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

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

[Sergey Senozhatsky]

On (26/08/06 16:27), xueyuan.chen@vivo.com wrote:
> On an Android device with 12 GB of RAM, observed zsmalloc compaction
> durations had a p95 of 38.86 ms and a maximum of 269.68 ms, motivating a
> per-scan reclaim goal.

Would it be possible to give a little more data?  What was the
fragmentation ratio, how much memory was saved during that
auto-compaction, etc.  If possible.

Somewhere in the back of my mind I was thinking about, maybe,
disabling (removing) zsmalloc shrinker callbacks, in other words
disabling auto-compaction.  We have a sysfs knob for pool compaction
for system that still want to run compaction.  So I'm leaning towards
removal of shrinker callbacks from zsmalloc.

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

[Xueyuan Chen]

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?

Thanks,
Xueyuan


On 8/7/2026 11:56 AM, Sergey Senozhatsky wrote:
> On (26/08/06 16:27), xueyuan.chen@vivo.com wrote:
>> On an Android device with 12 GB of RAM, observed zsmalloc compaction
>> durations had a p95 of 38.86 ms and a maximum of 269.68 ms, motivating a
>> per-scan reclaim goal.
> Would it be possible to give a little more data?  What was the
> fragmentation ratio, how much memory was saved during that
> auto-compaction, etc.  If possible.
>
> Somewhere in the back of my mind I was thinking about, maybe,
> disabling (removing) zsmalloc shrinker callbacks, in other words
> disabling auto-compaction.  We have a sysfs knob for pool compaction
> for system that still want to run compaction.  So I'm leaning towards
> removal of shrinker callbacks from zsmalloc.

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

[Sergey Senozhatsky]

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).

Any thoughts?

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

[Nhat Pham]

On Fri, Aug 7, 2026 at 5:12 AM Sergey Senozhatsky
<senozhatsky@chromium.org> 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.

>
> Any thoughts?