Re: [PATCH 1/2] mem_cgroup: optimize the atomic count of wb_completion

[brookxu.cn]

Thanks for your time.

On 2021/9/24 10:01 PM, Michal Hocko wrote:
> On Fri 24-09-21 21:02:52, brookxu wrote:
> > Thanks for your time.
> >
> > Michal Hocko wrote on 2021/9/24 17:34:
> > > On Fri 24-09-21 14:46:22, brookxu wrote:
> > > > From: Chunguang Xu <brookxu@xxxxxxxxxxx>
> > > >
> > > > In order to track inflight foreign writeback, we init
> > > > wb_completion.cnt to 1. For normal writeback, this cause
> > > > wb_wait_for_completion() to perform meaningless atomic
> > > > operations. Since foreign writebacks rarely occur in most
> > > > scenarios, we can init wb_completion.cnt to 0 and set
> > > > frn.done.cnt to 1. In this way we can avoid unnecessary
> > > > atomic operations.
> > >
> > > Does this lead to any measurable differences?
> >
> > I created multiple cgroups that performed IO on multiple disks,
> > then flushed the cache with sync command, and no measurable
> > differences have been observed so far.
>
> OK, so why do we want to optimize this code?

Just a optimization point discovered during the diagnosis, no
behavior change.

>