Re: too big min_free_kbytes

[Rik van Riel <riel@redhat.com>]

On 01/28/2011 11:46 AM, Mel Gorman wrote:
> On Fri, Jan 28, 2011 at 05:28:31PM +0100, Andrea Arcangeli wrote:

>> In previous email you asked me how kswapd get stuck in D state and
>> never stops working, and that it should stop earlier. This sounds
>> impossible, kswapd behavior can't possibly change, simply there is
>> less memory freed by lowering that "gap".
>
> There might be less memory freed by lowering that gap but it still needs to
> exit balance_pgdat() and go to sleep. Otherwise it'll keep freeing zones up
> to the high watermark + gap and calling congestion_wait (hence the D state).

The gap works because kswapd has different thresholds for
different things:

1) get woken up if every zone on an allocator's zone list
    is below the low watermark

2) exit the loop if _every_ zone is at or above the
    high watermark

3) skip a zone in the freeing loop if the zone has more
    than high + gap free memory

Continuing the loop as long as one zone is below the low
watermark is what equalizes memory pressure between zones.

Skipping the freeing of pages in a zone that already has
excessive amounts of free memory helps avoid memory waste
and excessive swapping.  We simply equalize the balance
between zones a little more slowly.  What matters is that
the memory pressure gets equalized over time.

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