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

[Minchan Kim]

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.

Thanks for the review, Michal!