[Cc linux-api]
On Mon 20-05-19 12:52:47, Minchan Kim wrote:
> - Background
>
> The Android terminology used for forking a new process and starting an app
> from scratch is a cold start, while resuming an existing app is a hot start.
> While we continually try to improve the performance of cold starts, hot
> starts will always be significantly less power hungry as well as faster so
> we are trying to make hot start more likely than cold start.
>
> To increase hot start, Android userspace manages the order that apps should
> be killed in a process called ActivityManagerService. ActivityManagerService
> tracks every Android app or service that the user could be interacting with
> at any time and translates that into a ranked list for lmkd(low memory
> killer daemon). They are likely to be killed by lmkd if the system has to
> reclaim memory. In that sense they are similar to entries in any other cache.
> Those apps are kept alive for opportunistic performance improvements but
> those performance improvements will vary based on the memory requirements of
> individual workloads.
>
> - Problem
>
> Naturally, cached apps were dominant consumers of memory on the system.
> However, they were not significant consumers of swap even though they are
> good candidate for swap. Under investigation, swapping out only begins
> once the low zone watermark is hit and kswapd wakes up, but the overall
> allocation rate in the system might trip lmkd thresholds and cause a cached
> process to be killed(we measured performance swapping out vs. zapping the
> memory by killing a process. Unsurprisingly, zapping is 10x times faster
> even though we use zram which is much faster than real storage) so kill
> from lmkd will often satisfy the high zone watermark, resulting in very
> few pages actually being moved to swap.
>
> - Approach
>
> The approach we chose was to use a new interface to allow userspace to
> proactively reclaim entire processes by leveraging platform information.
> This allowed us to bypass the inaccuracy of the kernel’s LRUs for pages
> that are known to be cold from userspace and to avoid races with lmkd
> by reclaiming apps as soon as they entered the cached state. Additionally,
> it could provide many chances for platform to use much information to
> optimize memory efficiency.
>
> IMHO we should spell it out that this patchset complements MADV_WONTNEED
> and MADV_FREE by adding non-destructive ways to gain some free memory
> space. MADV_COLD is similar to MADV_WONTNEED in a way that it hints the
> kernel that memory region is not currently needed and should be reclaimed
> immediately; MADV_COOL is similar to MADV_FREE in a way that it hints the
> kernel that memory region is not currently needed and should be reclaimed
> when memory pressure rises.
>
> To achieve the goal, the patchset introduce two new options for madvise.
> One is MADV_COOL which will deactive activated pages and the other is
> MADV_COLD which will reclaim private pages instantly. These new options
> complement MADV_DONTNEED and MADV_FREE by adding non-destructive ways to
> gain some free memory space. MADV_COLD is similar to MADV_DONTNEED in a way
> that it hints the kernel that memory region is not currently needed and
> should be reclaimed immediately; MADV_COOL is similar to MADV_FREE in a way
> that it hints the kernel that memory region is not currently needed and
> should be reclaimed when memory pressure rises.
>
> This approach is similar in spirit to madvise(MADV_WONTNEED), but the
> information required to make the reclaim decision is not known to the app.
> Instead, it is known to a centralized userspace daemon, and that daemon
> must be able to initiate reclaim on its own without any app involvement.
> To solve the concern, this patch introduces new syscall -
>
> struct pr_madvise_param {
> int size;
> const struct iovec *vec;
> }
>
> int process_madvise(int pidfd, ssize_t nr_elem, int *behavior,
> struct pr_madvise_param *restuls,
> struct pr_madvise_param *ranges,
> unsigned long flags);
>
> The syscall get pidfd to give hints to external process and provides
> pair of result/ranges vector arguments so that it could give several
> hints to each address range all at once.
>
> I guess others have different ideas about the naming of syscall and options
> so feel free to suggest better naming.
>
> - Experiment
>
> We did bunch of testing with several hundreds of real users, not artificial
> benchmark on android. We saw about 17% cold start decreasement without any
> significant battery/app startup latency issues. And with artificial benchmark
> which launches and switching apps, we saw average 7% app launching improvement,
> 18% less lmkd kill and good stat from vmstat.
>
> A is vanilla and B is process_madvise.
