Re: [PATCH] memcg: introduce per-memcg reclaim interface

[Shakeel Butt]

On Tue, Sep 22, 2020 at 11:31 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Tue 22-09-20 11:10:17, Shakeel Butt wrote:
> > On Tue, Sep 22, 2020 at 9:55 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> [...]
> > > So far I have learned that you are primarily working around an
> > > implementation detail in the zswap which is doing the swapout path
> > > directly in the pageout path.
> >
> > Wait how did you reach this conclusion? I have explicitly said that we
> > are not using uswapd like functionality in production. We are using
> > this interface for proactive reclaim and proactive reclaim is not a
> > workaround for implementation detail in the zswap.
>
> Hmm, I must have missed the distinction between the two you have
> mentioned. Correct me if I am wrong but "latency sensitive" workload is
> the one that cannot use the high limit, right.

Yes.

> For some reason I thought
> that your pro-active reclaim usecase is also not compatible with the
> throttling imposed by the high limit. Hence my conclusion above.
>

For proactive reclaim use-case, it is more about the weirdness of
using memory.high interface for proactive reclaim.

Let's suppose I want to reclaim 20 MiB from a job. To use memory.high,
I have to read memory.current and subtract 20MiB from it and then
write that to memory.high and once that is done, I have to set
memory.high to the previous value (job's original high limit).

There is a time window where the allocation of the target job can hit
the temporary memory.high which will cause uninteresting MEMCG_HIGH
event, PSI pressure and can potentially over reclaim. Also there is a
race between reading memory.current and setting the temporary
memory.high. There are many non-deterministic  variables added to the
request of reclaiming 20MiB from a job.

Shakeel