Re: [PATCH] vmscan: check all_unreclaimable in direct reclaim path

[Dave Young]

On Thu, Sep 9, 2010 at 6:19 AM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, 9 Sep 2010 00:45:27 +0900
> Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
>
>> +static inline bool zone_reclaimable(struct zone *zone)
>> +{
>> + Â Â return zone->pages_scanned < zone_reclaimable_pages(zone) * 6;
>> +}
>> +
>> +static inline bool all_unreclaimable(struct zonelist *zonelist,
>> + Â Â Â Â Â Â struct scan_control *sc)
>> +{
>> + Â Â struct zoneref *z;
>> + Â Â struct zone *zone;
>> + Â Â bool all_unreclaimable = true;
>> +
>> + Â Â if (!scanning_global_lru(sc))
>> + Â Â Â Â Â Â return false;
>> +
>> + Â Â for_each_zone_zonelist_nodemask(zone, z, zonelist,
>> + Â Â Â Â Â Â Â Â Â Â gfp_zone(sc->gfp_mask), sc->nodemask) {
>> + Â Â Â Â Â Â if (!populated_zone(zone))
>> + Â Â Â Â Â Â Â Â Â Â continue;
>> + Â Â Â Â Â Â if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
>> + Â Â Â Â Â Â Â Â Â Â continue;
>> + Â Â Â Â Â Â if (zone_reclaimable(zone)) {
>> + Â Â Â Â Â Â Â Â Â Â all_unreclaimable = false;
>> + Â Â Â Â Â Â Â Â Â Â break;
>> + Â Â Â Â Â Â }
>> + Â Â }
>> +
>> Â Â Â return all_unreclaimable;
>> Â}
>
> Could we have some comments over these functions please? ÂWhy they
> exist, what problem they solve, how they solve them, etc. ÂStuff which
> will be needed for maintaining this code three years from now.
>
> We may as well remove the `inline's too. Âgcc will tkae care of that.
>
>> - Â Â Â Â Â Â if (nr_slab == 0 &&
>> - Â Â Â Â Â Â Â Âzone->pages_scanned >= (zone_reclaimable_pages(zone) * 6))
>> + Â Â Â Â Â Â if (nr_slab == 0 && !zone_reclaimable(zone))
>
> Extra marks for working out and documenting how we decided on the value
> of "6". ÂSigh. ÂIt's hopefully in the git record somewhere.

Here it is (necessary to add additional comment?):

commit 4ff1ffb4870b007b86f21e5f27eeb11498c4c077
Author: Nick Piggin <npiggin@xxxxxxx>
Date:   Mon Sep 25 23:31:28 2006 -0700

    [PATCH] oom: reclaim_mapped on oom

    Potentially it takes several scans of the lru lists before we can even start
    reclaiming pages.

    mapped pages, with young ptes can take 2 passes on the active list + one on
    the inactive list.  But reclaim_mapped may not always kick in
instantly, so it
    could take even more than that.

    Raise the threshold for marking a zone as all_unreclaimable from a
factor of 4
    time the pages in the zone to 6.  Introduce a mechanism to force
    reclaim_mapped if we've reached a factor 3 and still haven't made progress.

    Previously, a customer doing stress testing was able to easily OOM the box
    after using only a small fraction of its swap (~100MB).  After the
patches, it
    would only OOM after having used up all swap (~800MB).

>
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-- 
Regards
dave
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