Re: [PATCH] mm: make fault_around_bytes configurable

From: Vinayak Menon
Date: Fri Apr 22 2016 - 04:45:28 EST

On 04/22/2016 05:31 AM, Andrew Morton wrote:
On Mon, 18 Apr 2016 20:47:16 +0530 Vinayak Menon <vinmenon@xxxxxxxxxxxxxx> 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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