On Fri, Apr 22, 2016 at 11:02:16PM +0900, Minchan Kim 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@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.
>
> The page_check_references makes difference only when pte has marked access_bit.
>
> enum page_references page_check_references(struct page *page)
> {
> referenced_ptes = page_referenced(page);
> if (referenced_ptes) {
> ...
> return PAGEREF_ACTIVATE
> }
> }
>
> But map_pages doesn't mark ahead pages as pte_mkyoung. IOW, ptes are already
> pte_mkold. So, I think page_check_reference shouldn't make any difference.
Actually, I've checked and mk_pte() produces young ptes for me. Not sure
why.
--
Kirill A. Shutemov
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