Re: [PATCH mm-unstable v1 1/4] mm/mglru: fix underprotected page cache

[Yu Zhao]

On Thu, Dec 14, 2023 at 11:38 AM Kairui Song <ryncsn@xxxxxxxxx> wrote:
>
> Yu Zhao <yuzhao@xxxxxxxxxx> 于2023年12月14日周四 11:09写道：
> > On Wed, Dec 13, 2023 at 12:59:14AM -0700, Yu Zhao wrote:
> > > On Tue, Dec 12, 2023 at 8:03 PM Kairui Song <ryncsn@xxxxxxxxx> wrote:
> > > >
> > > > Kairui Song <ryncsn@xxxxxxxxx> 于2023年12月12日周二 14:52写道：
> > > > >
> > > > > Yu Zhao <yuzhao@xxxxxxxxxx> 于2023年12月12日周二 06:07写道：
> > > > > >
> > > > > > On Fri, Dec 8, 2023 at 1:24 AM Kairui Song <ryncsn@xxxxxxxxx> wrote:
> > > > > > >
> > > > > > > Yu Zhao <yuzhao@xxxxxxxxxx> 于2023年12月8日周五 14:14写道：
> > > > > > > >
> > > > > > > > Unmapped folios accessed through file descriptors can be
> > > > > > > > underprotected. Those folios are added to the oldest generation based
> > > > > > > > on:
> > > > > > > > 1. The fact that they are less costly to reclaim (no need to walk the
> > > > > > > >    rmap and flush the TLB) and have less impact on performance (don't
> > > > > > > >    cause major PFs and can be non-blocking if needed again).
> > > > > > > > 2. The observation that they are likely to be single-use. E.g., for
> > > > > > > >    client use cases like Android, its apps parse configuration files
> > > > > > > >    and store the data in heap (anon); for server use cases like MySQL,
> > > > > > > >    it reads from InnoDB files and holds the cached data for tables in
> > > > > > > >    buffer pools (anon).
> > > > > > > >
> > > > > > > > However, the oldest generation can be very short lived, and if so, it
> > > > > > > > doesn't provide the PID controller with enough time to respond to a
> > > > > > > > surge of refaults. (Note that the PID controller uses weighted
> > > > > > > > refaults and those from evicted generations only take a half of the
> > > > > > > > whole weight.) In other words, for a short lived generation, the
> > > > > > > > moving average smooths out the spike quickly.
> > > > > > > >
> > > > > > > > To fix the problem:
> > > > > > > > 1. For folios that are already on LRU, if they can be beyond the
> > > > > > > >    tracking range of tiers, i.e., five accesses through file
> > > > > > > >    descriptors, move them to the second oldest generation to give them
> > > > > > > >    more time to age. (Note that tiers are used by the PID controller
> > > > > > > >    to statistically determine whether folios accessed multiple times
> > > > > > > >    through file descriptors are worth protecting.)
> > > > > > > > 2. When adding unmapped folios to LRU, adjust the placement of them so
> > > > > > > >    that they are not too close to the tail. The effect of this is
> > > > > > > >    similar to the above.
> > > > > > > >
> > > > > > > > On Android, launching 55 apps sequentially:
> > > > > > > >                            Before     After      Change
> > > > > > > >   workingset_refault_anon  25641024   25598972   0%
> > > > > > > >   workingset_refault_file  115016834  106178438  -8%
> > > > > > >
> > > > > > > Hi Yu,
> > > > > > >
> > > > > > > Thanks you for your amazing works on MGLRU.
> > > > > > >
> > > > > > > I believe this is the similar issue I was trying to resolve previously:
> > > > > > > https://lwn.net/Articles/945266/
> > > > > > > The idea is to use refault distance to decide if the page should be
> > > > > > > place in oldest generation or some other gen, which per my test,
> > > > > > > worked very well, and we have been using refault distance for MGLRU in
> > > > > > > multiple workloads.
> > > > > > >
> > > > > > > There are a few issues left in my previous RFC series, like anon pages
