Introduce minimal ZIL block commit delay

Despite all optimizations, tests on actual hardware show that FreeBSD
kernel can't sleep for less then ~2us.  Similar tests on Linux show
~50us delay at least from nanosleep() (haven't tested inside kernel).
It means that on very fast log device ZIL may not be able to satisfy
zfs_commit_timeout_pct block commit timeout, increasing log latency
more than desired.

Handle that by introduction of zil_min_commit_timeout parameter,
specifying minimal timeout value where additional delays to aggregate
writes may be skipped.  Also skip delays if the LWB is more than 7/8
full, that often happens if I/O sizes are constant and match one of
LWB sizes.  Both things are applied only if there were no already
outstanding log blocks, that may indicate single-threaded workload,
that by definition can not benefit from the commit delays.

While there, add short time moving average to zl_last_lwb_latency to
make it more stable.

Tests of single-threaded 4KB writes to NVDIMM SLOG on FreeBSD show IOPS
increase by 9% instead of expected 5%.  For zfs_commit_timeout_pct of
1 there IOPS increase by 5.5% instead of expected 1%.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Prakash Surya <prakash.surya@delphix.com>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #14418

man/man4/zfs.4

@@ -2126,6 +2126,13 @@ On very fragmented pools, lowering this
 .Pq typically to Sy 36kB
 can improve performance.
 .
+.It Sy zil_min_commit_timeout Ns = Ns Sy 5000 Pq u64
+This sets the minimum delay in nanoseconds ZIL care to delay block commit,
+waiting for more records.
+If ZIL writes are too fast, kernel may not be able sleep for so short interval,
+increasing log latency above allowed by
+.Sy zfs_commit_timeout_pct .
+.
 .It Sy zil_nocacheflush Ns = Ns Sy 0 Ns | Ns 1 Pq int
 Disable the cache flush commands that are normally sent to disk by
 the ZIL after an LWB write has completed.

module/zfs/zil.c

@@ -91,6 +91,14 @@
  */
 int 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 unsigned long zil_min_commit_timeout = 5000;
+
 /*
  * See zil.h for more information about these fields.
  */
@@ -1155,7 +1163,8 @@ zil_lwb_flush_vdevs_done(zio_t *zio)
 	lwb->lwb_tx = NULL;
 
 	ASSERT3U(lwb->lwb_issued_timestamp, >, 0);
-	zilog->zl_last_lwb_latency = gethrtime() - lwb->lwb_issued_timestamp;
+	zilog->zl_last_lwb_latency = (zilog->zl_last_lwb_latency * 3 +
+	    gethrtime() - lwb->lwb_issued_timestamp) / 4;
 
 	lwb->lwb_root_zio = NULL;
 
@@ -2283,8 +2292,9 @@ zil_process_commit_list(zilog_t *zilog)
 	spa_t *spa = zilog->zl_spa;
 	list_t nolwb_itxs;
 	list_t nolwb_waiters;
-	lwb_t *lwb;
+	lwb_t *lwb, *plwb;
 	itx_t *itx;
+	boolean_t first = B_TRUE;
 
 	ASSERT(MUTEX_HELD(&zilog->zl_issuer_lock));
 
@@ -2306,6 +2316,9 @@ zil_process_commit_list(zilog_t *zilog)
 		ASSERT3S(lwb->lwb_state, !=, LWB_STATE_ISSUED);
 		ASSERT3S(lwb->lwb_state, !=, LWB_STATE_WRITE_DONE);
 		ASSERT3S(lwb->lwb_state, !=, LWB_STATE_FLUSH_DONE);
+		first = (lwb->lwb_state != LWB_STATE_OPENED) &&
+		    ((plwb = list_prev(&zilog->zl_lwb_list, lwb)) == NULL ||
+		    plwb->lwb_state == LWB_STATE_FLUSH_DONE);
 	}
 
 	while ((itx = list_head(&zilog->zl_itx_commit_list)) != NULL) {
@@ -2476,7 +2489,23 @@ zil_process_commit_list(zilog_t *zilog)
 		 * 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_sz - lwb->lwb_nused < lwb->lwb_sz / 8) {
+				lwb = zil_lwb_write_issue(zilog, lwb);
+				zilog->zl_cur_used = 0;
+				if (lwb == NULL)
+					zil_commit_writer_stall(zilog);
+			}
+		}
 	}
 }
 
@@ -3726,6 +3755,9 @@ EXPORT_SYMBOL(zil_set_logbias);
 ZFS_MODULE_PARAM(zfs, zfs_, commit_timeout_pct, INT, ZMOD_RW,
 	"ZIL block open timeout percentage");
 
+ZFS_MODULE_PARAM(zfs_zil, zil_, min_commit_timeout, ULONG, 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");