On Thu 13-12-12 10:34:20, Mel Gorman wrote:
> On Wed, Dec 12, 2012 at 04:43:34PM -0500, Johannes Weiner wrote:
> > When a reclaim scanner is doing its final scan before giving up and
> > there is swap space available, pay no attention to swappiness
> > preference anymore. Just swap.
> >
> > Note that this change won't make too big of a difference for general
> > reclaim: anonymous pages are already force-scanned when there is only
> > very little file cache left, and there very likely isn't when the
> > reclaimer enters this final cycle.
> >
> > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
>
> Ok, I see the motivation for your patch but is the block inside still
> wrong for what you want? After your patch the block looks like this
>
> if (sc->priority || noswap) {
> scan >>= sc->priority;
> if (!scan && force_scan)
> scan = SWAP_CLUSTER_MAX;
> scan = div64_u64(scan * fraction[file], denominator);
> }
>
> if sc->priority == 0 and swappiness==0 then you enter this block but
> fraction[0] for anonymous pages will also be 0 and because of the ordering
> of statements there, scan will be
>
> scan = scan * 0 / denominator
>
> so you are still not reclaiming anonymous pages in the swappiness=0
> case. What did I miss?
Yes, now that you have mentioned that I realized that it really doesn't
make any sense. fraction[0] is _always_ 0 for swappiness==0. So we just
made a bigger pressure on file LRUs. So this sounds like a misuse of the
swappiness. This all has been introduced with fe35004f (mm: avoid
swapping out with swappiness==0).
I think that removing swappiness check make sense but I am not sure it
does what the changelog says. It should have said that checking
swappiness doesn't make any sense for small LRUs.
--
Michal Hocko
SUSE Labs
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