Re: [PATCH] mm: make fault_around_bytes configurable

[Minchan Kim]

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?