On Thu, Aug 18, 2011 at 04:54:28PM -0700, Andrew Morton wrote: > On Wed, 10 Aug 2011 11:47:19 +0100 > Mel Gorman <mgorman@xxxxxxx> wrote: > > > The percentage that must be in writeback depends on the priority. At > > default priority, all of them must be dirty. At DEF_PRIORITY-1, 50% > > of them must be, DEF_PRIORITY-2, 25% etc. i.e. as pressure increases > > the greater the likelihood the process will get throttled to allow > > the flusher threads to make some progress. > > It'd be nice if the code comment were to capture this piece of implicit > arithmetic. How about this? ==== CUT HERE ==== mm: vmscan: Throttle reclaim if encountering too many dirty pages under writeback -fix1 This patch expands on a comment on how we throttle from reclaim context. It should be merged with mm-vmscan-throttle-reclaim-if-encountering-too-many-dirty-pages-under-writeback.patch Signed-off-by: Mel Gorman <mgorman@xxxxxxx> --- mm/vmscan.c | 26 +++++++++++++++++++++----- 1 files changed, 21 insertions(+), 5 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index 33882a3..5ff3e26 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -1491,11 +1491,27 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone, putback_lru_pages(zone, sc, nr_anon, nr_file, &page_list); /* - * If we have encountered a high number of dirty pages under writeback - * then we are reaching the end of the LRU too quickly and global - * limits are not enough to throttle processes due to the page - * distribution throughout zones. Scale the number of dirty pages that - * must be under writeback before being throttled to priority. + * If reclaim is isolating dirty pages under writeback, it implies + * that the long-lived page allocation rate is exceeding the page + * laundering rate. Either the global limits are not being effective + * at throttling processes due to the page distribution throughout + * zones or there is heavy usage of a slow backing device. The + * only option is to throttle from reclaim context which is not ideal + * as there is no guarantee the dirtying process is throttled in the + * same way balance_dirty_pages() manages. + * + * This scales the number of dirty pages that must be under writeback + * before throttling depending on priority. It is a simple backoff + * function that has the most effect in the range DEF_PRIORITY to + * DEF_PRIORITY-2 which is the priority reclaim is considered to be + * in trouble and reclaim is considered to be in trouble. + * + * DEF_PRIORITY 100% isolated pages must be PageWriteback to throttle + * DEF_PRIORITY-1 50% must be PageWriteback + * DEF_PRIORITY-2 25% must be PageWriteback, kswapd in trouble + * ... + * DEF_PRIORITY-6 For SWAP_CLUSTER_MAX isolated pages, throttle if any + * isolated page is PageWriteback */ if (nr_writeback && nr_writeback >= (nr_taken >> (DEF_PRIORITY-priority))) wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10); -- 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>