ZIL: Detect single-threaded workloads

... by checking that previous block is fully written and flushed.
It allows to skip commit delays since we can give up on aggregation
in that case.  This removes zil_min_commit_timeout parameter, since
for single-threaded workloads it is not needed at all, while on very
fast devices even some multi-threaded workloads may get detected as
single-threaded and still bypass the wait.  To give multi-threaded
workloads more aggregation chances increase zfs_commit_timeout_pct
from 5 to 10%, as they should suffer less from additional latency.

Also single-threaded workloads detection allows in perspective better
prediction of the next block size.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Prakash Surya <[email protected]>
Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.
Closes #15381 

1 files changed, 40 insertions, 51 deletions

diff --git a/module/zfs/zil.c b/module/zfs/zil.c
index 5642f082b..8742fc662 100644
--- a/module/zfs/zil.c
+++ b/module/zfs/zil.c
@@ -91,15 +91,7 @@
  * committed to stable storage. Please refer to the zil_commit_waiter()
  * function (and the comments within it) for more details.
  */
-static uint_t zfs_commit_timeout_pct = 5;
-
-/*
- * Minimal time we care to delay commit waiting for more ZIL records.
- * At least FreeBSD kernel can't sleep for less than 2us at its best.
- * So requests to sleep for less then 5us is a waste of CPU time with
- * a risk of significant log latency increase due to oversleep.
- */
-static uint64_t zil_min_commit_timeout = 5000;
+static uint_t zfs_commit_timeout_pct = 10;
 
 /*
  * See zil.h for more information about these fields.
@@ -2732,6 +2724,19 @@ zil_commit_writer_stall(zilog_t *zilog)
 	ASSERT(list_is_empty(&zilog->zl_lwb_list));
 }
 
+static void
+zil_burst_done(zilog_t *zilog)
+{
+	if (!list_is_empty(&zilog->zl_itx_commit_list) ||
+	    zilog->zl_cur_used == 0)
+		return;
+
+	if (zilog->zl_parallel)
+		zilog->zl_parallel--;
+
+	zilog->zl_cur_used = 0;
+}
+
 /*
  * This function will traverse the commit list, creating new lwbs as
  * needed, and committing the itxs from the commit list to these newly
@@ -2746,7 +2751,6 @@ zil_process_commit_list(zilog_t *zilog, zil_commit_waiter_t *zcw, list_t *ilwbs)
 	list_t nolwb_waiters;
 	lwb_t *lwb, *plwb;
 	itx_t *itx;
-	boolean_t first = B_TRUE;
 
 	ASSERT(MUTEX_HELD(&zilog->zl_issuer_lock));
 
@@ -2772,9 +2776,22 @@ zil_process_commit_list(zilog_t *zilog, zil_commit_waiter_t *zcw, list_t *ilwbs)
 		zil_commit_activate_saxattr_feature(zilog);
 		ASSERT(lwb->lwb_state == LWB_STATE_NEW ||
 		    lwb->lwb_state == LWB_STATE_OPENED);
-		first = (lwb->lwb_state == LWB_STATE_NEW) &&
-		    ((plwb = list_prev(&zilog->zl_lwb_list, lwb)) == NULL ||
-		    plwb->lwb_state == LWB_STATE_FLUSH_DONE);
+
+		/*
+		 * If the lwb is still opened, it means the workload is really
+		 * multi-threaded and we won the chance of write aggregation.
+		 * If it is not opened yet, but previous lwb is still not
+		 * flushed, it still means the workload is multi-threaded, but
+		 * there was too much time between the commits to aggregate, so
+		 * we try aggregation next times, but without too much hopes.
+		 */
+		if (lwb->lwb_state == LWB_STATE_OPENED) {
+			zilog->zl_parallel = ZIL_BURSTS;
+		} else if ((plwb = list_prev(&zilog->zl_lwb_list, lwb))
+		    != NULL && plwb->lwb_state != LWB_STATE_FLUSH_DONE) {
+			zilog->zl_parallel = MAX(zilog->zl_parallel,
+			    ZIL_BURSTS / 2);
+		}
 	}
 
