From d7831a0bdf06b9f722b947bb0c205ff7d77cebd8 Mon Sep 17 00:00:00 2001 From: Richard Kennedy Date: Tue, 30 Jun 2009 11:41:35 -0700 Subject: mm: prevent balance_dirty_pages() from doing too much work balance_dirty_pages can overreact and move all of the dirty pages to writeback unnecessarily. balance_dirty_pages makes its decision to throttle based on the number of dirty plus writeback pages that are over the calculated limit,so it will continue to move pages even when there are plenty of pages in writeback and less than the threshold still dirty. This allows it to overshoot its limits and move all the dirty pages to writeback while waiting for the drives to catch up and empty the writeback list. A simple fio test easily demonstrates this problem. fio --name=f1 --directory=/disk1 --size=2G -rw=write --name=f2 --directory=/disk2 --size=1G --rw=write --startdelay=10 This is the simplest fix I could find, but I'm not entirely sure that it alone will be enough for all cases. But it certainly is an improvement on my desktop machine writing to 2 disks. Do we need something more for machines with large arrays where bdi_threshold * number_of_drives is greater than the dirty_ratio ? Signed-off-by: Richard Kennedy Acked-by: Peter Zijlstra Cc: Jens Axboe Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/page-writeback.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 7b0dcea..7687879 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -541,8 +541,11 @@ static void balance_dirty_pages(struct address_space *mapping) * filesystems (i.e. NFS) in which data may have been * written to the server's write cache, but has not yet * been flushed to permanent storage. + * Only move pages to writeback if this bdi is over its + * threshold otherwise wait until the disk writes catch + * up. */ - if (bdi_nr_reclaimable) { + if (bdi_nr_reclaimable > bdi_thresh) { writeback_inodes(&wbc); pages_written += write_chunk - wbc.nr_to_write; get_dirty_limits(&background_thresh, &dirty_thresh, -- cgit v1.1