[PATCH 1/3] writeback: max, min and target dirty pause time
From: Wu Fengguang
Date: Mon Dec 12 2011 - 21:26:39 EST
Control the pause time and the call intervals to balance_dirty_pages()
with three parameters:
1) max_pause, limited by bdi_dirty and MAX_PAUSE
2) the target pause time, grows with the number of dd tasks
and is normally limited by max_pause/2
3) the minimal pause, set to half the target pause
and is used to skip short sleeps and accumulate them into bigger ones
The typical behaviors after patch:
- if ever task_ratelimit is far below dirty_ratelimit, the pause time
will remain constant at max_pause and nr_dirtied_pause will be
fluctuating with task_ratelimit
- in the normal cases, nr_dirtied_pause will remain stable (keep in the
same pace with dirty_ratelimit) and the pause time will be fluctuating
with task_ratelimit
In summary, someone has to fluctuate with task_ratelimit, because
task_ratelimit = nr_dirtied_pause / pause
We normally prefer a stable nr_dirtied_pause, until reaching max_pause.
The notable behavior changes are:
- in stable workloads, there will no longer be sudden big trajectory
switching of nr_dirtied_pause as concerned by Peter. It will be as
smooth as dirty_ratelimit and changing proportionally with it (as
always, assuming bdi bandwidth does not fluctuate across 2^N lines,
otherwise nr_dirtied_pause will show up in 2+ parallel trajectories)
- in the rare cases when something keeps task_ratelimit far below
dirty_ratelimit, the smoothness can no longer be retained and
nr_dirtied_pause will be "dancing" with task_ratelimit. This fixes a
(not that destructive but still not good) bug that
dirty_ratelimit gets brought down undesirably
<= balanced_dirty_ratelimit is under estimated
<= weakly executed task_ratelimit
<= pause goes too large and gets trimmed down to max_pause
<= nr_dirtied_pause (based on dirty_ratelimit) is set too large
<= dirty_ratelimit being much larger than task_ratelimit
- introduce min_pause to avoid small pause sleeps
- when pause is trimmed down to max_pause, try to compensate it at the
next pause time
The "refactor" type of changes are:
The max_pause equation is slightly transformed to make it slightly more
efficient.
We now scale target_pause by (N * 10ms) on 2^N concurrent tasks, which
is effectively equal to the original scaling max_pause by (N * 20ms)
because the original code does implicit target_pause ~= max_pause / 2.
Based on the same implicit ratio, target_pause starts with 10ms on 1 dd.
CC: Jan Kara <jack@xxxxxxx>
CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
mm/page-writeback.c | 125 +++++++++++++++++++++++++++---------------
1 file changed, 81 insertions(+), 44 deletions(-)
--- linux-next.orig/mm/page-writeback.c 2011-12-11 21:49:27.000000000 +0800
+++ linux-next/mm/page-writeback.c 2011-12-12 19:22:49.000000000 +0800
@@ -955,40 +955,81 @@ static unsigned long dirty_poll_interval
return 1;
}
-static unsigned long bdi_max_pause(struct backing_dev_info *bdi,
- unsigned long bdi_dirty)
+static long bdi_max_pause(struct backing_dev_info *bdi,
+ unsigned long bdi_dirty)
{
- unsigned long bw = bdi->avg_write_bandwidth;
- unsigned long hi = ilog2(bw);
- unsigned long lo = ilog2(bdi->dirty_ratelimit);
- unsigned long t;
+ long bw = bdi->avg_write_bandwidth;
+ long t;
- /* target for 20ms max pause on 1-dd case */
- t = HZ / 50;
+ /*
+ * Limit pause time for small memory systems. If sleeping for too long
+ * time, a small pool of dirty/writeback pages may go empty and disk go
+ * idle.
+ *
+ * 8 serves as the safety ratio.
+ */
+ t = bdi_dirty / (1 + bw / roundup_pow_of_two(1 + HZ / 8));
+ t++;
+
+ return min_t(long, t, MAX_PAUSE);
+}
+
+static long bdi_min_pause(struct backing_dev_info *bdi,
+ long max_pause,
+ unsigned long task_ratelimit,
+ unsigned long dirty_ratelimit,
+ int *nr_dirtied_pause)
+{
+ long hi = ilog2(bdi->avg_write_bandwidth);
+ long lo = ilog2(bdi->dirty_ratelimit);
+ long t; /* target pause */
+ long pause; /* estimated next pause */
+ int pages; /* target nr_dirtied_pause */
+
+ /* target for 10ms pause on 1-dd case */
+ t = max(1, HZ / 100);
/*
* Scale up pause time for concurrent dirtiers in order to reduce CPU
* overheads.
*
- * (N * 20ms) on 2^N concurrent tasks.
+ * (N * 10ms) on 2^N concurrent tasks.
*/
if (hi > lo)
- t += (hi - lo) * (20 * HZ) / 1024;
+ t += (hi - lo) * (10 * HZ) / 1024;
/*
- * Limit pause time for small memory systems. If sleeping for too long
- * time, a small pool of dirty/writeback pages may go empty and disk go
- * idle.
+ * This is a bit convoluted. We try to base the next nr_dirtied_pause
+ * on the much more stable dirty_ratelimit. However the next pause time
+ * will be computed based on task_ratelimit and the two rate limits may
+ * depart considerably at some time. Especially if task_ratelimit goes
+ * below dirty_ratelimit/2 and the target pause is max_pause, the next
+ * pause time will be max_pause*2 _trimmed down_ to max_pause. As a
+ * result task_ratelimit won't be executed faithfully, which could
+ * eventually bring down dirty_ratelimit.
