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    SubjectRe: [patch 3/8] mm: vmscan: save work scanning (almost) empty LRU lists
    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...

    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? :-)


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