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Message-ID: <5719E494.20302@codeaurora.org>
Date: Fri, 22 Apr 2016 14:15:08 +0530
From: Vinayak Menon <vinmenon@...eaurora.org>
To: Andrew Morton <akpm@...ux-foundation.org>
CC: 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 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.
>
> Do you have any data on actual wall-time changes? How much faster do
> things become with the patch? If it is "0.1%" then I'd say "umm, no".
>
=== Without swap ====
3.18 3.18-fault_around_bytes=4096
Avg launch latency 1695ms 1300ms (23.3%)
Max launch latency 5097ms 3135ms (38.49%)
>> Make fault_around_bytes configurable so that it can be tuned to avoid
>> performance degradation.
>
> It sounds like we need to be smarter about auto-tuning this thing.
> Maybe the refault code could be taught to provide the feedback path but
> that sounds hard.
>
> Still. I do think it would be better to make this configurable at
> runtime. Move the existing debugfs tunable into /proc/sys/vm (and
> document it!). I do dislkie adding even more tunables but this one
> does make sense. People will want to run their workloads with various
> values until they find the peak throughput, and requiring a kernel
> rebuild for that is a huge pain.
>
I can send a v2 to do this runtime via /proc/sys/vm.
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