On 23/06/11 12:46, Mel Gorman wrote:
> Based on the information you have provided from sysrq and the profile,
> I put together a theory as to what is going wrong for your machine at
> least although I somehow doubt the same fix will work for Dan. Can you
> try out the following please? It's against 2.6.38.8 (and presumably
> Fedora) but will apply with offset against 2.6.39 and 3.0-rc4.
>
> ==== CUT HERE ====
> mm: vmscan: Correct check for kswapd sleeping in sleeping_prematurely
>
> 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 that keeps kswapd awake.
> 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 before sleep
> 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 the zone is not
> balanced. all_zones_ok is false so kswapd stays awake
>
> The impact is that kswapd chews up a lot of CPU as it avoids most of
> the scheduling points and reclaims excessively from the lower zones.
> This patch corrects the behaviour of sleeping_prematurely to check
> the zones balance_pgdat() checked.
>
> Reported-by: Pádraig Brady <P@xxxxxxxxxxxxxx>
> Not-signed-off-awaiting-confirmation: Mel Gorman <mgorman@xxxxxxx>
> ---
> mm/vmscan.c | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a74bf72..a578535 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2261,7 +2261,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
> return true;
>
> /* Check the watermark levels */
> - for (i = 0; i < pgdat->nr_zones; i++) {
> + for (i = 0; i <= classzone_idx; i++) {
> struct zone *zone = pgdat->node_zones + i;
>
> if (!populated_zone(zone))
No joy :(
cheers,
Pádraig.
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