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Date: Mon, 25 Apr 2016 17:21:11 +0530
From: Vinayak Menon <vinmenon@...eaurora.org>
To: "Kirill A. Shutemov" <kirill@...temov.name>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, dan.j.williams@...el.com,
mgorman@...e.de, vbabka@...e.cz, kirill.shutemov@...ux.intel.com,
dave.hansen@...ux.intel.com, hughd@...gle.com
Subject: Re: [PATCH] mm: make fault_around_bytes configurable
On 4/22/2016 3:14 PM, Kirill A. Shutemov wrote:
> On Fri, Apr 22, 2016 at 02:15:08PM +0530, Vinayak Menon wrote:
>> On 04/22/2016 05:31 AM, Andrew Morton wrote:
>>> On Mon, 18 Apr 2016 20:47:16 +0530 Vinayak Menon <vinmenon@...eaurora.org> wrote:
>>>
>>>> Mapping pages around fault is found to cause performance degradation
>>>> in certain use cases. The test performed here is launch of 10 apps
>>>> one by one, doing something with the app each time, and then repeating
>>>> the same sequence once more, on an ARM 64-bit Android device with 2GB
>>>> of RAM. The time taken to launch the apps is found to be better when
>>>> fault around feature is disabled by setting fault_around_bytes to page
>>>> size (4096 in this case).
>>> Well that's one workload, and a somewhat strange one. What is the
>>> effect on other workloads (of which there are a lot!).
>>>
>> This workload emulates the way a user would use his mobile device, opening
>> an application, using it for some time, switching to next, and then coming
>> back to the same application later. Another stat which shows significant
>> degradation on Android with fault_around is device boot up time. I have not
>> tried any other workload other than these.
>>
>>>> The tests were done on 3.18 kernel. 4 extra vmstat counters were added
>>>> for debugging. pgpgoutclean accounts the clean pages reclaimed via
>>>> __delete_from_page_cache. pageref_activate, pageref_activate_vm_exec,
>>>> and pageref_keep accounts the mapped file pages activated and retained
>>>> by page_check_references.
>>>>
>>>> === Without swap ===
>>>> 3.18 3.18-fault_around_bytes=4096
>>>> -----------------------------------------------------------------------
>>>> workingset_refault 691100 664339
>>>> workingset_activate 210379 179139
>>>> pgpgin 4676096 4492780
>>>> pgpgout 163967 96711
>>>> pgpgoutclean 1090664 990659
>>>> pgalloc_dma 3463111 3328299
>>>> pgfree 3502365 3363866
>>>> pgactivate 568134 238570
>>>> pgdeactivate 752260 392138
>>>> pageref_activate 315078 121705
>>>> pageref_activate_vm_exec 162940 55815
>>>> pageref_keep 141354 51011
>>>> pgmajfault 24863 23633
>>>> pgrefill_dma 1116370 544042
>>>> pgscan_kswapd_dma 1735186 1234622
>>>> pgsteal_kswapd_dma 1121769 1005725
>>>> pgscan_direct_dma 12966 1090
>>>> pgsteal_direct_dma 6209 967
>>>> slabs_scanned 1539849 977351
>>>> pageoutrun 1260 1333
>>>> allocstall 47 7
>>>>
>>>> === With swap ===
>>>> 3.18 3.18-fault_around_bytes=4096
>>>> -----------------------------------------------------------------------
>>>> workingset_refault 597687 878109
>>>> workingset_activate 167169 254037
>>>> pgpgin 4035424 5157348
>>>> pgpgout 162151 85231
>>>> pgpgoutclean 928587 1225029
>>>> pswpin 46033 17100
>>>> pswpout 237952 127686
>>>> pgalloc_dma 3305034 3542614
>>>> pgfree 3354989 3592132
>>>> pgactivate 626468 355275
>>>> pgdeactivate 990205 771902
>>>> pageref_activate 294780 157106
>>>> pageref_activate_vm_exec 141722 63469
>>>> pageref_keep 121931 63028
>>>> pgmajfault 67818 45643
>>>> pgrefill_dma 1324023 977192
>>>> pgscan_kswapd_dma 1825267 1720322
>>>> pgsteal_kswapd_dma 1181882 1365500
>>>> pgscan_direct_dma 41957 9622
>>>> pgsteal_direct_dma 25136 6759
>>>> slabs_scanned 689575 542705
>>>> pageoutrun 1234 1538
>>>> allocstall 110 26
>>>>
>>>> Looks like with fault_around, there is more pressure on reclaim because
>>>> of the presence of more mapped pages, resulting in more IO activity,
>>>> more faults, more swapping, and allocstalls.
>>> A few of those things did get a bit worse?
>> I think some numbers (like workingset, pgpgin, pgpgoutclean etc) looks
>> better with fault_around because, increased number of mapped pages is
>> resulting in less number of file pages being reclaimed (pageref_activate,
>> pageref_activate_vm_exec, pageref_keep above), but increased swapping.
>> Latency numbers are far bad with fault_around_bytes + swap, possibly because
>> of increased swapping, decrease in kswapd efficiency and increase in
>> allocstalls.
>> So the problem looks to be that unwanted pages are mapped around the fault
>> and page_check_references is unaware of this.
> Hm. It makes me think we should make ptes setup by faultaround old.
>
> Although, it would defeat (to some extend) purpose of faultaround on
> architectures without HW accessed bit :-/
>
> Could you check if the patch below changes the situation?
> It would require some more work to not mark the pte we've got fault for old.
Column at the end shows the values with the patch
3.18 3.18-fab=4096 3.18-Kirill's-fix
---------------------------------------------------------
workingset_refault 597687 878109 790207
workingset_activate 167169 254037 207912
pgpgin 4035424 5157348 4793116
pgpgout 162151 85231 85539
pgpgoutclean 928587 1225029 1129088
pswpin 46033 17100 8926
pswpout 237952 127686 103435
pgalloc_dma 3305034 3542614 3401000
pgfree 3354989 3592132 3457783
pgactivate 626468 355275 326716
pgdeactivate 990205 771902 697392
pageref_activate 294780 157106 138451
pageref_activate_vm_exec 141722 63469 64585
pageref_keep 121931 63028 65811
pgmajfault 67818 45643 34944
pgrefill_dma 1324023 977192 874497
pgscan_kswapd_dma 1825267 1720322 1577483
pgsteal_kswapd_dma 1181882 1365500 1243968
pgscan_direct_dma 41957 9622 9387
pgsteal_direct_dma 25136 6759 7108
slabs_scanned 689575 542705 618839
pageoutrun 1234 1538 1450
allocstall 110 26 13
Everything seems to have improved except slabs_scanned, possibly because
of this check which Minchan pointed out, that results in higher pressure on slabs.
if (page_mapped(page) || PageSwapCache(page))
sc->nr_scanned++;
I had added some traces to monitor the vmpressure values. Those also seems to
be high, possibly because of the same reason.
Should the pressure be doubled only if page is mapped and referenced ?
There is big improvement in avg latency, but still 5% higher than with fault_around
disabled. I will try to debug this further.
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