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.
Other thing it can make the difference about reclaiming is that it can
make more pressure slab shrinking.
unsigned long shrink_page_list()
{
..
/* Double the slab pressure for mapped and swapcache pages */
if (page_mapped(page) || PageSwapCache(page))
sc->nr_scanned++;
..
}
But I'm not sure it can make such difference.
Could you explain why I am missing?
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