Re: [PATCH] mm/vmscan.c: avoid recording the original scan targets in shrink_lruvec()

Chen Yucong
2014-06-11T02:10:14Z

On Tue, 2014-06-10 at 16:33 -0700, Andrew Morton wrote:
> On Mon, 9 Jun 2014 21:27:16 +0800 Chen Yucong <slaoub@gmail.com> wrote:
>
> > Via https://lkml.org/lkml/2013/4/10/334 , we can find that recording the
> > original scan targets introduces extra 40 bytes on the stack. This patch
> > is able to avoid this situation and the call to memcpy(). At the same time,
> > it does not change the relative design idea.
> >
> > ratio = original_nr_file / original_nr_anon;
> >
> > If (nr_file > nr_anon), then ratio = (nr_file - x) / nr_anon.
> > x = nr_file - ratio * nr_anon;
> >
> > if (nr_file <= nr_anon), then ratio = nr_file / (nr_anon - x).
> > x = nr_anon - nr_file / ratio;
> >
> > ...
> >
>
> Are you sure this is an equivalent-to-before change? If so, then I
> can't immediately see why :(
>
The relative design idea is to keep

ratio
== scan_target[anon] : scan_target[file]
== really_scanned_num[anon] : really_scanned_num[file]

The original implementation is
ratio
== (scan_target[anon] * percentage_anon) /
(scan_target[file] * percentage_file)

To keep the original ratio, percentage_anon should equal to
percentage_file. In other word, we need to calculate the difference
value between percentage_anon and percentage_file, we also have to
record the original scan targets for this.

Instead, we can calculate the *ratio* at the beginning of
shrink_lruvec(). As a result, this can avoid introducing the extra 40
bytes.

In short, we have the same goal: keep the same *ratio* from beginning to
end.

thx!
cyc