Re: [RFC 6/7] mm: extend process_madvise syscall to support vector arrary

[Daniel Colascione]

On Mon, May 27, 2019 at 12:49 AM Minchan Kim <minchan@xxxxxxxxxx> wrote:
>
> On Tue, May 21, 2019 at 12:37:26PM +0200, Michal Hocko wrote:
> > On Tue 21-05-19 19:26:13, Minchan Kim wrote:
> > > On Tue, May 21, 2019 at 08:24:21AM +0200, Michal Hocko wrote:
> > > > On Tue 21-05-19 11:48:20, Minchan Kim wrote:
> > > > > On Mon, May 20, 2019 at 11:22:58AM +0200, Michal Hocko wrote:
> > > > > > [Cc linux-api]
> > > > > >
> > > > > > On Mon 20-05-19 12:52:53, Minchan Kim wrote:
> > > > > > > Currently, process_madvise syscall works for only one address range
> > > > > > > so user should call the syscall several times to give hints to
> > > > > > > multiple address range.
> > > > > >
> > > > > > Is that a problem? How big of a problem? Any numbers?
> > > > >
> > > > > We easily have 2000+ vma so it's not trivial overhead. I will come up
> > > > > with number in the description at respin.
> > > >
> > > > Does this really have to be a fast operation? I would expect the monitor
> > > > is by no means a fast path. The system call overhead is not what it used
> > > > to be, sigh, but still for something that is not a hot path it should be
> > > > tolerable, especially when the whole operation is quite expensive on its
> > > > own (wrt. the syscall entry/exit).
> > >
> > > What's different with process_vm_[readv|writev] and vmsplice?
> > > If the range needed to be covered is a lot, vector operation makes senese
> > > to me.
> >
> > I am not saying that the vector API is wrong. All I am trying to say is
> > that the benefit is not really clear so far. If you want to push it
> > through then you should better get some supporting data.
>
> I measured 1000 madvise syscall vs. a vector range syscall with 1000
> ranges on ARM64 mordern device. Even though I saw 15% improvement but
> absoluate gain is just 1ms so I don't think it's worth to support.
> I will drop vector support at next revision.

Please do keep the vector support. Absolute timing is misleading,
since in a tight loop, you're not going to contend on mmap_sem. We've
seen tons of improvements in things like camera start come from
coalescing mprotect calls, with the gains coming from taking and
releasing various locks a lot less often and bouncing around less on
the contended lock paths. Raw throughput doesn't tell the whole story,
especially on mobile.