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

Kairui Song
2023-12-12T06:52:35Z

Yu Zhao <yuzhao@google.com> 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@gmail.com> wrote:
> >
> > Yu Zhao <yuzhao@google.com> 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

Thanks!

> lot easier for me if you could share
> 1. your latest tree, preferably based on the mainline, and
> 2. your VM image containing the above test.

Sure, I'll update the RFC and try to provide an easier test reproducer.