Refactor Log Size Limit.

Original Log Size Limit implementation blocked all writes in case of
limit reached until the TXG is committed and the log is freed.  It
caused huge delays and following speed spikes in application writes.

This implementation instead smoothly throttles writes, using exactly
the same mechanism as used for dirty data.

Signed-off-by: Alexander Motin <[email protected]>
Sponsored-By: iXsystems, Inc.

Author: amotin

include/sys/dmu_tx.h

@@ -124,8 +124,8 @@ typedef struct dmu_tx_stats {
 	kstat_named_t dmu_tx_dirty_throttle;
 	kstat_named_t dmu_tx_dirty_delay;
 	kstat_named_t dmu_tx_dirty_over_max;
-	kstat_named_t dmu_tx_wrlog_over_max;
 	kstat_named_t dmu_tx_dirty_frees_delay;
+	kstat_named_t dmu_tx_wrlog_delay;
 	kstat_named_t dmu_tx_quota;
 } dmu_tx_stats_t;
 

include/sys/dsl_pool.h

@@ -164,7 +164,7 @@ uint64_t dsl_pool_unreserved_space(dsl_pool_t *dp,
     zfs_space_check_t slop_policy);
 uint64_t dsl_pool_deferred_space(dsl_pool_t *dp);
 void dsl_pool_wrlog_count(dsl_pool_t *dp, int64_t size, uint64_t txg);
-boolean_t dsl_pool_wrlog_over_max(dsl_pool_t *dp);
+boolean_t dsl_pool_need_wrlog_delay(dsl_pool_t *dp);
 void dsl_pool_dirty_space(dsl_pool_t *dp, int64_t space, dmu_tx_t *tx);
 void dsl_pool_undirty_space(dsl_pool_t *dp, int64_t space, uint64_t txg);
 void dsl_free(dsl_pool_t *dp, uint64_t txg, const blkptr_t *bpp);

man/man4/zfs.4

@@ -1098,10 +1098,9 @@ This should be less than
 .
 .It Sy zfs_wrlog_data_max Ns = Pq int
 The upper limit of write-transaction zil log data size in bytes.
-Once it is reached, write operation is blocked, until log data is cleared out
-after transaction group sync.
-Because of some overhead, it should be set
-at least 2 times the size of
+Write operations are throttled when approaching the limit until log data is
+cleared out after transaction group sync.
+Because of some overhead, it should be set at least 2 times the size of
 .Sy zfs_dirty_data_max
 .No to prevent harming normal write throughput.
 It also should be smaller than the size of the slog device if slog is present.

module/zfs/dmu_tx.c

@@ -53,8 +53,8 @@ dmu_tx_stats_t dmu_tx_stats = {
 	{ "dmu_tx_dirty_throttle",	KSTAT_DATA_UINT64 },
 	{ "dmu_tx_dirty_delay",		KSTAT_DATA_UINT64 },
 	{ "dmu_tx_dirty_over_max",	KSTAT_DATA_UINT64 },
-	{ "dmu_tx_wrlog_over_max",	KSTAT_DATA_UINT64 },
 	{ "dmu_tx_dirty_frees_delay",	KSTAT_DATA_UINT64 },
+	{ "dmu_tx_wrlog_delay",		KSTAT_DATA_UINT64 },
 	{ "dmu_tx_quota",		KSTAT_DATA_UINT64 },
 };
 
@@ -779,34 +779,49 @@ static void
 dmu_tx_delay(dmu_tx_t *tx, uint64_t dirty)
 {
 	dsl_pool_t *dp = tx->tx_pool;
-	uint64_t delay_min_bytes =
+	uint64_t delay_min_bytes, wrlog;
+	hrtime_t wakeup, tx_time = 0, now;
+
+	/* Calculate minimum transaction time for the dirty data amount. */
+	delay_min_bytes =
 	    zfs_dirty_data_max * zfs_delay_min_dirty_percent / 100;
-	hrtime_t wakeup, min_tx_time, now;
+	if (dirty > delay_min_bytes) {
+		/*
+		 * The caller has already waited until we are under the max.
+		 * We make them pass us the amount of dirty data so we don't
+		 * have to handle the case of it being >= the max, which
+		 * could cause a divide-by-zero if it's == the max.
+		 */
+		ASSERT3U(dirty, <, zfs_dirty_data_max);
 
-	if (dirty <= delay_min_bytes)
-		return;
+		tx_time = zfs_delay_scale * (dirty - delay_min_bytes) /
+		    (zfs_dirty_data_max - dirty);
+	}
 
