/* Rate limiting */
static uint64_t __calc_target_rate(struct cached_dev *dc)
{
struct cache_set *c = dc->disk.c;
/*
* This is the size of the cache, minus the amount used for
* flash-only devices
*/
uint64_t cache_sectors = c->nbuckets * c->sb.bucket_size -
atomic_long_read(&c->flash_dev_dirty_sectors);
/*
* Unfortunately there is no control of global dirty data. If the
* user states that they want 10% dirty data in the cache, and has,
* e.g., 5 backing volumes of equal size, we try and ensure each
* backing volume uses about 2% of the cache for dirty data.
*/
uint32_t bdev_share =
div64_u64(bdev_sectors(dc->bdev) << WRITEBACK_SHARE_SHIFT,
c->cached_dev_sectors);
uint64_t cache_dirty_target =
div_u64(cache_sectors * dc->writeback_percent, 100);
/* Ensure each backing dev gets at least one dirty share */
if (bdev_share < 1)
bdev_share = 1;
return (cache_dirty_target * bdev_share) >> WRITEBACK_SHARE_SHIFT;
}
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;
}
