Re: [RFC PATCH 0/2] mm: zsmalloc: make shrinker compaction budget-aware

[Xueyuan Chen]

On Wed, Aug 12, 2026 at 12:25 AM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
>
> On Tue, Aug 11, 2026 at 2:04 AM Xueyuan Chen <xueyuan.chen@xxxxxxxx> wrote:
> >
> >
> > On 8/8/2026 12:25 AM, Nhat Pham wrote:
> > > On Fri, Aug 7, 2026 at 5:12 AM Sergey Senozhatsky
> > > <senozhatsky@xxxxxxxxxxxx> wrote:
> > >> On (26/08/07 18:57), Xueyuan Chen wrote:
> > >>> Hi Sergey,
> > >>>
> > >>> Here is some additional data:
> > >>>
> > >>> I used the following definitions:
> > >>>    compactable ratio = freeable_pages / total_pages
> > >>>    memory reclaimed  = pages_freed * PAGE_SIZE
> > >>>
> > >>> freeable_pages is the estimate before compaction, based on the same
> > >>> calculation as zs_shrinker_count(), while pages_freed is the actual
> > >>> number of backing pages released.
> > >>>
> > >>> There were 264 callbacks in the trace:
> > >>>    callback elapsed time:
> > >>>      median:    5.77 ms
> > >>>      p95:      55.82 ms
> > >>>      maximum: 271.36 ms
> > >>>
> > >>>    compactable ratio before compaction:
> > >>>      median:   0.32%
> > >>>      p95:      2.86%
> > >>>      maximum:  8.33%
> > >>>
> > >>>    memory reclaimed per callback:
> > >>>      median:    3.80 MiB
> > >>>      p95:      30.45 MiB
> > >>>      maximum:  92.73 MiB
> > >>>
> > >>> The longest callback took 271.36 ms. Its compactable ratio was 3.25%,
> > >>> and it released 7,650 pages, or about 29.88 MiB.
> > >>>
> > >>> There was also a 241.91 ms callback (with 30 schedule-outs) with a
> > >>> compactable ratio of 0.44%. It released 1,019 pages, or about
> > >>> 3.98 MiB.
> > >>>
> > >>> Based on this data, it seems better to remove the shrinker.
> > >>>
> > >>> Would you prefer that I change v2 to remove the zsmalloc shrinker
> > >>> callbacks directly?
> > >> Let's bring in heavy artillery to this discussion, in addition to Andrew
> > >> and Minchan, adding Nhat, Yosry, Barry, Johannes, Brian (random order).
> > >>
> > >> Folks, I'm bullish on removal of zsmalloc shrinker callbacks.
> > >> I don't think those buy us much apart from memcpy-s and lock
> > >> contention.  Systems that want to compact zsmalloc have a sysfs
> > >> knob (and API) to do so (based on zram mm_stat numbers).
> > > Hmm we'd need to collect more data in our workloads to determine, but
> > > if we can compute compactable ratio (freeable_pages / total_pages) on
> > > a per size class basis, can we just skip the size class whose ratio is
> > > too bad? Would that at least cut down on the vast majority of
> > > fruitless memcpys and lock acquisitions etc?
> > >
> > > I'm always a bit hesitant to over-rely on userspace, especially when
> > > kernel has information to do something smart about it. It might not
> > > react in time, and many proactive reclaiming schemes back off under
> > > heavy memory pressure.
> >
> > Hi Nhat,
> >
> > I collected per size-class data. 53.7% of size classes have freeable==0,
> > so skipping them saves some time. However, 94.7% of zs_compact() time is
> > spent on classes where freeable > 0 — meaning the cost is dominated by
> > the actual compaction work, not by scanning empty classes.
>

Hi Nhat,

Sorry for the late reply.

> What's the distribution of these? Say if I were to skip all size class
> with compactable ratio below 20%, or those where we can free at least
> one full zspage? (IIUC, we only free any memory at all if a zspage got
> freed up from our internal compaction, correct?).

You are right, we only free the backing pages when a full zspage is emptied
by compaction.

My test device had some problems, I need some time to collect new data.
The good news is the per-size-class distribution should be ready next week.

Thanks