From: Vlastimil Babka <vbabka@suse.cz>
To: Xishi Qiu <qiuxishi@huawei.com>
Cc: linux-mm@kvack.org, Johannes Weiner <hannes@cmpxchg.org>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
David Rientjes <rientjes@google.com>,
Mel Gorman <mgorman@techsingularity.net>,
linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH v2 04/10] mm, page_alloc: count movable pages when stealing from pageblock
Date: Fri, 17 Feb 2017 17:21:19 +0100 [thread overview]
Message-ID: <d1427480-12ee-d848-5ac4-f76a5090e75b@suse.cz> (raw)
In-Reply-To: <58A441F8.9060909@huawei.com>
On 02/15/2017 12:56 PM, Xishi Qiu wrote:
> On 2017/2/15 18:47, Vlastimil Babka wrote:
>
>> On 02/14/2017 11:07 AM, Xishi Qiu wrote:
>>> On 2017/2/11 1:23, Vlastimil Babka wrote:
>>>
>>>> When stealing pages from pageblock of a different migratetype, we count how
>>>> many free pages were stolen, and change the pageblock's migratetype if more
>>>> than half of the pageblock was free. This might be too conservative, as there
>>>> might be other pages that are not free, but were allocated with the same
>>>> migratetype as our allocation requested.
>>>>
>>>> While we cannot determine the migratetype of allocated pages precisely (at
>>>> least without the page_owner functionality enabled), we can count pages that
>>>> compaction would try to isolate for migration - those are either on LRU or
>>>> __PageMovable(). The rest can be assumed to be MIGRATE_RECLAIMABLE or
>>>> MIGRATE_UNMOVABLE, which we cannot easily distinguish. This counting can be
>>>> done as part of free page stealing with little additional overhead.
>>>>
>>>> The page stealing code is changed so that it considers free pages plus pages
>>>> of the "good" migratetype for the decision whether to change pageblock's
>>>> migratetype.
>>>>
>>>> The result should be more accurate migratetype of pageblocks wrt the actual
>>>> pages in the pageblocks, when stealing from semi-occupied pageblocks. This
>>>> should help the efficiency of page grouping by mobility.
>>>>
>>>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>>>
>>> Hi Vlastimil,
>>>
>>> How about these two changes?
>>>
>>> 1. If we steal some free pages, we will add these page at the head of start_migratetype
>>> list, it will cause more fixed, because these pages will be allocated more easily.
>>
>> What do you mean by "more fixed" here?
>>
>>> So how about use list_move_tail instead of list_move?
>>
>> Hmm, not sure if it can make any difference. We steal because the lists
>> are currently empty (at least for the order we want), so it shouldn't
>> matter if we add to head or tail.
>>
>
> Hi Vlastimil,
>
> Please see the following case, I am not sure if it is right.
>
> MIGRATE_MOVABLE
> order: 0 1 2 3 4 5 6 7 8 9 10
> free num: 1 1 1 1 1 1 1 1 1 1 0 // one page(e.g. page A) was allocated before
>
> MIGRATE_UNMOVABLE
> order: 0 1 2 3 4 5 6 7 8 9 10
> free num: x x x x 0 0 0 0 0 0 0 // we want order=4, so steal from MIGRATE_MOVABLE
>
> We alloc order=4 in MIGRATE_UNMOVABLE, then it will fallback to steal pages from
> MIGRATE_MOVABLE, and we will move free pages form MIGRATE_MOVABLE list to
> MIGRATE_UNMOVABLE list.
>
> List of order 4-9 in MIGRATE_UNMOVABLE is empty, so add head or tail is the same.
> But order 0-3 is not empty, so if we add to the head, we will allocate pages which
> stolen from MIGRATE_MOVABLE first later. So we will have less chance to make a large
> block(order=10) when the one page(page A) free again.
I see. But do we know that page A, and the order-4 page we just allocated, are
both going to be freed soon? It's not a clear win to me, so maybe you can try
implementing it and see if it makes any difference?
> Also we will split order=9 which from MIGRATE_MOVABLE to alloc order=4 in expand(),
Yes, for pageblock order == 9.
> so if we add to the head, we will allocate pages which split from order=9 first later.
> So we will have less chance to make a large block(order=9) when the order=4 page
> free again.
Again that assumes our order-4 allocation is temporary. Is there a significant
chance of this?
>>> __rmqueue_fallback
>>> steal_suitable_fallback
>>> move_freepages_block
>>> move_freepages
>>> list_move
>>>
>>> 2. When doing expand() - list_add(), usually the list is empty, but in the
>>> following case, the list is not empty, because we did move_freepages_block()
>>> before.
