Re: [PATCH] mm: make fault_around_bytes configurable

From: Kirill A. Shutemov
Date: Fri Apr 22 2016 - 10:17:24 EST

On Fri, Apr 22, 2016 at 05:11:41PM +0300, Kirill A. Shutemov wrote:
> 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.

Ah. Okay, _PAGE_ACCESSED included into pgprot mask, which is reasonable to
have if you handle page fault for the address. But it should be adjusted
for faultaround.

--
Kirill A. Shutemov