Allow metaslab to be unloaded even when not freed from

On large systems, the memory used by loaded metaslabs can become
a concern. While range trees are a fairly efficient data structure, 
on heavily fragmented pools they can still consume a significant 
amount of memory. This problem is amplified when we fail to unload 
metaslabs that we aren't using. Currently, we only unload a metaslab 
during metaslab_sync_done; in order for that function to be called 
on a given metaslab in a given txg, we have to have dirtied that 
metaslab in that txg. If the dirtying was the result of an allocation, 
we wouldn't be unloading it (since it wouldn't be 8 txgs since it 
was selected), so in effect we only unload a metaslab during txgs 
where it's being freed from.

We move the unload logic from sync_done to a new function, and 
call that function on all metaslabs in a given vdev during 
vdev_sync_done().

Reviewed-by: Richard Elling <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Paul Dagnelie <[email protected]>
Closes #8837 

1 files changed, 25 insertions, 22 deletions

diff --git a/module/zfs/metaslab.c b/module/zfs/metaslab.c
index d1d5a243f..41cbaad5f 100644
--- a/module/zfs/metaslab.c
+++ b/module/zfs/metaslab.c
@@ -20,7 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2011, 2018 by Delphix. All rights reserved.
+ * Copyright (c) 2011, 2019 by Delphix. All rights reserved.
  * Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
  * Copyright (c) 2017, Intel Corporation.
  */
@@ -2949,6 +2949,30 @@ metaslab_sync(metaslab_t *msp, uint64_t txg)
 	dmu_tx_commit(tx);
 }
 
+void
+metaslab_potentially_unload(metaslab_t *msp, uint64_t txg)
+{
+	/*
+	 * If the metaslab is loaded and we've not tried to load or allocate
+	 * from it in 'metaslab_unload_delay' txgs, then unload it.
+	 */
+	if (msp->ms_loaded &&
+	    msp->ms_disabled == 0 &&
+	    msp->ms_selected_txg + metaslab_unload_delay < txg) {
+		for (int t = 1; t < TXG_CONCURRENT_STATES; t++) {
+			VERIFY0(range_tree_space(
+			    msp->ms_allocating[(txg + t) & TXG_MASK]));
+		}
+		if (msp->ms_allocator != -1) {
+			metaslab_passivate(msp, msp->ms_weight &
+			    ~METASLAB_ACTIVE_MASK);
+		}
+
+		if (!metaslab_debug_unload)
+			metaslab_unload(msp);
+	}
+}
+
 /*
  * Called after a transaction group has completely synced to mark
  * all of the metaslab's free space as usable.
@@ -3086,27 +3110,6 @@ metaslab_sync_done(metaslab_t *msp, uint64_t txg)
 	 */
 	metaslab_recalculate_weight_and_sort(msp);
 
-	/*
-	 * If the metaslab is loaded and we've not tried to load or allocate
-	 * from it in 'metaslab_unload_delay' txgs, then unload it.
-	 */
-	if (msp->ms_loaded &&
-	    msp->ms_disabled == 0 &&
-	    msp->ms_selected_txg + metaslab_unload_delay < txg) {
-
-		for (int t = 1; t < TXG_CONCURRENT_STATES; t++) {
-			VERIFY0(range_tree_space(
-			    msp->ms_allocating[(txg + t) & TXG_MASK]));
-		}
-		if (msp->ms_allocator != -1) {
-			metaslab_passivate(msp, msp->ms_weight &
-			    ~METASLAB_ACTIVE_MASK);
-		}
-
-		if (!metaslab_debug_unload)
-			metaslab_unload(msp);
-	}
-
 	ASSERT0(range_tree_space(msp->ms_allocating[txg & TXG_MASK]));
 	ASSERT0(range_tree_space(msp->ms_freeing));
 	ASSERT0(range_tree_space(msp->ms_freed));