On Thu, 2007-08-02 at 19:17 +0100, Andy Whitcroft wrote:
> [This is a re-spin based on feedback from akpm.]
>
> As pointed out by Mel when reclaim is applied at higher orders a
> significant amount of IO may be started. As this takes finite time
> to drain reclaim will consider more areas than ultimatly needed
> to satisfy the request. This leads to more reclaim than strictly
> required and reduced success rates.
>
> I was able to confirm Mel's test results on systems locally.
> These show that even under light load the success rates drop off far
> more than expected. Testing with a modified version of his patch
> (which follows) I was able to allocate almost all of ZONE_MOVABLE
> with a near idle system. I ran 5 test passes sequentially following
> system boot (the system has 29 hugepages in ZONE_MOVABLE):
>
> 2.6.23-rc1 11 8 6 7 7
> sync_lumpy 28 28 29 29 26
>
> These show that although hugely better than the near 0% success
> normally expected we can only allocate about a 1/4 of the zone.
> Using synchronous reclaim for these allocations we get close to 100%
> as expected.
>
> I have also run our standard high order tests and these show no
> regressions in allocation success rates at rest, and some significant
> improvements under load.
>
> Following this email are two patches, both should be considered as
> bug fixes to lumpy reclaim for 2.6.23:
>
> ensure-we-count-pages-transitioning-inactive-via-clear_active_flags:
> this a bug fix for Lumpy Reclaim fixing up a bug in VM Event
> accounting when it marks pages inactive, and
>
> Wait-for-page-writeback-when-directly-reclaiming-contiguous-areas:
> updates reclaim making direct reclaim synchronous when applied
> at orders above PAGE_ALLOC_COSTLY_ORDER.
>
> Patches against 2.6.23-rc1. Andrew please consider for -mm and
> for pushing to mainline.
Acked-by: Peter Zijlstra <[email protected]>
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