [PATCH v3 7/8] mm, compaction: restrict async compaction to pageblocks of same migratetype
Date: Thu, 4 May 2017 08:12:56 +0200	[thread overview]
Message-ID: <45323114-97d7-f34a-8336-51efff26bc8b@suse.cz> (raw)
In-Reply-To: <20170407003851.GA17231@js1304-P5Q-DELUXE>

On 04/07/2017 02:38 AM, Joonsoo Kim wrote:
> On Wed, Mar 29, 2017 at 06:06:41PM +0200, Vlastimil Babka wrote:
>> On 03/16/2017 03:14 AM, Joonsoo Kim wrote:
>>> On Tue, Mar 07, 2017 at 02:15:44PM +0100, Vlastimil Babka wrote:
>>>> The migrate scanner in async compaction is currently limited to MIGRATE_MOVABLE
>>>> pageblocks. This is a heuristic intended to reduce latency, based on the
>>>> assumption that non-MOVABLE pageblocks are unlikely to contain movable pages.
>>>>
>>>> However, with the exception of THP's, most high-order allocations are not
>>>> movable. Should the async compaction succeed, this increases the chance that
>>>> the non-MOVABLE allocations will fallback to a MOVABLE pageblock, making the
>>>> long-term fragmentation worse.
>>>
>>> I agree with this idea but have some concerns on this change.
>>>
>>> *ASYNC* compaction is designed for reducing latency and this change
>>> doesn't fit it. If everything works fine, there is a few movable pages
>>> in non-MOVABLE pageblocks as you noted above. Moreover, there is quite
>>> less the number of non-MOVABLE pageblock than MOVABLE one so finding
>>> non-MOVABLE pageblock takes long time. These two factors will increase
>>> the latency of *ASYNC* compaction.
>>
>> Right. I lately started to doubt the whole idea of async compaction (for
>> non-movable allocations). Seems it's one of the compaction heuristics tuned
>> towards the THP usecase. But for non-movable allocations, we just can't have
>> both the low latency and long-term fragmentation avoidance. I see now even my
>> own skip_on_failure mode in isolate_migratepages_block() as a mistake for
>> non-movable allocations.
> 
> Why do you think that skip_on_failure mode is a mistake? I think that
> it would lead to reduce the latency and it fits the goal of async
> compaction.

Yes, but the downside is that compaction will create just the single
high-order page that is requested, while previously it would also
migrate away some more lower-order pages. When compacting for
MIGRATE_UNMOVABLE allocation, we then can't steal extra pages, so next
allocation might pollute a different pageblock. It's not a good tradeoff.

>>
>> Ideally I'd like to make async compaction redundant by kcompactd, and direct
>> compaction would mean a serious situation which should warrant sync compaction.
>> Meanwhile I see several options to modify this patch
>> - async compaction for non-movable allocations will stop doing the
>> skip_on_failure mode, and won't restrict the pageblock at all. patch 8/8 will
>> make sure that also this kind of compaction finishes the whole pageblock
>> - non-movable allocations will skip async compaction completely and go for sync
>> compaction immediately
> 
> IMO, concept of async compaction is also important for non-movable allocation.
> Non-movable allocation is essential for some workload and they hope
> the low latency.

The low latency should not be at the expense of making long-term
fragmentation worse.

> Thanks.
> 

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  reply	other threads:[~2017-05-04  6:13 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
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 [this message]
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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