Re: [PATCH v4 4/4] mm: memcg: use non-unified stats flushing for userspace reads

[Wei Xu]

On Mon, Sep 11, 2023 at 6:11 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Thu 07-09-23 17:52:12, Wei Xu wrote:
> [...]
> > I tested this patch on a machine with 384 CPUs using a microbenchmark
> > that spawns 10K threads, each reading its memory.stat every 100
> > milliseconds.
>
> This is rather extreme case but I wouldn't call it utterly insane
> though.
>
> > Most of memory.stat reads take 5ms-10ms in kernel, with
> > ~5% reads even exceeding 1 second.
>
> Just curious, what would numbers look like if the mutex is removed and
> those threads would be condending on the existing spinlock with lock
> dropping in place and removed. Would you be willing to give it a shot?

Without the mutex and with the spinlock only, the common read latency
of memory.stat is still 5ms-10ms in kernel. There are very few reads
(<0.003%) going above 10ms and none more than 1 second.

> --
> Michal Hocko
> SUSE Labs