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    SubjectRe: [patch 05/19] split LRU lists into anon & file sets
    On Wed, 30 Jan 2008 17:57:54 +0900
    KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

    > I found number of scan pages calculation bug.

    My latest version of get_scan_ratio() works differently, with the
    percentages always adding up to 100. However, your patch gave me
    the inspiration to (hopefully) find the bug in my version of the
    code.

    > 1. wrong calculation order

    I do not believe my new code has this problem. Of course, this
    is purely due to luck :)

    > 2. wrong fraction omission
    >
    > nr[l] = zone->nr_scan[l] * percent[file] / 100;
    >
    > when percent is very small,
    > nr[l] become 0.

    This is probably where the problem is. Kind of.

    I believe that the problem is that we scale nr[l] by the percentage,
    instead of scaling the amount we add to zone->nr_scan[l] by the
    percentage!

    > @@ -1409,7 +1410,11 @@ static unsigned long shrink_zone(int pri
    > */
    > zone->nr_scan[l] += (zone_page_state(zone,
    > NR_INACTIVE_ANON + l) >> priority) + 1;
    > - nr[l] = zone->nr_scan[l] * percent[file] / 100;
    > + nr[l] = (zone->nr_scan[l] * percent[file] / 100) + 1;
    > + nr_max_scan = zone_page_state(zone, NR_INACTIVE_ANON+l);
    > + if (nr[l] > nr_max_scan)
    > + nr[l] = nr_max_scan;
    > +
    > if (nr[l] >= sc->swap_cluster_max)
    > zone->nr_scan[l] = 0;
    > else

    With the fix below (against my latest tree), we always add at least one
    to zone->nr_scan[l] and always make that increment count later on!

    I am still recovering from my trip home (thanks to the airline companies
    I spent 25 hours travelling, from door to door), so I may not get around
    to actually testing this today:

    Index: linux-2.6.24-rc6-mm1/mm/vmscan.c
    ===================================================================
    --- linux-2.6.24-rc6-mm1.orig/mm/vmscan.c 2008-02-06 19:23:16.000000000 -0500
    +++ linux-2.6.24-rc6-mm1/mm/vmscan.c 2008-02-06 19:22:55.000000000 -0500
    @@ -1275,13 +1275,17 @@ static unsigned long shrink_zone(int pri
    for_each_lru(l) {
    if (scan_global_lru(sc)) {
    int file = is_file_lru(l);
    + int scan;
    /*
    * Add one to nr_to_scan just to make sure that the
    - * kernel will slowly sift through the active list.
    + * kernel will slowly sift through each list.
    */
    - zone->nr_scan[l] += (zone_page_state(zone,
    - NR_INACTIVE_ANON + l) >> priority) + 1;
    - nr[l] = zone->nr_scan[l] * percent[file] / 100;
    + scan = zone_page_state(zone, NR_INACTIVE_ANON + l);
    + scan >>= priority;
    + scan = (scan * percent[file]) / 100;
    +
    + zone->nr_scan[l] += scan + 1;
    + nr[l] = zone->nr_scan[l];
    if (nr[l] >= sc->swap_cluster_max)
    zone->nr_scan[l] = 0;
    else


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