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SubjectRe: [patch 3/8] mm: vmscan: save work scanning (almost) empty LRU lists
On Thu 13-12-12 14:38:20, Johannes Weiner wrote:
> On Thu, Dec 13, 2012 at 04:43:46PM +0100, Michal Hocko wrote:
> > On Wed 12-12-12 16:43:35, Johannes Weiner wrote:
> > > In certain cases (kswapd reclaim, memcg target reclaim), a fixed
> > > minimum amount of pages is scanned from the LRU lists on each
> > > iteration, to make progress.
> > >
> > > Do not make this minimum bigger than the respective LRU list size,
> > > however, and save some busy work trying to isolate and reclaim pages
> > > that are not there.
> > >
> > > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> >
> > Hmm, shrink_lruvec would do:
> > nr_to_scan = min_t(unsigned long,
> > nr[lru], SWAP_CLUSTER_MAX);
> > nr[lru] -= nr_to_scan;
> > and isolate_lru_pages does
> > for (scan = 0; scan < nr_to_scan && !list_empty(src); scan++)
> > so it shouldn't matter and we shouldn't do any additional loops, right?
> >
> > Anyway it would be beter if get_scan_count wouldn't ask for more than is
> > available.
>
> Consider the inactive_list_is_low() check (especially expensive for
> memcg anon), lru_add_drain(), lru lock acquisition...

Ohh, I totally missed that. Thanks for pointing out (maybe s/some busy
wok/$WITH_ALL_THIS/)?

Thanks for clarification!

> And as I wrote to Mel in the other email, this can happen a lot when
> you have memory cgroups in a multi-node environment.
>
> > Reviewed-by: Michal Hocko <mhocko@suse.cz>
>
> Thanks!
>
> > > @@ -1748,15 +1748,17 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
> > > out:
> > > for_each_evictable_lru(lru) {
> > > int file = is_file_lru(lru);
> > > + unsigned long size;
> > > unsigned long scan;
> > >
> > > - scan = get_lru_size(lruvec, lru);
> > > + size = get_lru_size(lruvec, lru);
> > + size = scan = get_lru_size(lruvec, lru);
> >
> > > if (sc->priority || noswap) {
> > > - scan >>= sc->priority;
> > > + scan = size >> sc->priority;
> > > if (!scan && force_scan)
> > > - scan = SWAP_CLUSTER_MAX;
> > > + scan = min(size, SWAP_CLUSTER_MAX);
> > > scan = div64_u64(scan * fraction[file], denominator);
> > > - }
> > > + } else
> > > + scan = size;
> >
> > And this is not necessary then but this is totally nit.
>
> Do you actually find this more readable? Setting size = scan and then
> later scan = size >> sc->priority? :-)

--
Michal Hocko
SUSE Labs


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