Re: [PATCH v12 00/31] Speculative page faults

[Laurent Dufour]

Le 14/12/2020 à 03:03, Joel Fernandes a écrit :
> On Tue, Jul 07, 2020 at 01:31:37PM +0800, Chinwen Chang wrote:
> [..]
> > > > Hi Laurent,
> > > >
> > > > We merged SPF v11 and some patches from v12 into our platforms. After
> > > > several experiments, we observed SPF has obvious improvements on the
> > > > launch time of applications, especially for those high-TLP ones,
> > > >
> > > > # launch time of applications(s):
> > > >
> > > > package           version      w/ SPF      w/o SPF      improve(%)
> > > > ------------------------------------------------------------------
> > > > Baidu maps        10.13.3      0.887       0.98         9.49
> > > > Taobao            8.4.0.35     1.227       1.293        5.10
> > > > Meituan           9.12.401     1.107       1.543        28.26
> > > > WeChat            7.0.3        2.353       2.68         12.20
> > > > Honor of Kings    1.43.1.6     6.63        6.713        1.24
> > >
> > > That's great news, thanks for reporting this!
> > >
> > > > By the way, we have verified our platforms with those patches and
> > > > achieved the goal of mass production.
> > >
> > > Another good news!
> > > For my information, what is your targeted hardware?
> > >
> > > Cheers,
> > > Laurent.
> >
> > Hi Laurent,
> >
> > Our targeted hardware belongs to ARM64 multi-core series.
>
> Hello!
>
> I was trying to develop an intuition about why does SPF give improvement for
> you on small CPU systems. This is just a high-level theory but:
>
> 1. Assume the improvement is because of elimination of "blocking" on
> mmap_sem.
> Could it be that the mmap_sem is acquired in write-mode unnecessarily in some
> places, thus causing blocking on mmap_sem in other paths? If so, is it
> feasible to convert such usages to acquiring them in read-mode?

That's correct, and the goal of this series is to try not holding the mmap_sem 
in read mode during page fault processing.

Converting mmap_sem holder from write to read mode is not so easy and that work 
as already been done in some places. If you think there are areas where this 
could be done, you're welcome to send patches fixing that.

> 2. Assume the improvement is because of lesser read-side contention on
> mmap_sem.
> On small CPU systems, I would not expect reducing cache-line bouncing to give
> such a dramatic improvement in performance as you are seeing.

I don't think cache line bouncing reduction is the main sourcec of performance 
improvement, I would rather think this is the lower part here.
I guess this is mainly because during loading time a lot of page fault is 
occuring and thus SPF is reducing the contention on the mmap_sem.

> Thanks for any insight on this!
>
> - Joel