On 28/06/11 22:49, Andrew Morton wrote: > On Fri, 24 Jun 2011 15:44:54 +0100 > Mel Gorman <mgorman@xxxxxxx> wrote: > >> 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? > >> Reported-and-tested-by: Pádraig Brady <P@xxxxxxxxxxxxxx> > > 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, Pádraig. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>