writeback: introduce max-pause and pass-good dirty limits
The max-pause limit helps to keep the sleep time inside
balance_dirty_pages() within MAX_PAUSE=200ms. The 200ms max sleep means
per task rate limit of 8pages/200ms=160KB/s when dirty exceeded, which
normally is enough to stop dirtiers from continue pushing the dirty
pages high, unless there are a sufficient large number of slow dirtiers
(eg. 500 tasks doing 160KB/s will still sum up to 80MB/s, exceeding the
write bandwidth of a slow disk and hence accumulating more and more dirty
pages).
The pass-good limit helps to let go of the good bdi's in the presence of
a blocked bdi (ie. NFS server not responding) or slow USB disk which for
some reason build up a large number of initial dirty pages that refuse
to go away anytime soon.
For example, given two bdi's A and B and the initial state
bdi_thresh_A = dirty_thresh / 2
bdi_thresh_B = dirty_thresh / 2
bdi_dirty_A = dirty_thresh / 2
bdi_dirty_B = dirty_thresh / 2
Then A get blocked, after a dozen seconds
bdi_thresh_A = 0
bdi_thresh_B = dirty_thresh
bdi_dirty_A = dirty_thresh / 2
bdi_dirty_B = dirty_thresh / 2
The (bdi_dirty_B < bdi_thresh_B) test is now useless and the dirty pages
will be effectively throttled by condition (nr_dirty < dirty_thresh).
This has two problems:
(1) we lose the protections for light dirtiers
(2) balance_dirty_pages() effectively becomes IO-less because the
(bdi_nr_reclaimable > bdi_thresh) test won't be true. This is good
for IO, but balance_dirty_pages() loses an important way to break
out of the loop which leads to more spread out throttle delays.
DIRTY_PASSGOOD_AREA can eliminate the above issues. The only problem is,
DIRTY_PASSGOOD_AREA needs to be defined as 2 to fully cover the above
example while this patch uses the more conservative value 8 so as not to
surprise people with too many dirty pages than expected.
The max-pause limit won't noticeably impact the speed dirty pages are
knocked down when there is a sudden drop of global/bdi dirty thresholds.
Because the heavy dirties will be throttled below 160KB/s which is slow
enough. It does help to avoid long dirty throttle delays and especially
will make light dirtiers more responsive.
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index da95995..798842a 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -37,6 +37,11 @@
#include <trace/events/writeback.h>
/*
+ * Sleep at most 200ms at a time in balance_dirty_pages().
+ */
+#define MAX_PAUSE max(HZ/5, 1)
+
+/*
* Estimate write bandwidth at 200ms intervals.
*/
#define BANDWIDTH_INTERVAL max(HZ/5, 1)
@@ -399,6 +404,11 @@
return x + 1; /* Ensure that we never return 0 */
}
+static unsigned long hard_dirty_limit(unsigned long thresh)
+{
+ return max(thresh, global_dirty_limit);
+}
+
/*
* global_dirty_limits - background-writeback and dirty-throttling thresholds
*
@@ -723,6 +733,29 @@
io_schedule_timeout(pause);
trace_balance_dirty_wait(bdi);
+ dirty_thresh = hard_dirty_limit(dirty_thresh);
+ /*
+ * max-pause area. If dirty exceeded but still within this
+ * area, no need to sleep for more than 200ms: (a) 8 pages per
+ * 200ms is typically more than enough to curb heavy dirtiers;
+ * (b) the pause time limit makes the dirtiers more responsive.
+ */
+ if (nr_dirty < dirty_thresh +
+ dirty_thresh / DIRTY_MAXPAUSE_AREA &&
+ time_after(jiffies, start_time + MAX_PAUSE))
+ break;
+ /*
+ * pass-good area. When some bdi gets blocked (eg. NFS server
+ * not responding), or write bandwidth dropped dramatically due
+ * to concurrent reads, or dirty threshold suddenly dropped and
+ * the dirty pages cannot be brought down anytime soon (eg. on
+ * slow USB stick), at least let go of the good bdi's.
+ */
+ if (nr_dirty < dirty_thresh +
+ dirty_thresh / DIRTY_PASSGOOD_AREA &&
+ bdi_dirty < bdi_thresh)
+ break;
+
/*
* Increase the delay for each loop, up to our previous
* default of taking a 100ms nap.