On 28/06/11 22:49, Andrew Morton wrote:

On Fri, 24 Jun 2011 15:44:54 +0100
Mel Gorman [off-list ref] wrote:

quoted

During allocator-intensive workloads, kswapd will be woken frequently
causing free memory to oscillate between the high and min watermark.
This is expected behaviour.

A problem occurs if the highest zone is small.  balance_pgdat()
only considers unreclaimable zones when priority is DEF_PRIORITY
but sleeping_prematurely considers all zones. It's possible for this
sequence to occur

  1. kswapd wakes up and enters balance_pgdat()
  2. At DEF_PRIORITY, marks highest zone unreclaimable
  3. At DEF_PRIORITY-1, ignores highest zone setting end_zone
  4. At DEF_PRIORITY-1, calls shrink_slab freeing memory from
        highest zone, clearing all_unreclaimable. Highest zone
        is still unbalanced
  5. kswapd returns and calls sleeping_prematurely
  6. sleeping_prematurely looks at *all* zones, not just the ones
     being considered by balance_pgdat. The highest small zone
     has all_unreclaimable cleared but but the zone is not
     balanced. all_zones_ok is false so kswapd stays awake

This patch corrects the behaviour of sleeping_prematurely to check
the zones balance_pgdat() checked.

But kswapd is making progress: it's reclaiming slab.  Eventually that
won't work any more and all_unreclaimable will not be cleared and the
condition will fix itself up?



btw,

	if (!sleeping_prematurely(...))
		sleep();

hurts my brain.  My brain would prefer

	if (kswapd_should_sleep(...))
		sleep();

no?

quoted

Reported-and-tested-by: Padraig Brady <redacted>

But what were the before-and-after observations?  I don't understand
how this can cause a permanent cpuchew by kswapd.

Context:
  http://marc.info/?t=130865025500001&r=1&w=2
  https://bugzilla.redhat.com/show_bug.cgi?id=712019

Summary:

This will spin kswapd0 on my SNB laptop with 3GB RAM (with small normal zone):

    dd bs=1M count=3000 if=/dev/zero of=spin.test

Basically once a certain amount of data is cached,
kswapd0 will start spinning, until the data
is removed from cache (by `rm spin.test` for example).

cheers,
Padraig.

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