> > > > > > > in MGLRU shouldn't be considered, I wanted to collect feedback or test
> > > > > > > cases, but unfortunately it seems didn't get too much attention
> > > > > > > upstream.
> > > > > > >
> > > > > > > I think both this patch and my previous series are for solving the
> > > > > > > file pages underpertected issue, and I did a quick test using this
> > > > > > > series, for mongodb test, refault distance seems still a better
> > > > > > > solution (I'm not saying these two optimization are mutually exclusive
> > > > > > > though, just they do have some conflicts in implementation and solving
> > > > > > > similar problem):
> > > > > > >
> > > > > > > Previous result:
> > > > > > > ==================================================================
> > > > > > > Execution Results after 905 seconds
> > > > > > > ------------------------------------------------------------------
> > > > > > >                   Executed        Time (µs)       Rate
> > > > > > >   STOCK_LEVEL     2542            27121571486.2   0.09 txn/s
> > > > > > > ------------------------------------------------------------------
> > > > > > >   TOTAL           2542            27121571486.2   0.09 txn/s
> > > > > > >
> > > > > > > This patch:
> > > > > > > ==================================================================
> > > > > > > Execution Results after 900 seconds
> > > > > > > ------------------------------------------------------------------
> > > > > > >                   Executed        Time (µs)       Rate
> > > > > > >   STOCK_LEVEL     1594            27061522574.4   0.06 txn/s
> > > > > > > ------------------------------------------------------------------
> > > > > > >   TOTAL           1594            27061522574.4   0.06 txn/s
> > > > > > >
> > > > > > > Unpatched version is always around ~500.
> > > > > >
> > > > > > Thanks for the test results!
> > > > > >
> > > > > > > I think there are a few points here:
> > > > > > > - Refault distance make use of page shadow so it can better
> > > > > > > distinguish evicted pages of different access pattern (re-access
> > > > > > > distance).
> > > > > > > - Throttled refault distance can help hold part of workingset when
> > > > > > > memory is too small to hold the whole workingset.
> > > > > > >
> > > > > > > So maybe part of this patch and the bits of previous series can be
> > > > > > > combined to work better on this issue, how do you think?
> > > > > >
> > > > > > I'll try to find some time this week to look at your RFC. It'd be a
> > > >
> > > > Hi Yu,
> > > >
> > > > I'm working on V4 of the RFC now, which just update some comments, and
> > > > skip anon page re-activation in refault path for mglru which was not
> > > > very helpful, only some tiny adjustment.
> > > > And I found it easier to test with fio, using following test script:
> > > >
> > > > #!/bin/bash
> > > > swapoff -a
> > > >
> > > > modprobe brd rd_nr=1 rd_size=16777216
> > > > mkfs.ext4 /dev/ram0
> > > > mount /dev/ram0 /mnt
> > > >
> > > > mkdir -p /sys/fs/cgroup/benchmark
> > > > cd /sys/fs/cgroup/benchmark
> > > >
> > > > echo 4G > memory.max
> > > > echo $$ > cgroup.procs
> > > > echo 3 > /proc/sys/vm/drop_caches
> > > >
> > > > fio -name=mglru --numjobs=12 --directory=/mnt --size=1024m \
> > > >           --buffered=1 --ioengine=io_uring --iodepth=128 \
> > > >           --iodepth_batch_submit=32 --iodepth_batch_complete=32 \
> > > >           --rw=randread --random_distribution=zipf:0.5 --norandommap \
> > > >           --time_based --ramp_time=5m --runtime=5m --group_reporting
> > > >
> > > > zipf:0.5 is used here to simulate a cached read with slight bias
> > > > towards certain pages.
> > > > Unpatched 6.7-rc4:
