On Wed, 24 Feb 2016 16:38:50 -0500 Rik van Riel <riel@xxxxxxxxxx> wrote:
> For multi page allocations smaller than PAGE_ALLOC_COSTLY_ORDER,
> the kernel will do direct reclaim if compaction failed for any
> reason. This worked fine when Linux systems had 128MB RAM, but
> on my 24GB system I frequently see higher order allocations
> free up over 3GB of memory, pushing all kinds of things into
> swap, and slowing down applications.
hm. Seems a pretty obvious flaw - why didn't we notice+fix it earlier?
> It would be much better to limit the amount of reclaim done,
> rather than cause excessive pageout activity.
>
> When enough memory is free to do compaction for the highest order
> allocation possible, bail out of the direct page reclaim code.
>
> On smaller systems, this may be enough to obtain contiguous
> free memory areas to satisfy small allocations, continuing our
> strategy of relying on luck occasionally. On larger systems,
> relying on luck like that has not been working for years.
>
It would be nice to see some solid testing results on real-world
workloads?
(patch retained for linux-mm)
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index fc62546096f9..8dd15d514761 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2584,20 +2584,17 @@ static bool shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
> continue; /* Let kswapd poll it */
>
> /*
> - * If we already have plenty of memory free for
> - * compaction in this zone, don't free any more.
> - * Even though compaction is invoked for any
> - * non-zero order, only frequent costly order
> - * reclamation is disruptive enough to become a
> - * noticeable problem, like transparent huge
> - * page allocations.
> + * For higher order allocations, free enough memory
> + * to be able to do compaction for the largest possible
> + * allocation. On smaller systems, this may be enough
> + * that smaller allocations can skip compaction, if
> + * enough adjacent pages get freed.
> */
> - if (IS_ENABLED(CONFIG_COMPACTION) &&
> - sc->order > PAGE_ALLOC_COSTLY_ORDER &&
> + if (IS_ENABLED(CONFIG_COMPACTION) && sc->order &&
> zonelist_zone_idx(z) <= requested_highidx &&
> - compaction_ready(zone, sc->order)) {
> + compaction_ready(zone, MAX_ORDER)) {
> sc->compaction_ready = true;
> - continue;
> + return true;
> }
>
> /*
--
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