Fixes for the DMU free throttle

This patch fixes 2 issues with the DMU free throttle implemented
in dmu_free_long_range(). The first issue is that get_next_chunk()
was calculating the number of L1 blocks the free would dirty
incorrectly. In some cases involving extremely large files, this
code would greatly overestimate the number of effected L1 blocks,
causing excessive calls to txg_wait_open(). This patch corrects
the calculation.

The second issue is that the free throttle uses the total number
of free'd blocks in all (open, quiescing, and syncing) txgs to
determine whether to throttle. This causes large frees (such as
those created by the first issue) to cause 4 txg syncs before
any further frees were allowed to proceed. This patch ensures
that the accounting is done entirely in a per-txg fashion, so
that frees from a given txg don't affect those that immediately
follow it.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Matthew Ahrens <[email protected]>
Signed-off-by: Tom Caputi <[email protected]>
Closes #8655 

1 files changed, 36 insertions, 29 deletions

diff --git a/module/zfs/dmu.c b/module/zfs/dmu.c
index b047a93ef..1697a6320 100644
--- a/module/zfs/dmu.c
+++ b/module/zfs/dmu.c
@@ -724,13 +724,18 @@ get_next_chunk(dnode_t *dn, uint64_t *start, uint64_t minimum, uint64_t *l1blks)
 
 	ASSERT3U(minimum, <=, *start);
 
-	if (*start - minimum <= iblkrange * maxblks) {
+	/*
+	 * Check if we can free the entire range assuming that all of the
+	 * L1 blocks in this range have data. If we can, we use this
+	 * worst case value as an estimate so we can avoid having to look
+	 * at the object's actual data.
+	 */
+	uint64_t total_l1blks =
+	    (roundup(*start, iblkrange) - (minimum / iblkrange * iblkrange)) /
+	    iblkrange;
+	if (total_l1blks <= maxblks) {
+		*l1blks = total_l1blks;
 		*start = minimum;
-		/*
-		 * Assume full L1 blocks and 128k recordsize to approximate the
-		 * expected number of L1 blocks in this chunk
-		 */
-		*l1blks = minimum / (1024 * 128 * 1024);
 		return (0);
 	}
 	ASSERT(ISP2(iblkrange));
@@ -745,6 +750,7 @@ get_next_chunk(dnode_t *dn, uint64_t *start, uint64_t minimum, uint64_t *l1blks)
 		 * to search.
 		 */
 		(*start)--;
+
 		err = dnode_next_offset(dn,
 		    DNODE_FIND_BACKWARDS, start, 2, 1, 0);
 
@@ -763,6 +769,7 @@ get_next_chunk(dnode_t *dn, uint64_t *start, uint64_t minimum, uint64_t *l1blks)
 	if (*start < minimum)
 		*start = minimum;
 	*l1blks = blks;
+
 	return (0);
 }
 
@@ -809,7 +816,6 @@ dmu_free_long_range_impl(objset_t *os, dnode_t *dn, uint64_t offset,
 
 	while (length != 0) {
 		uint64_t chunk_end, chunk_begin, chunk_len;
-		uint64_t long_free_dirty_all_txgs = 0;
 		uint64_t l1blks;
 		dmu_tx_t *tx;
 
@@ -827,25 +833,6 @@ dmu_free_long_range_impl(objset_t *os, dnode_t *dn, uint64_t offset,
 
 		chunk_len = chunk_end - chunk_begin;
 
-		mutex_enter(&dp->dp_lock);
-		for (int t = 0; t < TXG_SIZE; t++) {
-			long_free_dirty_all_txgs +=
-			    dp->dp_long_free_dirty_pertxg[t];
-		}
-		mutex_exit(&dp->dp_lock);
-
-		/*
-		 * To avoid filling up a TXG with just frees wait for
-		 * the next TXG to open before freeing more chunks if
-		 * we have reached the threshold of frees
-		 */
-		if (dirty_frees_threshold != 0 &&
-		    long_free_dirty_all_txgs >= dirty_frees_threshold) {
-			DMU_TX_STAT_BUMP(dmu_tx_dirty_frees_delay);
-			txg_wait_open(dp, 0, B_TRUE);
-			continue;
-		}
-
 		tx = dmu_tx_create(os);
 		dmu_tx_hold_free(tx, dn->dn_object, chunk_begin, chunk_len);
 
@@ -860,6 +847,26 @@ dmu_free_long_range_impl(objset_t *os, dnode_t *dn, uint64_t offset,
 			return (err);
 		}
 
+		uint64_t txg = dmu_tx_get_txg(tx);
+
+		mutex_enter(&dp->dp_lock);
+		uint64_t long_free_dirty =
+		    dp->dp_long_free_dirty_pertxg[txg & TXG_MASK];
+		mutex_exit(&dp->dp_lock);
+
+		/*
+		 * To avoid filling up a TXG with just frees, wait for
+		 * the next TXG to open before freeing more chunks if
+		 * we have reached the threshold of frees.
+		 */
+		if (dirty_frees_threshold != 0 &&
+		    long_free_dirty >= dirty_frees_threshold) {
+			DMU_TX_STAT_BUMP(dmu_tx_dirty_frees_delay);
+			dmu_tx_commit(tx);
+			txg_wait_open(dp, 0, B_TRUE);
+			continue;
+		}
+
 		/*
 		 * In order to prevent unnecessary write throttling, for each
 		 * TXG, we track the cumulative size of L1 blocks being dirtied
@@ -871,12 +878,12 @@ dmu_free_long_range_impl(objset_t *os, dnode_t *dn, uint64_t offset,
 		 * blocks taking up a large percentage of zfs_dirty_data_max.
 		 */
 		mutex_enter(&dp->dp_lock);
-		dp->dp_long_free_dirty_pertxg[dmu_tx_get_txg(tx) & TXG_MASK] +=
+		dp->dp_long_free_dirty_pertxg[txg & TXG_MASK] +=
 		    l1blks << dn->dn_indblkshift;
 		mutex_exit(&dp->dp_lock);
 		DTRACE_PROBE3(free__long__range,
-		    uint64_t, long_free_dirty_all_txgs, uint64_t, chunk_len,
-		    uint64_t, dmu_tx_get_txg(tx));
+		    uint64_t, long_free_dirty, uint64_t, chunk_len,
+		    uint64_t, txg);
 		dnode_free_range(dn, chunk_begin, chunk_len, tx);
 
 		dmu_tx_commit(tx);