 	while ((itx = list_remove_head(&zilog->zl_itx_commit_list)) != NULL) {
@@ -2849,7 +2866,7 @@ zil_process_commit_list(zilog_t *zilog, zil_commit_waiter_t *zcw, list_t *ilwbs)
 					 * Our lwb is done, leave the rest of
 					 * itx list to somebody else who care.
 					 */
-					first = B_FALSE;
+					zilog->zl_parallel = ZIL_BURSTS;
 					break;
 				}
 			} else {
@@ -2941,28 +2958,15 @@ zil_process_commit_list(zilog_t *zilog, zil_commit_waiter_t *zcw, list_t *ilwbs)
 		 * try and pack as many itxs into as few lwbs as
 		 * possible, without significantly impacting the latency
 		 * of each individual itx.
-		 *
-		 * If we had no already running or open LWBs, it can be
-		 * the workload is single-threaded.  And if the ZIL write
-		 * latency is very small or if the LWB is almost full, it
-		 * may be cheaper to bypass the delay.
 		 */
-		if (lwb->lwb_state == LWB_STATE_OPENED && first) {
-			hrtime_t sleep = zilog->zl_last_lwb_latency *
-			    zfs_commit_timeout_pct / 100;
-			if (sleep < zil_min_commit_timeout ||
-			    lwb->lwb_nmax - lwb->lwb_nused <
-			    lwb->lwb_nmax / 8) {
-				list_insert_tail(ilwbs, lwb);
-				lwb = zil_lwb_write_close(zilog, lwb,
-				    LWB_STATE_NEW);
-				zilog->zl_cur_used = 0;
-				if (lwb == NULL) {
-					while ((lwb = list_remove_head(ilwbs))
-					    != NULL)
-						zil_lwb_write_issue(zilog, lwb);
-					zil_commit_writer_stall(zilog);
-				}
+		if (lwb->lwb_state == LWB_STATE_OPENED && !zilog->zl_parallel) {
+			list_insert_tail(ilwbs, lwb);
+			lwb = zil_lwb_write_close(zilog, lwb, LWB_STATE_NEW);
+			zil_burst_done(zilog);
+			if (lwb == NULL) {
+				while ((lwb = list_remove_head(ilwbs)) != NULL)
+					zil_lwb_write_issue(zilog, lwb);
+				zil_commit_writer_stall(zilog);
 			}
 		}
 	}
@@ -3120,19 +3124,7 @@ zil_commit_waiter_timeout(zilog_t *zilog, zil_commit_waiter_t *zcw)
 
 	ASSERT3S(lwb->lwb_state, ==, LWB_STATE_CLOSED);
 
-	/*
-	 * Since the lwb's zio hadn't been issued by the time this thread
-	 * reached its timeout, we reset the zilog's "zl_cur_used" field
-	 * to influence the zil block size selection algorithm.
-	 *
-	 * By having to issue the lwb's zio here, it means the size of the
-	 * lwb was too large, given the incoming throughput of itxs.  By
-	 * setting "zl_cur_used" to zero, we communicate this fact to the
-	 * block size selection algorithm, so it can take this information
-	 * into account, and potentially select a smaller size for the
-	 * next lwb block that is allocated.
-	 */
-	zilog->zl_cur_used = 0;
+	zil_burst_done(zilog);
 
 	if (nlwb == NULL) {
 		/*
@@ -4250,9 +4242,6 @@ EXPORT_SYMBOL(zil_kstat_values_update);
 ZFS_MODULE_PARAM(zfs, zfs_, commit_timeout_pct, UINT, ZMOD_RW,
 	"ZIL block open timeout percentage");
 
-ZFS_MODULE_PARAM(zfs_zil, zil_, min_commit_timeout, U64, ZMOD_RW,
-	"Minimum delay we care for ZIL block commit");
-
 ZFS_MODULE_PARAM(zfs_zil, zil_, replay_disable, INT, ZMOD_RW,
 	"Disable intent logging replay");