On Thu, Jan 27, 2011 at 06:18:07PM -0500, Rik van Riel wrote:

On 01/27/2011 04:31 PM, Mel Gorman wrote:

quoted

Whatever the final solution, it both needs to prevent too much memory
being reclaimed and allow kswapd to go to sleep if there is no
indication from the page allocator that it should stay awake.

A third requirement:

If one zone has a lot lower memory pressure than another zone,
we want to do relatively more memory allocations from that zone,
than from a zone where the memory is heavily used.

Risky. Allocations could end up using a lower zone than required causing
a form of lowmem pressure when highmem should have been used. Worse,
it'll be unnoticable on x86-64 but potentially cause problems on x86-32
that are easily missed.

If kswapd only ever goes up to the high watermark, and also uses
that as its sleep point, the allocations end up corresponding to
zone size alone and not to memory pressure.

hmm.

Going a little bit above the high watermark (1% of zone size?
high + min watermark?) will help balance things out between zones.

quoted

quoted

  			if (!zone_watermark_ok_safe(zone, order,
-					8*high_wmark_pages(zone), end_zone, 0))
+					(zone->present_pages +
+					 KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
+					 KSWAPD_ZONE_BALANCE_GAP_RATIO +
+					high_wmark_pages(zone), end_zone, 0))

Rik has already pointed out that this potentially is a very large gap
but that is an addressable problem if the final decision goes this
direction.

I was wrong.  I guess on some systems the min watermark can be less
than 1% and (high + min) may be better, but on most systems the
number of pages should be about the same.

Maybe we should use high_wmark_pages(zone) + low_wmark_pages(zone)
for easy readability?

I'd be ok with high+low as a starting point to solve the immediate
problem of way too much memory being free and then treat "kswapd must go
to sleep" as a separate problem. I'm less keen on 1% but only because it
could be too large a value.

-- 
Mel Gorman
Linux Technology Center
IBM Dublin Software Lab

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