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SubjectRe: [RFC 0/7] introduce memory hinting API for external process
On Mon, May 20, 2019 at 04:42:00PM +0200, Oleksandr Natalenko wrote:
> Hi.
>
> On Mon, May 20, 2019 at 12:52:47PM +0900, 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
> >
>
> Please Cc me for the next iteration since I was working on the very same
> thing recently [1].
>
> Thank you.
>
> [1] https://gitlab.com/post-factum/pf-kernel/commits/remote-madvise-v3

Sure, I'm happy to see others have similar requirement.

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