Re: [PATCH 7/7] mm: compaction: add compaction to zone_reclaim_mode

[Andrea Arcangeli <aarcange@redhat.com>]

Hi,

On Fri, Jul 12, 2013 at 06:26:37AM -0600, Hush Bensen wrote:
> > This isn't VM reclaim, we're not globally low on memory and we can't
> > wake kswapd until we expired all memory from all zones or we risk to
> > screw the lru rotation even further (by having kswapd and the thread
> 
> What's the meaning of lru rotation?

I mean the per-zone LRU walks to shrink the memory (they rotate pages
through the LRU). To provide for better global working set information
in the LRUs, we should walk all the zone LRUs in a fair
way.

zone_reclaim_mode however makes it non fair by always shrinking from
the first NUMA local zone even if the other zones could be shrunk
too.

When zone_reclaim_mode is disabled instead (default for most hardware
out there), we wait all candidate zones to be at the low wmark before
starting the shrinking from any zone (and then we shrink all zones,
not just one). So when zone_reclaim_mode is disabled, we don't insist
aging a single zone indefinitely, while leaving the others un-aged.

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