Re: [PATCH] vmscan: evict use-once pages first (v2)

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 30 Apr 2009 21:50:34 -0400 Rik van Riel <riel@redhat.com> wrote:

> > Which would cause exactly the problem Elladan saw?
> 
> Yes.  It was not noticable in the initial split LRU code,
> but after we decided to ignore the referenced bit on active
> file pages and deactivate pages regardless, it has gotten
> exacerbated.
> 
> That change was very good for scalability, so we should not
> undo it.  However, we do need to put something in place to
> protect the working set from streaming IO.
> 
> > > Currently the kernel has no effective code to protect the 
> > > page cache working set from streaming IO.  Elladan's bug
> > > report shows that we do need some kind of protection...
> > 
> > Seems to me that reclaim should treat swapcache-backed mapped mages in
> > a similar fashion to file-backed mapped pages?
> 
> Swapcache-backed pages are not on the same set of LRUs as
> file-backed mapped pages.

yup.

> Furthermore, there is no streaming IO on the anon LRUs like
> there is on the file LRUs. Only the file LRUs need (and want)
> use-once replacement, which means that we only need special
> protection of the working set for file-backed pages.

OK.

> When we implement working set protection, we might as well
> do it for frequently accessed unmapped pages too.  There is
> no reason to restrict this protection to mapped pages.

Well.  Except for empirical observation, which tells us that biasing
reclaim to prefer to retain mapped memory produces a better result.