@@ -323,12 +323,6 @@ struct cached_dev {
struct bch_ratelimit writeback_rate;
struct delayed_work writeback_rate_update;
- /*
- * Internal to the writeback code, so read_dirty() can keep track of
- * where it's at.
- */
- sector_t last_read;
-
/* Limit number of writeback bios in flight */
struct semaphore in_flight;
struct task_struct *writeback_thread;
@@ -248,10 +248,25 @@ static void read_dirty_submit(struct closure *cl)
continue_at(cl, write_dirty, io->dc->writeback_write_wq);
}
+static inline bool keys_contiguous(struct cached_dev *dc,
+ struct keybuf_key *first, struct keybuf_key *second)
+{
+ if (KEY_INODE(&second->key) != KEY_INODE(&first->key))
+ return false;
+
+ if (KEY_OFFSET(&second->key) !=
+ KEY_OFFSET(&first->key) + KEY_SIZE(&first->key))
+ return false;
+
+ return true;
+}
+
static void read_dirty(struct cached_dev *dc)
{
unsigned delay = 0;
- struct keybuf_key *w;
+ struct keybuf_key *next, *keys[MAX_WRITEBACKS_IN_PASS], *w;
+ size_t size;
+ int nk, i;
struct dirty_io *io;
struct closure cl;
@@ -262,45 +277,87 @@ static void read_dirty(struct cached_dev *dc)
* mempools.
*/
- while (!kthread_should_stop()) {
-
- w = bch_keybuf_next(&dc->writeback_keys);
- if (!w)
- break;
-
- BUG_ON(ptr_stale(dc->disk.c, &w->key, 0));
-
- if (KEY_START(&w->key) != dc->last_read ||
- jiffies_to_msecs(delay) > 50)
- while (!kthread_should_stop() && delay)
- delay = schedule_timeout_interruptible(delay);
-
- dc->last_read = KEY_OFFSET(&w->key);
-
- io = kzalloc(sizeof(struct dirty_io) + sizeof(struct bio_vec)
- * DIV_ROUND_UP(KEY_SIZE(&w->key), PAGE_SECTORS),
- GFP_KERNEL);
- if (!io)
- goto err;
-
- w->private = io;
- io->dc = dc;
-
- dirty_init(w);
- bio_set_op_attrs(&io->bio, REQ_OP_READ, 0);
- io->bio.bi_iter.bi_sector = PTR_OFFSET(&w->key, 0);
- bio_set_dev(&io->bio, PTR_CACHE(dc->disk.c, &w->key, 0)->bdev);
- io->bio.bi_end_io = read_dirty_endio;
-
- if (bio_alloc_pages(&io->bio, GFP_KERNEL))
- goto err_free;
-
- trace_bcache_writeback(&w->key);
+ next = bch_keybuf_next(&dc->writeback_keys);
+
+ while (!kthread_should_stop() && next) {
+ size = 0;
+ nk = 0;
+
+ do {
+ BUG_ON(ptr_stale(dc->disk.c, &next->key, 0));
+
+ /*
+ * Don't combine too many operations, even if they
+ * are all small.
+ */
+ if (nk >= MAX_WRITEBACKS_IN_PASS)
+ break;
+
+ /*
+ * If the current operation is very large, don't
+ * further combine operations.
+ */
+ if (size >= MAX_WRITESIZE_IN_PASS)
+ break;
+
+ /*
+ * Operations are only eligible to be combined
+ * if they are contiguous.
+ *
+ * TODO: add a heuristic willing to fire a
+ * certain amount of non-contiguous IO per pass,
+ * so that we can benefit from backing device
+ * command queueing.
+ */
+ if (nk != 0 && !keys_contiguous(dc, keys[nk-1], next))
+ break;
+
+ size += KEY_SIZE(&next->key);
+ keys[nk++] = next;
+ } while ((next = bch_keybuf_next(&dc->writeback_keys)));
+
+ /* Now we have gathered a set of 1..5 keys to write back. */
+
+ for (i = 0; i < nk; i++) {
+ w = keys[i];
+
+ io = kzalloc(sizeof(struct dirty_io) +
+ sizeof(struct bio_vec) *
+ DIV_ROUND_UP(KEY_SIZE(&w->key), PAGE_SECTORS),
+ GFP_KERNEL);
+ if (!io)
+ goto err;
+
+ w->private = io;
+ io->dc = dc;
+
+ dirty_init(w);
+ bio_set_op_attrs(&io->bio, REQ_OP_READ, 0);
+ io->bio.bi_iter.bi_sector = PTR_OFFSET(&w->key, 0);
+ bio_set_dev(&io->bio,
+ PTR_CACHE(dc->disk.c, &w->key, 0)->bdev);
+ io->bio.bi_end_io = read_dirty_endio;
+
+ if (bio_alloc_pages(&io->bio, GFP_KERNEL))
+ goto err_free;
+
+ trace_bcache_writeback(&w->key);
+
+ down(&dc->in_flight);
+
+ /* We've acquired a semaphore for the maximum
+ * simultaneous number of writebacks; from here
+ * everything happens asynchronously.
+ */
+ closure_call(&io->cl, read_dirty_submit, NULL, &cl);
+ }
- down(&dc->in_flight);
- closure_call(&io->cl, read_dirty_submit, NULL, &cl);
+ delay = writeback_delay(dc, size);
- delay = writeback_delay(dc, KEY_SIZE(&w->key));
+ while (!kthread_should_stop() && delay) {
+ schedule_timeout_interruptible(delay);
+ delay = writeback_delay(dc, 0);
+ }
}
if (0) {
@@ -5,6 +5,9 @@
#define CUTOFF_WRITEBACK 40
#define CUTOFF_WRITEBACK_SYNC 70
+#define MAX_WRITEBACKS_IN_PASS 5
+#define MAX_WRITESIZE_IN_PASS 5000 /* *512b */
+
static inline uint64_t bcache_dev_sectors_dirty(struct bcache_device *d)
{
uint64_t i, ret = 0;
Previously, there was some logic that attempted to immediately issue writeback of backing-contiguous blocks when the writeback rate was fast. The previous logic did not have any limits on the aggregate size it would issue, nor the number of keys it would combine at once. It would also discard the chance to do a contiguous write when the writeback rate was low-- e.g. at "background" writeback of target rate = 8, it would not combine two adjacent 4k writes and would instead seek the disk twice. This patch imposes limits and explicitly understands the size of contiguous I/O during issue. It also will combine contiguous I/O in all circumstances, not just when writeback is requested to be relatively fast. It is a win on its own, but also lays the groundwork for skip writes to short keys to make the I/O more sequential/contiguous. It also gets ready to start using blk_*_plug, and to allow issuing of non-contig I/O in parallel if requested by the user (to make use of disk throughput benefits available from higher queue depths). This patch fixes a previous version where the contiguous information was not calculated properly. Signed-off-by: Michael Lyle <mlyle@lyle.org> --- drivers/md/bcache/bcache.h | 6 -- drivers/md/bcache/writeback.c | 133 ++++++++++++++++++++++++++++++------------ drivers/md/bcache/writeback.h | 3 + 3 files changed, 98 insertions(+), 44 deletions(-)