writeback: max, min and target dirty pause time

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@suse.cz>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
This commit is contained in:
Wu Fengguang 2011-11-30 11:08:55 -06:00
parent 83712358ba
commit 7ccb9ad536

View file

@ -962,25 +962,11 @@ static unsigned long dirty_poll_interval(unsigned long dirty,
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;
/* target for 20ms max pause on 1-dd case */
t = HZ / 50;
/*
* Scale up pause time for concurrent dirtiers in order to reduce CPU
* overheads.
*
* (N * 20ms) on 2^N concurrent tasks.
*/
if (hi > lo)
t += (hi - lo) * (20 * HZ) / 1024;
long bw = bdi->avg_write_bandwidth;
long t;
/*
* Limit pause time for small memory systems. If sleeping for too long
@ -989,13 +975,68 @@ static unsigned long bdi_max_pause(struct backing_dev_info *bdi,
*
* 8 serves as the safety ratio.
*/
t = min(t, bdi_dirty * HZ / (8 * bw + 1));
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);
/*
* 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.
* Scale up pause time for concurrent dirtiers in order to reduce CPU
* overheads.
*
* (N * 10ms) on 2^N concurrent tasks.
*/
return clamp_val(t, 4, MAX_PAUSE);
if (hi > lo)
t += (hi - lo) * (10 * HZ) / 1024;
/*
* 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.
*
* 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, 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 minimal pause time will normally be half the target pause time.
*/
return 1 + t / 2;
}
/*
@ -1017,11 +1058,13 @@ static void balance_dirty_pages(struct address_space *mapping,
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;
@ -1051,6 +1094,8 @@ static void balance_dirty_pages(struct address_space *mapping,
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;
}
@ -1101,14 +1146,17 @@ static void balance_dirty_pages(struct address_space *mapping,
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;
@ -1125,7 +1173,7 @@ static void balance_dirty_pages(struct address_space *mapping,
* 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,
@ -1136,7 +1184,7 @@ static void balance_dirty_pages(struct address_space *mapping,
task_ratelimit,
pages_dirtied,
period,
pause,
min(pause, 0L),
start_time);
if (pause < -HZ) {
current->dirty_paused_when = now;
@ -1144,11 +1192,15 @@ static void balance_dirty_pages(struct address_space *mapping,
} 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,
@ -1168,6 +1220,7 @@ 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
@ -1196,22 +1249,6 @@ 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;