*
- * 8 serves as the safety ratio.
+ * We apply two rules to fix it up:
+ * 1) try to estimate the next pause time and if necessary, use a lower
+ * nr_dirtied_pause so as not to exceed max_pause. When this happens,
+ * nr_dirtied_pause will be "dancing" with task_ratelimit.
+ * 2) limit the target pause time to max_pause/2, so that the normal
+ * small fluctuations of task_ratelimit won't trigger rule (1) and
+ * nr_dirtied_pause will remain as stable as dirty_ratelimit.
*/
- t = min(t, bdi_dirty * HZ / (8 * bw + 1));
+ t = min(t, 1 + max_pause / 2);
+ pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
+
+ pause = HZ * pages / (task_ratelimit + 1);
+ if (pause > max_pause) {
+ t = max_pause;
+ pages = task_ratelimit * t / roundup_pow_of_two(HZ);
+ }
+ *nr_dirtied_pause = pages;
/*
- * The pause time will be settled within range (max_pause/4, max_pause).
- * Apply a minimal value of 4 to get a non-zero max_pause/4.
+ * The minimal pause time will normally be half the target pause time.
*/
- return clamp_val(t, 4, MAX_PAUSE);
+ return 1 + t / 2;
}
/*
@@ -1010,11 +1051,13 @@ static void balance_dirty_pages(struct a
unsigned long dirty_thresh;
unsigned long bdi_thresh;
long period;
- long pause = 0;
- long uninitialized_var(max_pause);
+ long pause;
+ long max_pause;
+ long min_pause;
+ int nr_dirtied_pause;
bool dirty_exceeded = false;
unsigned long task_ratelimit;
- unsigned long uninitialized_var(dirty_ratelimit);
+ unsigned long dirty_ratelimit;
unsigned long pos_ratio;
struct backing_dev_info *bdi = mapping->backing_dev_info;
unsigned long start_time = jiffies;
@@ -1044,6 +1087,8 @@ static void balance_dirty_pages(struct a
if (nr_dirty <= freerun) {
current->dirty_paused_when = now;
current->nr_dirtied = 0;
+ current->nr_dirtied_pause =
+ dirty_poll_interval(nr_dirty, dirty_thresh);
break;
}
@@ -1094,14 +1139,17 @@ static void balance_dirty_pages(struct a
nr_dirty, bdi_thresh, bdi_dirty,
start_time);
- max_pause = bdi_max_pause(bdi, bdi_dirty);
-
dirty_ratelimit = bdi->dirty_ratelimit;
pos_ratio = bdi_position_ratio(bdi, dirty_thresh,
background_thresh, nr_dirty,
bdi_thresh, bdi_dirty);
task_ratelimit = ((u64)dirty_ratelimit * pos_ratio) >>
RATELIMIT_CALC_SHIFT;
+ max_pause = bdi_max_pause(bdi, bdi_dirty);
+ min_pause = bdi_min_pause(bdi, max_pause,
+ task_ratelimit, dirty_ratelimit,
+ &nr_dirtied_pause);
+
if (unlikely(task_ratelimit == 0)) {
period = max_pause;
pause = max_pause;
@@ -1118,7 +1166,7 @@ static void balance_dirty_pages(struct a
* future periods by updating the virtual time; otherwise just
* do a reset, as it may be a light dirtier.
*/
- if (unlikely(pause <= 0)) {
+ if (pause < min_pause) {
trace_balance_dirty_pages(bdi,
dirty_thresh,
background_thresh,
@@ -1129,7 +1177,7 @@ static void balance_dirty_pages(struct a
task_ratelimit,
pages_dirtied,
period,
- pause,
+ min(pause, 0L),
start_time);
if (pause < -HZ) {
current->dirty_paused_when = now;
@@ -1137,11 +1185,15 @@ static void balance_dirty_pages(struct a
} else if (period) {
current->dirty_paused_when += period;
current->nr_dirtied = 0;
- }
- pause = 1; /* avoid resetting nr_dirtied_pause below */
+ } else if (current->nr_dirtied_pause <= pages_dirtied)
+ current->nr_dirtied_pause += pages_dirtied;
break;
}
- pause = min(pause, max_pause);
+ if (unlikely(pause > max_pause)) {
+ /* for occasional dropped task_ratelimit */
+ now += min(pause - max_pause, max_pause);
+ pause = max_pause;
+ }
pause:
trace_balance_dirty_pages(bdi,
@@ -1161,6 +1213,7 @@ pause:
current->dirty_paused_when = now + pause;
current->nr_dirtied = 0;
+ current->nr_dirtied_pause = nr_dirtied_pause;
/*
* This is typically equal to (nr_dirty < dirty_thresh) and can
@@ -1189,22 +1242,6 @@ pause:
if (!dirty_exceeded && bdi->dirty_exceeded)
bdi->dirty_exceeded = 0;
- if (pause == 0) { /* in freerun area */
- current->nr_dirtied_pause =
- dirty_poll_interval(nr_dirty, dirty_thresh);
- } else if (period <= max_pause / 4 &&
- pages_dirtied >= current->nr_dirtied_pause) {
- current->nr_dirtied_pause = clamp_val(
- dirty_ratelimit * (max_pause / 2) / HZ,
- pages_dirtied + pages_dirtied / 8,
- pages_dirtied * 4);
- } else if (pause >= max_pause) {
- current->nr_dirtied_pause = 1 | clamp_val(
- dirty_ratelimit * (max_pause / 2) / HZ,
- pages_dirtied / 4,
- pages_dirtied - pages_dirtied / 8);
- }
-
if (writeback_in_progress(bdi))
return;
--
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