Re: [PATCH 2/2] mm/zswap: Support batch writeback in shrink_memcg()

[Yosry Ahmed <yosry@kernel.org>]

> >> Test Setup:
> >> Total memory: 32 GB.
> >> zswap settings: max_pool_percent=1, accept_threshold_percent=50,
> >> shrinker_enabled=N.
> >> Allocate 512MB of anonymous pages and fill them with random data (to avoid
> >> compression), then use cgroup memory.reclaim to force a large amount of
> >> anonymous pages into zswap. At an interval of 2ms, allocate a 4K anonymous
> >> page where the first 4 bytes are random numbers and the rest are zeros, and
> >> then trigger a reclamation of this 4K anonymous page through cgroup
> >> memory.reclaim. When the pool threshold is reached, shrink_memcg() will
> >> be triggered.
> >>
> >> The test data after running for 120s is as follows:
> >>                             Baseline         Patched
> >> shrink_worker wakeups          5363             85
> >> shrink_memcg calls       11,345,012        188,264
> >> written_back                  40214          40275
> >>
> >> Conclusion:
> >> Under the same workload and run duration, the patched kernel shows a
> >> significant reduction in both shrink_worker wakeups and shrink_memcg calls.
> >
> > Please also include data from the case where zswap store failures are
> > observed and pages go to disk, and compare before and after this
> > patch. I think that part is also really important.
>
> I retested and added some collected information. Perhaps
> `pool_limit_hit` and `pswpout` can explain that batch shrinking of zswap
> can reduce the number of pages that fail to be stored due to the pool
> limit, allowing zswap to skip zswap and go directly to disk.

Oh I meant the other test case with high memory pressure where we saw
a lot more writeback with this patch. Do you have similar data from
that test case?

>
>                                Baseline      Patched
> shrink_worker wakeups         5,363            85
> shrink_memcg calls       11,373,201      180,928
> written_back pages            40,212       40,236
> zswap_store calls            161,190      168,741
>    store succeeded (ret=1)    102,743      127,644
>    store rejected (ret=0)      58,447       41,097
>    store reject rate            ~36%         ~24%
> pool_limit_hit delta          55,826       14,062
> pswpout                       98,659       81,333
> pswpin                            2            1
>
>
> Thanks,
> Hao