static void __update_writeback_rate(struct cached_dev *dc)
{
/*
* 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 target = __calc_target_rate(dc);
int64_t dirty = bcache_dev_sectors_dirty(&dc->disk);
int64_t error = dirty - target;
int64_t proportional_scaled =
div_s64(error, dc->writeback_rate_p_term_inverse);
int64_t integral_scaled;
uint32_t 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;
}
integral_scaled = div_s64(dc->writeback_rate_integral,
dc->writeback_rate_i_term_inverse);
new_rate = clamp_t(int32_t, (proportional_scaled + integral_scaled),
dc->writeback_rate_minimum, NSEC_PER_SEC);
dc->writeback_rate_proportional = proportional_scaled;
dc->writeback_rate_integral_scaled = integral_scaled;
dc->writeback_rate_change = new_rate -
atomic_long_read(&dc->writeback_rate.rate);
atomic_long_set(&dc->writeback_rate.rate, new_rate);
dc->writeback_rate_target = target;
}
static void __update_writeback_rate(struct cached_dev *dc)
{
struct cache_set *c = dc->disk.c;
uint64_t cache_sectors = c->nbuckets * c->sb.bucket_size;
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 */
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;
/* 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;
dc->writeback_rate.rate =
clamp_t(int64_t, (int64_t) dc->writeback_rate.rate + change,
1, NSEC_PER_MSEC);
dc->writeback_rate_proportional = proportional;
dc->writeback_rate_derivative = derivative;
dc->writeback_rate_change = change;
dc->writeback_rate_target = target;
}
