diff options
Diffstat (limited to 'drivers/md/bcache/writeback.c')
| -rw-r--r-- | drivers/md/bcache/writeback.c | 91 |
1 files changed, 52 insertions, 39 deletions
diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c index 5e65a392287d..cac8678da5d0 100644 --- a/drivers/md/bcache/writeback.c +++ b/drivers/md/bcache/writeback.c @@ -25,48 +25,62 @@ static void __update_writeback_rate(struct cached_dev *dc) bcache_flash_devs_sectors_dirty(c); uint64_t cache_dirty_target = div_u64(cache_sectors * dc->writeback_percent, 100); - int64_t target = div64_u64(cache_dirty_target * bdev_sectors(dc->bdev), c->cached_dev_sectors); - /* PD controller */ - + /* + * PI controller: + * Figures out the amount that should be written per second. + * + * First, the error (number of sectors that are dirty beyond our + * target) is calculated. The error is accumulated (numerically + * integrated). + * + * Then, the proportional value and integral value are scaled + * based on configured values. These are stored as inverses to + * avoid fixed point math and to make configuration easy-- e.g. + * the default value of 40 for writeback_rate_p_term_inverse + * attempts to write at a rate that would retire all the dirty + * blocks in 40 seconds. + * + * The writeback_rate_i_inverse value of 10000 means that 1/10000th + * of the error is accumulated in the integral term per second. + * This acts as a slow, long-term average that is not subject to + * variations in usage like the p term. + */ int64_t dirty = bcache_dev_sectors_dirty(&dc->disk); - int64_t derivative = dirty - dc->disk.sectors_dirty_last; - int64_t proportional = dirty - target; - int64_t change; - - dc->disk.sectors_dirty_last = dirty; - - /* Scale to sectors per second */ - - proportional *= dc->writeback_rate_update_seconds; - proportional = div_s64(proportional, dc->writeback_rate_p_term_inverse); - - derivative = div_s64(derivative, dc->writeback_rate_update_seconds); - - derivative = ewma_add(dc->disk.sectors_dirty_derivative, derivative, - (dc->writeback_rate_d_term / - dc->writeback_rate_update_seconds) ?: 1, 0); - - derivative *= dc->writeback_rate_d_term; - derivative = div_s64(derivative, dc->writeback_rate_p_term_inverse); - - change = proportional + derivative; + int64_t error = dirty - target; + int64_t proportional_scaled = + div_s64(error, dc->writeback_rate_p_term_inverse); + int64_t integral_scaled, new_rate; + + if ((error < 0 && dc->writeback_rate_integral > 0) || + (error > 0 && time_before64(local_clock(), + dc->writeback_rate.next + NSEC_PER_MSEC))) { + /* + * Only decrease the integral term if it's more than + * zero. Only increase the integral term if the device + * is keeping up. (Don't wind up the integral + * ineffectively in either case). + * + * It's necessary to scale this by + * writeback_rate_update_seconds to keep the integral + * term dimensioned properly. + */ + dc->writeback_rate_integral += error * + dc->writeback_rate_update_seconds; + } - /* Don't increase writeback rate if the device isn't keeping up */ - if (change > 0 && - time_after64(local_clock(), - dc->writeback_rate.next + NSEC_PER_MSEC)) - change = 0; + integral_scaled = div_s64(dc->writeback_rate_integral, + dc->writeback_rate_i_term_inverse); - dc->writeback_rate.rate = - clamp_t(int64_t, (int64_t) dc->writeback_rate.rate + change, - 1, NSEC_PER_MSEC); + new_rate = clamp_t(int64_t, (proportional_scaled + integral_scaled), + dc->writeback_rate_minimum, NSEC_PER_MSEC); - dc->writeback_rate_proportional = proportional; - dc->writeback_rate_derivative = derivative; - dc->writeback_rate_change = change; + dc->writeback_rate_proportional = proportional_scaled; + dc->writeback_rate_integral_scaled = integral_scaled; + dc->writeback_rate_change = new_rate - dc->writeback_rate.rate; + dc->writeback_rate.rate = new_rate; dc->writeback_rate_target = target; } @@ -499,8 +513,6 @@ void bch_sectors_dirty_init(struct bcache_device *d) bch_btree_map_keys(&op.op, d->c, &KEY(op.inode, 0, 0), sectors_dirty_init_fn, 0); - - d->sectors_dirty_last = bcache_dev_sectors_dirty(d); } void bch_cached_dev_writeback_init(struct cached_dev *dc) @@ -514,10 +526,11 @@ void bch_cached_dev_writeback_init(struct cached_dev *dc) dc->writeback_percent = 10; dc->writeback_delay = 30; dc->writeback_rate.rate = 1024; + dc->writeback_rate_minimum = 1; dc->writeback_rate_update_seconds = 5; - dc->writeback_rate_d_term = 30; - dc->writeback_rate_p_term_inverse = 6000; + dc->writeback_rate_p_term_inverse = 40; + dc->writeback_rate_i_term_inverse = 10000; INIT_DELAYED_WORK(&dc->writeback_rate_update, update_writeback_rate); } |