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SubjectRe: [RFC PATCH 0/2] avoid external fragmentation related to migration fallback
On 2016/1/30 5:03, Vlastimil Babka wrote:

> On 01/29/2016 08:23 PM, ChengYi He wrote:
>
> [...]
>
>> Below is the root cause of this external fragmentation which could be
>> observed in devices which have only one memory zone, such as some arm64
>> android devices:
>>
>> 1) In arm64, the first 4GB physical address space is of DMA zone. If the
>> size of physical memory is less than 4GB and the whole memory is in the
>> first 4GB address space, then the system will have only one DMA zone.
>> 2) By default, all pageblocks are Movable.
>> 3) Allocators such as slab, ion, graphics preferably allocate pages of
>> Unmvoable migration type. It might fallback to allocate Movable pages
>> and changes Movable pageblocks into Unmovable ones.
>> 4) Movable pagesblocks will become less and less due to above reason.
>> However, in android system, AnonPages request is always high. The
>> Movable pages will be easily exhausted.
>> 5) While Movable pages are exhausted, the Movable allocations will
>> frequently fallback to allocate the largest feasiable pages of the other
>> migration types. The order-2 and order-3 Unmovable pages will be split
>> into smaller ones easily.
>>
>> This symptom doesn't appear in arm32 android which usually has two
>> memory zones including Highmem and Normal. The slab, ion, and graphics
>> allocators allocate pages with flag GFP_KERNEL. Only Movable pageblocks
>> in Normal zone become less, and the Movable pages in Highmem zone are
>> still a lot. Thus, the Movable pages will not be easily exhausted, and
>> there will not be frequent fallbacks.
>
> Hm, this 1 zone vs 2 zones shouldn't make that much difference, unless
> a) you use zone reclaim mode, or b) you have an old kernel without fair
> zone allocation policy?
>

Hi Vlastimil,

I agree with you.

I think if we have a normal zone and a movable zone, then the effect of
compaction will be better.

e.g.
U: unmovable page
M: movable page
F: free page

one zone(DMA):
paddr: 0 max
ZONE_DMA: U M F U M F ... U M F U M F
after compact: U F F U F F ... U M M U M M

two zone(DMA and MOVABLE)
paddr: 0 max
ZONE_DMA: the same as above
ZONE_MOVABLE: M F M F M F ... M F M F M F
after compact: F F F F F F ... M M M M M M // we get large block than above


>> Since the root cause is that fallbacks might frequently split order-2
>> and order-3 pages of the other migration types. This patch tweaks
>> fallback mechanism to avoid splitting order-2 and order-3 pages. while
>> fallbacks happen, if the largest feasible pages are less than or queal to
>> COSTLY_ORDER, i.e. 3, then try to select the smallest feasible pages. The
>> reason why fallbacks prefer the largest feasiable pages is to increase
>> fallback efficiency since fallbacks are likely to happen again. By
>> stealing the largest feasible pages, it could reduce the oppourtunities
>> of antoher fallback. Besides, it could make consecutive allocations more
>> approximate to each other and make system less fragment. However, if the
>> largest feasible pages are less than or equal to order-3, fallbacks might
>> split it and make the upcoming order-3 page allocations fail.
>
> In theory I don't see immediately why preferring smaller pages for
> fallback should be a clear win. If it's Unmovable allocations stealing
> from Movable pageblocks, the allocations will spread over larger areas
> instead of being grouped together. Maybe, for Movable allocations
> stealing from Unmovable allocations, preferring smallest might make
> sense and be safe, as any extra fragmentation is fixable bycompaction.
> Maybe it was already tried (by Joonsoo?) at some point, I'm not sure
> right now.
>
>> My test is against arm64 android devices with kernel 3.10.49. I set the
>> same account and install the same applications in both deivces and use
>> them synchronously.
>
> 3.10 is wayyyyyy old. There were numerous patches to compaction and
> anti-fragmentation since then. IIRC the fallback decisions were quite
> suboptimal at that point. I'm not even sure how you could apply your
> patches to both recent kernel for posting them, and 3.10 for testing?
> Is it possible to test on 4.4?
>

I think it's hard to update the drivers on android smart phone.

