Re: [PATCH v4 00/17] mm/mglru: frequency guided promotion (MGLRU-FG) and flag cleanup

[Barry Song (Xiaomi)]

On Fri, Oct 16, 2026 at 2:15 AM Kairui Song via B4 Relay <devnull+kasong.tencent.com@xxxxxxxxxx> wrote:
[...]
>
> LevelDB Scan/Get
> ================
> I also retested the LevelDB benchmark from the cache_ext paper [5].
> Interestingly, mainline MGLRU already beats CLRU on this one after a
> recent change in lru_gen_folio_seq that bumps new folios with refs == 1
> to the second-oldest generation as discussed. That change accidentally
> gave random reads a higher hotness level while making sequential reads
> much colder as described before [14]. However it's unreliable and
> incorrect because it depends on LRU drain timing, and hurts some workloads
> as reported, or where the sequential part is actually hotter (any workload
> involving hotter large file and many small cold files will be affected)
> which is reproduced below.
>
> This series improves it while covering ordinary workloads too and should
> fix regressions:
>
> LevelDB Scan / Get, Throughput Total:
> CLRU:         4708.3 ops/s
> MGLRU:        5026.9 ops/s (faster than CLRU, but hurts other workloads)
> MGLRU After:  5179.2 ops/s (even better in all cases, and no regression)
>
> The hot-sequential and cold-random workload can be easily reproduced with
> SQLite and grep. SQLite continuously scans and looks up a small hot
> portion of a DB file, while grep iterates over a set of small files much
> larger than RAM [6]:
>
>          SQLite scan & lookup time:       Grep iterate time:
> CLRU:                       14.51ms               13281.37ms
> MGLRU mainline:            567.05ms               13694.47ms
> MGLRU After this series:    10.58ms               12930.43ms
>

Thanks, Kairui. I'll do a thorough review of the whole patchset over
the next two weeks.

BTW, do you think we need a quick fix patch like the one below
for the above problem before this patchset, so that it can be
backported to stable? I assume your patchset is too large to
backport?