> > > > Run status group 0 (all jobs):
> > > >    READ: bw=6548MiB/s (6866MB/s), 6548MiB/s-6548MiB/s
> > > > (6866MB/s-6866MB/s), io=1918GiB (2060GB), run=300001-300001msec
> > > >
> > > > Patched with RFC v4:
> > > > Run status group 0 (all jobs):
> > > >    READ: bw=7270MiB/s (7623MB/s), 7270MiB/s-7270MiB/s
> > > > (7623MB/s-7623MB/s), io=2130GiB (2287GB), run=300001-300001msec
> > > >
> > > > Patched with this series:
> > > > Run status group 0 (all jobs):
> > > >    READ: bw=7098MiB/s (7442MB/s), 7098MiB/s-7098MiB/s
> > > > (7442MB/s-7442MB/s), io=2079GiB (2233GB), run=300002-300002msec
> > > >
> > > > MGLRU off:
> > > > Run status group 0 (all jobs):
> > > >    READ: bw=6525MiB/s (6842MB/s), 6525MiB/s-6525MiB/s
> > > > (6842MB/s-6842MB/s), io=1912GiB (2052GB), run=300002-300002msec
> > > >
> > > > - If I change zipf:0.5 to random:
> > > > Unpatched 6.7-rc4:
> > > > Patched with this series:
> > > > Run status group 0 (all jobs):
> > > >    READ: bw=5975MiB/s (6265MB/s), 5975MiB/s-5975MiB/s
> > > > (6265MB/s-6265MB/s), io=1750GiB (1879GB), run=300002-300002msec
> > > >
> > > > Patched with RFC v4:
> > > > Run status group 0 (all jobs):
> > > >    READ: bw=5987MiB/s (6278MB/s), 5987MiB/s-5987MiB/s
> > > > (6278MB/s-6278MB/s), io=1754GiB (1883GB), run=300001-300001msec
> > > >
> > > > Patched with this series:
> > > > Run status group 0 (all jobs):
> > > >    READ: bw=5839MiB/s (6123MB/s), 5839MiB/s-5839MiB/s
> > > > (6123MB/s-6123MB/s), io=1711GiB (1837GB), run=300001-300001msec
> > > >
> > > > MGLRU off:
> > > > Run status group 0 (all jobs):
> > > >    READ: bw=5689MiB/s (5965MB/s), 5689MiB/s-5689MiB/s
> > > > (5965MB/s-5965MB/s), io=1667GiB (1790GB), run=300003-300003msec
> > > >
> > > > fio uses ramdisk so LRU accuracy will have smaller impact. The Mongodb
> > > > test I provided before uses a SATA SSD so it will have a much higher
> > > > impact. I'll provides a script to setup the test case and run it, it's
> > > > more complex to setup than fio since involving setting up multiple
> > > > replicas and auth and hundreds of GB of test fixtures, I'm currently
> > > > occupied by some other tasks but will try best to send them out as
> > > > soon as possible.
> > >
> > > Thanks! Apparently your RFC did show better IOPS with both access
> > > patterns, which was a surprise to me because it had higher refaults
> > > and usually higher refautls result in worse performance.
> > >
> > > So I'm still trying to figure out why it turned out the opposite. My
> > > current guess is that:
> > > 1. It had a very small but stable inactive LRU list, which was able to
> > > fit into the L3 cache entirely.
> > > 2. It counted few folios as workingset and therefore incurred less
> > > overhead from CONFIG_PSI and/or CONFIG_TASK_DELAY_ACCT.
> > >
> > > Did you save workingset_refault_file when you ran the test? If so, can
> > > you check the difference between this series and your RFC?
> >
> >
> > It seems I was right about #1 above. After I scaled your test up by 20x,
> > I saw my series performed ~5% faster with zipf and ~9% faster with random
> > accesses.
>
> Hi Yu,
>
> Thank you so much for testing and sharing this result.
>
> I'm not sure about #1, the ramdisk size, access data, are far larger
> than L3 (16M on my CPU) even in down scaled test, and both random/zipf
> shows similar result.

It's the LRU list not pages. IOW, the kernel data structure, not the
content in LRU pages. Does it make sense?

> > IOW, I made rd_size from 16GB to 320GB, memory.max from 4GB to 80GB,
> > --numjobs from 12 to 60 and --size from 1GB to 4GB.

Would you be able to try a larger configuration like above instead?