Thanks,
Xishi Qiu

>>
>> Test result:
>> 1) Test without this patch:
>> Most free pages are order-0 Unmovable ones. allocstall and compact_stall
>> in /proc/vmstat are relatively high. And most occurances of allocstall
>> are due to order-2 and order-3 allocations.
>> 2) Test with this patch:
>> There are more order-2 and order-3 free pages. allocstall and
>> compact_stall in /proc/vmstat are relatively low. And most occurances of
>> allocstall are due to order-0 allocations.
>>
>> Log:
>> 1) Test without this patch:
>> ------ TIME (date) ------
>> Fri Jul 3 16:52:55 CST 2015
>> ------ UPTIME (uptime) ------
>> up time: 2 days, 12:06:52, idle time: 8 days, 14:48:55, sleep time: 16:43:56
>> ------ MEMORY INFO (/proc/meminfo) ------
>> MemTotal: 2792568 kB
>> MemFree: 194524 kB
>> Buffers: 3788 kB
>> Cached: 380872 kB
>> ------ PAGETYPEINFO (/proc/pagetypeinfo) ------
>> Free pages count per migrate type at order 0 1 2 3 4
>> Node 0, zone DMA, type Unmovable 43852 701 0 0 0
>> Node 0, zone DMA, type Reclaimable 3357 0 0 0 0
>> Node 0, zone DMA, type Movable 0 5 0 0 0
>> Node 0, zone DMA, type Reserve 0 1 5 0 0
>> Node 0, zone DMA, type CMA 2 0 0 0 0
>> Node 0, zone DMA, type Isolate 0 0 0 0 0
>> Number of blocks type Unmovable Reclaimable Movable Reserve CMA Isolate
>> Node 0, zone DMA 362 80 170 2 113 0
>> ------ VIRTUAL MEMORY STATS (/proc/vmstat) ------
>> pgsteal_kswapd_dma 31755040
>> pgsteal_direct_dma 34597394
>> pgscan_kswapd_dma 36427664
>> pgscan_direct_dma 39490711
>> kswapd_low_wmark_hit_quickly 201929
>> kswapd_high_wmark_hit_quickly 4858
>> allocstall 664269
>> allocstall_order_0 9738
>> allocstall_order_1 1787
>> allocstall_order_2 637608
>> allocstall_order_3 15136
>> pgmigrate_success 2941956
>> pgmigrate_fail 1033
>> compact_migrate_scanned 142985157
>> compact_free_scanned 4734040109
>> compact_isolated 7720362
>> compact_stall 65978
>> compact_fail 46084
>> compact_success 11717
>>
>> 2) Test with this patch:
>> ------ TIME (date) ------
>> Fri Jul 3 16:52:31 CST 2015
>> ------ UPTIME (uptime) ------
>> up time: 2 days, 12:06:30
>> ------ MEMORY INFO (/proc/meminfo) ------
>> MemTotal: 2792568 kB
>> MemFree: 47612 kB
>> Buffers: 3732 kB
>> Cached: 387048 kB
>> ------ PAGETYPEINFO (/proc/pagetypeinfo) ------
>> Free pages count per migrate type at order 0 1 2 3 4
>> Node 0, zone DMA, type Unmovable 272 243 126 1 0
>> Node 0, zone DMA, type Reclaimable 0 361 168 46 0
>> Node 0, zone DMA, type Movable 4103 1782 130 3 0
>> Node 0, zone DMA, type Reserve 0 0 0 0 0
>> Node 0, zone DMA, type CMA 563 2 0 0 0
>> Node 0, zone DMA, type Isolate 0 0 0 0 0
>> Number of blocks type Unmovable Reclaimable Movable Reserve CMA Isolate
>> Node 0, zone DMA 183 12 417 2 113 0
>> ------ VIRTUAL MEMORY STATS (/proc/vmstat) ------
>> pgsteal_kswapd_dma 50710868
>> pgsteal_direct_dma 1756780
>> pgscan_kswapd_dma 58281837
>> pgscan_direct_dma 2022049
>> kswapd_low_wmark_hit_quickly 37599
>> kswapd_high_wmark_hit_quickly 13564
>> allocstall 27510
>> allocstall_order_0 26101
>> allocstall_order_1 23
>> allocstall_order_2 1224
>> allocstall_order_3 162
>> pgmigrate_success 63751
>> pgmigrate_fail 7
>> compact_migrate_scanned 278170
>> compact_free_scanned 6155410
>> compact_isolated 140762
>> compact_stall 749
>> compact_fail 54
>> compact_success 22
>> unevictable_pgs_culled 794
>>
>> Below is the status of another device with this patch.
>> /proc/pagetypeinfo shows that even if there are no Movable pages, there
>> are lots of order-2 and order-3 Unmovable pages. For this case, if the
>> patch is not applied, then order-2 and order-3 Unmovable pages will be
>> split easily. It's likely that system perforamnce will become low due to
>> severe external fragmentation.
>>
>> ------ UPTIME (uptime) ------
>> up time: 33 days, 08:10:58
>> ------ MEMORY INFO (/proc/meminfo) ------
>> MemTotal: 2792568 kB
>> MemFree: 37340 kB
>> Buffers: 13412 kB
>> Cached: 655456 kB
>> ------ PAGETYPEINFO (/proc/pagetypeinfo) ------
>> Free pages count per migrate type at order 0 1 2 3 4
>> Node 0, zone DMA, type Unmovable 718 628 1116 301 0
>> Node 0, zone DMA, type Reclaimable 198 93 0 0 0
>> Node 0, zone DMA, type Movable 0 0 0 0 0
>> Node 0, zone DMA, type Reserve 0 0 0 0 0
>> Node 0, zone DMA, type CMA 89 11 3 0 0
>> Node 0, zone DMA, type Isolate 0 0 0 0 0
>> Number of blocks type Unmovable Reclaimable Movable Reserve CMA Isolate
>> Node 0, zone DMA 377 115 120 2 113 0
>> ------ VIRTUAL MEMORY STATS (/proc/vmstat) ------
>> pgsteal_direct_dma 28575192
>> pgsteal_kswapd_dma 378357910
>> pgscan_kswapd_dma 422765699
>> pgscan_direct_dma 31860747
>> kswapd_low_wmark_hit_quickly 947979
>> kswapd_high_wmark_hit_quickly 139901
>> allocstall 592989
>> compact_migrate_scanned 149884903
>> compact_free_scanned 6629299888
>> compact_isolated 7699012
>> compact_stall 52550
>> compact_fail 45155
>> compact_success 6057
>>
>> ChengYi He (2):
>> mm/page_alloc: let migration fallback support pages of requested order
>> mm/page_alloc: avoid splitting pages of order 2 and 3 in migration
>> fallback
>>
>> mm/page_alloc.c | 92 ++++++++++++++++++++++++++++++++++++---------------------
>> 1 file changed, 59 insertions(+), 33 deletions(-)
>>
>
>
> .
>



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