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

[Michal Hocko]

On Mon 11-09-23 12:15:24, Wei Xu wrote:
> 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.

Is this with the existing spinlock dropping and same 10k potentially
contending readers?
-- 
Michal Hocko
SUSE Labs