diff options
Diffstat (limited to 'mm/page-writeback.c')
| -rw-r--r-- | mm/page-writeback.c | 83 |
1 files changed, 81 insertions, 2 deletions
diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 4b954c9fe84..1721b6523c0 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -777,6 +777,79 @@ static void global_update_bandwidth(unsigned long thresh, spin_unlock(&dirty_lock); } +/* + * Maintain bdi->dirty_ratelimit, the base dirty throttle rate. + * + * Normal bdi tasks will be curbed at or below it in long term. + * Obviously it should be around (write_bw / N) when there are N dd tasks. + */ +static void bdi_update_dirty_ratelimit(struct backing_dev_info *bdi, + unsigned long thresh, + unsigned long bg_thresh, + unsigned long dirty, + unsigned long bdi_thresh, + unsigned long bdi_dirty, + unsigned long dirtied, + unsigned long elapsed) +{ + unsigned long write_bw = bdi->avg_write_bandwidth; + unsigned long dirty_ratelimit = bdi->dirty_ratelimit; + unsigned long dirty_rate; + unsigned long task_ratelimit; + unsigned long balanced_dirty_ratelimit; + unsigned long pos_ratio; + + /* + * The dirty rate will match the writeout rate in long term, except + * when dirty pages are truncated by userspace or re-dirtied by FS. + */ + dirty_rate = (dirtied - bdi->dirtied_stamp) * HZ / elapsed; + + pos_ratio = bdi_position_ratio(bdi, thresh, bg_thresh, dirty, + bdi_thresh, bdi_dirty); + /* + * task_ratelimit reflects each dd's dirty rate for the past 200ms. + */ + task_ratelimit = (u64)dirty_ratelimit * + pos_ratio >> RATELIMIT_CALC_SHIFT; + task_ratelimit++; /* it helps rampup dirty_ratelimit from tiny values */ + + /* + * A linear estimation of the "balanced" throttle rate. The theory is, + * if there are N dd tasks, each throttled at task_ratelimit, the bdi's + * dirty_rate will be measured to be (N * task_ratelimit). So the below + * formula will yield the balanced rate limit (write_bw / N). + * + * Note that the expanded form is not a pure rate feedback: + * rate_(i+1) = rate_(i) * (write_bw / dirty_rate) (1) + * but also takes pos_ratio into account: + * rate_(i+1) = rate_(i) * (write_bw / dirty_rate) * pos_ratio (2) + * + * (1) is not realistic because pos_ratio also takes part in balancing + * the dirty rate. Consider the state + * pos_ratio = 0.5 (3) + * rate = 2 * (write_bw / N) (4) + * If (1) is used, it will stuck in that state! Because each dd will + * be throttled at + * task_ratelimit = pos_ratio * rate = (write_bw / N) (5) + * yielding + * dirty_rate = N * task_ratelimit = write_bw (6) + * put (6) into (1) we get + * rate_(i+1) = rate_(i) (7) + * + * So we end up using (2) to always keep + * rate_(i+1) ~= (write_bw / N) (8) + * regardless of the value of pos_ratio. As long as (8) is satisfied, + * pos_ratio is able to drive itself to 1.0, which is not only where + * the dirty count meet the setpoint, but also where the slope of + * pos_ratio is most flat and hence task_ratelimit is least fluctuated. + */ + balanced_dirty_ratelimit = div_u64((u64)task_ratelimit * write_bw, + dirty_rate | 1); + + bdi->dirty_ratelimit = max(balanced_dirty_ratelimit, 1UL); +} + void __bdi_update_bandwidth(struct backing_dev_info *bdi, unsigned long thresh, unsigned long bg_thresh, @@ -787,6 +860,7 @@ void __bdi_update_bandwidth(struct backing_dev_info *bdi, { unsigned long now = jiffies; unsigned long elapsed = now - bdi->bw_time_stamp; + unsigned long dirtied; unsigned long written; /* @@ -795,6 +869,7 @@ void __bdi_update_bandwidth(struct backing_dev_info *bdi, if (elapsed < BANDWIDTH_INTERVAL) return; + dirtied = percpu_counter_read(&bdi->bdi_stat[BDI_DIRTIED]); written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]); /* @@ -804,12 +879,16 @@ void __bdi_update_bandwidth(struct backing_dev_info *bdi, if (elapsed > HZ && time_before(bdi->bw_time_stamp, start_time)) goto snapshot; - if (thresh) + if (thresh) { global_update_bandwidth(thresh, dirty, now); - + bdi_update_dirty_ratelimit(bdi, thresh, bg_thresh, dirty, + bdi_thresh, bdi_dirty, + dirtied, elapsed); + } bdi_update_write_bandwidth(bdi, elapsed, written); snapshot: + bdi->dirtied_stamp = dirtied; bdi->written_stamp = written; bdi->bw_time_stamp = now; } |