>>>
>>> __rmqueue_fallback
>>> steal_suitable_fallback
>>> move_freepages_block // move to the list of start_migratetype
>>> expand // split the largest order
>>> list_add // add to the list of start_migratetype
>>>
>>> So how about use list_add_tail instead of list_add? Then we can merge the large
>>> block again as soon as the page freed.
>>
>> Same here. The lists are not empty, but contain probably just the pages
>> from our stolen pageblock. It shouldn't matter how we order them within
>> the same block.
>>
>> So maybe it could make some difference for higher-order allocations, but
>> it's unclear to me. Making e.g. expand() more complex with a flag to
>> tell it the head vs tail add could mean extra overhead in allocator fast
>> path that would offset any gains.
>>
>>> Thanks,
>>> Xishi Qiu
>>>
>>
>>
>> .
>>
>
>
>
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next prev parent reply other threads:[~2017-02-17 16:21 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-02-10 17:23 [PATCH v2 00/10] try to reduce fragmenting fallbacks Vlastimil Babka
2017-02-10 17:23 ` [PATCH v2 01/10] mm, compaction: reorder fields in struct compact_control Vlastimil Babka
2017-02-13 10:49 ` Mel Gorman
2017-02-14 16:33 ` Johannes Weiner
2017-02-10 17:23 ` [PATCH v2 02/10] mm, compaction: remove redundant watermark check in compact_finished() Vlastimil Babka
2017-02-13 10:49 ` Mel Gorman
2017-02-14 16:34 ` Johannes Weiner
2017-02-10 17:23 ` [PATCH v2 03/10] mm, page_alloc: split smallest stolen page in fallback Vlastimil Babka
2017-02-13 10:51 ` Mel Gorman
2017-02-13 10:54 ` Vlastimil Babka
2017-02-14 16:59 ` Johannes Weiner
2017-02-10 17:23 ` [PATCH v2 04/10] mm, page_alloc: count movable pages when stealing from pageblock Vlastimil Babka
2017-02-13 10:53 ` Mel Gorman
2017-02-14 10:07 ` Xishi Qiu
2017-02-15 10:47 ` Vlastimil Babka
2017-02-15 11:56 ` Xishi Qiu
2017-02-17 16:21 ` Vlastimil Babka [this message]
2017-02-14 18:10 ` Johannes Weiner
2017-02-17 16:09 ` Vlastimil Babka
2017-02-10 17:23 ` [PATCH v2 05/10] mm, compaction: change migrate_async_suitable() to suitable_migration_source() Vlastimil Babka
2017-02-13 10:53 ` Mel Gorman
2017-02-14 18:12 ` Johannes Weiner
2017-02-10 17:23 ` [PATCH v2 06/10] mm, compaction: add migratetype to compact_control Vlastimil Babka
2017-02-13 10:53 ` Mel Gorman
2017-02-14 18:15 ` Johannes Weiner
2017-02-10 17:23 ` [PATCH v2 07/10] mm, compaction: restrict async compaction to pageblocks of same migratetype Vlastimil Babka
2017-02-13 10:56 ` Mel Gorman
2017-02-14 20:10 ` Johannes Weiner
2017-02-17 16:32 ` Vlastimil Babka
2017-02-17 17:39 ` Johannes Weiner
2017-02-10 17:23 ` [PATCH v2 08/10] mm, compaction: finish whole pageblock to reduce fragmentation Vlastimil Babka
2017-02-13 10:57 ` Mel Gorman
2017-02-16 11:44 ` Johannes Weiner
2017-02-10 17:23 ` [RFC v2 09/10] mm, page_alloc: disallow migratetype fallback in fastpath Vlastimil Babka
2017-02-10 17:23 ` [RFC v2 10/10] mm, page_alloc: introduce MIGRATE_MIXED migratetype Vlastimil Babka
2017-03-08 2:16 ` Yisheng Xie
2017-03-08 7:07 ` Vlastimil Babka
2017-03-13 2:16 ` Yisheng Xie
2017-02-13 11:07 ` [PATCH v2 00/10] try to reduce fragmenting fallbacks Mel Gorman
2017-02-15 14:29 ` Vlastimil Babka
2017-02-15 16:11 ` Vlastimil Babka
2017-02-15 20:11 ` Vlastimil Babka
2017-02-16 15:12 ` Vlastimil Babka
2017-02-17 15:24 ` Vlastimil Babka
2017-02-20 12:30 ` Vlastimil Babka
2017-02-23 16:01 ` Mel Gorman
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