-	/*
-	 * The caller has already waited until we are under the max.
-	 * We make them pass us the amount of dirty data so we don't
-	 * have to handle the case of it being >= the max, which could
-	 * cause a divide-by-zero if it's == the max.
-	 */
-	ASSERT3U(dirty, <, zfs_dirty_data_max);
+	/* Calculate minimum transaction time for the TX_WRITE log size. */
+	wrlog = aggsum_upper_bound(&dp->dp_wrlog_total);
+	delay_min_bytes =
+	    zfs_wrlog_data_max * zfs_delay_min_dirty_percent / 100;
+	if (wrlog >= zfs_wrlog_data_max) {
+		tx_time = zfs_delay_max_ns;
+	} else if (wrlog > delay_min_bytes) {
+		tx_time = MAX(zfs_delay_scale * (wrlog - delay_min_bytes) /
+		    (zfs_wrlog_data_max - wrlog), tx_time);
+	}
 
+	if (tx_time == 0)
+		return;
+
+	tx_time = MIN(tx_time, zfs_delay_max_ns);
 	now = gethrtime();
-	min_tx_time = zfs_delay_scale *
-	    (dirty - delay_min_bytes) / (zfs_dirty_data_max - dirty);
-	min_tx_time = MIN(min_tx_time, zfs_delay_max_ns);
-	if (now > tx->tx_start + min_tx_time)
+	if (now > tx->tx_start + tx_time)
 		return;
 
 	DTRACE_PROBE3(delay__mintime, dmu_tx_t *, tx, uint64_t, dirty,
-	    uint64_t, min_tx_time);
+	    uint64_t, tx_time);
 
 	mutex_enter(&dp->dp_lock);
-	wakeup = MAX(tx->tx_start + min_tx_time,
-	    dp->dp_last_wakeup + min_tx_time);
+	wakeup = MAX(tx->tx_start + tx_time, dp->dp_last_wakeup + tx_time);
 	dp->dp_last_wakeup = wakeup;
 	mutex_exit(&dp->dp_lock);
 
@@ -884,8 +899,9 @@ dmu_tx_try_assign(dmu_tx_t *tx, uint64_t txg_how)
 	}
 
 	if (!tx->tx_dirty_delayed &&
-	    dsl_pool_wrlog_over_max(tx->tx_pool)) {
-		DMU_TX_STAT_BUMP(dmu_tx_wrlog_over_max);
+	    dsl_pool_need_wrlog_delay(tx->tx_pool)) {
+		tx->tx_wait_dirty = B_TRUE;
+		DMU_TX_STAT_BUMP(dmu_tx_wrlog_delay);
 		return (SET_ERROR(ERESTART));
 	}
 

module/zfs/dsl_pool.c

@@ -105,9 +105,8 @@ int zfs_dirty_data_max_percent = 10;
 int zfs_dirty_data_max_max_percent = 25;
 
 /*
- * zfs_wrlog_data_max, the upper limit of TX_WRITE log data.
- * Once it is reached, write operation is blocked,
- * until log data is cleared out after txg sync.
+ * The upper limit of TX_WRITE log data.  Write operations are throttled
+ * when approaching the limit until log data is cleared out after txg sync.
  * It only counts TX_WRITE log with WR_COPIED or WR_NEED_COPY.
  */
 unsigned long zfs_wrlog_data_max = 0;
@@ -623,15 +622,18 @@ dsl_pool_wrlog_count(dsl_pool_t *dp, int64_t size, uint64_t txg)
 
 	/* Choose a value slightly bigger than min dirty sync bytes */
 	uint64_t sync_min =
-	    zfs_dirty_data_max * (zfs_dirty_data_sync_percent + 10) / 100;
+	    zfs_wrlog_data_max * (zfs_dirty_data_sync_percent + 10) / 200;
 	if (aggsum_compare(&dp->dp_wrlog_pertxg[txg & TXG_MASK], sync_min) > 0)
 		txg_kick(dp, txg);
 }
 
 boolean_t
-dsl_pool_wrlog_over_max(dsl_pool_t *dp)
+dsl_pool_need_wrlog_delay(dsl_pool_t *dp)
 {
-	return (aggsum_compare(&dp->dp_wrlog_total, zfs_wrlog_data_max) > 0);
+	uint64_t delay_min_bytes =
+	    zfs_wrlog_data_max * zfs_delay_min_dirty_percent / 100;
+
+	return (aggsum_compare(&dp->dp_wrlog_total, delay_min_bytes) > 0);
 }
 
 static void
@@ -641,6 +643,9 @@ dsl_pool_wrlog_clear(dsl_pool_t *dp, uint64_t txg)
 	delta = -(int64_t)aggsum_value(&dp->dp_wrlog_pertxg[txg & TXG_MASK]);
 	aggsum_add(&dp->dp_wrlog_pertxg[txg & TXG_MASK], delta);
 	aggsum_add(&dp->dp_wrlog_total, delta);
+	/* Compact per-CPU sums after the big change. */
+	(void) aggsum_value(&dp->dp_wrlog_pertxg[txg & TXG_MASK]);
+	(void) aggsum_value(&dp->dp_wrlog_total);
 }
 
 #ifdef ZFS_DEBUG