>
>
> A B delta ratio(%)
> allocstall_dma 0 0 0 0.00
> allocstall_movable 1464 457 -1007 -69.00
> allocstall_normal 263210 190763 -72447 -28.00
> allocstall_total 264674 191220 -73454 -28.00
> compact_daemon_wake 26912 25294 -1618 -7.00
> compact_fail 17885 14151 -3734 -21.00
> compact_free_scanned 4204766409 3835994922 -368771487 -9.00
> compact_isolated 3446484 2967618 -478866 -14.00
> compact_migrate_scanned 1621336411 1324695710 -296640701 -19.00
> compact_stall 19387 15343 -4044 -21.00
> compact_success 1502 1192 -310 -21.00
> kswapd_high_wmark_hit_quickly 234 184 -50 -22.00
> kswapd_inodesteal 221635 233093 11458 5.00
> kswapd_low_wmark_hit_quickly 66065 54009 -12056 -19.00
> nr_dirtied 259934 296476 36542 14.00
> nr_vmscan_immediate_reclaim 2587 2356 -231 -9.00
> nr_vmscan_write 1274232 2661733 1387501 108.00
> nr_written 1514060 2937560 1423500 94.00
> pageoutrun 67561 55133 -12428 -19.00
> pgactivate 2335060 1984882 -350178 -15.00
> pgalloc_dma 13743011 14096463 353452 2.00
> pgalloc_movable 0 0 0 0.00
> pgalloc_normal 18742440 16802065 -1940375 -11.00
> pgalloc_total 32485451 30898528 -1586923 -5.00
> pgdeactivate 4262210 2930670 -1331540 -32.00
> pgfault 30812334 31085065 272731 0.00
> pgfree 33553970 31765164 -1788806 -6.00
> pginodesteal 33411 15084 -18327 -55.00
> pglazyfreed 0 0 0 0.00
> pgmajfault 551312 1508299 956987 173.00
> pgmigrate_fail 43927 29330 -14597 -34.00
> pgmigrate_success 1399851 1203922 -195929 -14.00
> pgpgin 24141776 19032156 -5109620 -22.00
> pgpgout 959344 1103316 143972 15.00
> pgpgoutclean 4639732 3765868 -873864 -19.00
> pgrefill 4884560 3006938 -1877622 -39.00
> pgrotated 37828 25897 -11931 -32.00
> pgscan_direct 1456037 957567 -498470 -35.00
> pgscan_direct_throttle 0 0 0 0.00
> pgscan_kswapd 6667767 5047360 -1620407 -25.00
> pgscan_total 8123804 6004927 -2118877 -27.00
> pgskip_dma 0 0 0 0.00
> pgskip_movable 0 0 0 0.00
> pgskip_normal 14907 25382 10475 70.00
> pgskip_total 14907 25382 10475 70.00
> pgsteal_direct 1118986 690215 -428771 -39.00
> pgsteal_kswapd 4750223 3657107 -1093116 -24.00
> pgsteal_total 5869209 4347322 -1521887 -26.00
> pswpin 417613 1392647 975034 233.00
> pswpout 1274224 2661731 1387507 108.00
> slabs_scanned 13686905 10807200 -2879705 -22.00
> workingset_activate 668966 569444 -99522 -15.00
> workingset_nodereclaim 38957 32621 -6336 -17.00
> workingset_refault 2816795 2179782 -637013 -23.00
> workingset_restore 294320 168601 -125719 -43.00
>
> pgmajfault is increased by 173% because swapin is increased by 200% by
> process_madvise hint. However, swap read based on zram is much cheaper
> than file IO in performance point of view and app hot start by swapin is
> also cheaper than cold start from the beginning of app which needs many IO
> from storage and initialization steps.
>
> This patchset is against on next-20190517.
>
> Minchan Kim (7):
> mm: introduce MADV_COOL
> mm: change PAGEREF_RECLAIM_CLEAN with PAGE_REFRECLAIM
> mm: introduce MADV_COLD
> mm: factor out madvise's core functionality
> mm: introduce external memory hinting API
> mm: extend process_madvise syscall to support vector arrary
> mm: madvise support MADV_ANONYMOUS_FILTER and MADV_FILE_FILTER
>
> arch/x86/entry/syscalls/syscall_32.tbl | 1 +
> arch/x86/entry/syscalls/syscall_64.tbl | 1 +
> include/linux/page-flags.h | 1 +
> include/linux/page_idle.h | 15 +
> include/linux/proc_fs.h | 1 +
> include/linux/swap.h | 2 +
> include/linux/syscalls.h | 2 +
> include/uapi/asm-generic/mman-common.h | 12 +
> include/uapi/asm-generic/unistd.h | 2 +
> kernel/signal.c | 2 +-
> kernel/sys_ni.c | 1 +
> mm/madvise.c | 600 +++++++++++++++++++++----
> mm/swap.c | 43 ++
> mm/vmscan.c | 80 +++-
> 14 files changed, 680 insertions(+), 83 deletions(-)
>
> --
> 2.21.0.1020.gf2820cf01a-goog
>
--
Michal Hocko
SUSE Labs
