Improved dnode allocation and dmu_hold_impl() (#6611)

Refactor dmu_object_alloc_dnsize() and dnode_hold_impl() to simplify the
code, fix errors introduced by commit dbeb879 (PR #6117) interacting
badly with large dnodes, and improve performance.

* When allocating a new dnode in dmu_object_alloc_dnsize(), update the
percpu object ID for the core's metadnode chunk immediately.  This
eliminates most lock contention when taking the hold and creating the
dnode.

* Correct detection of the chunk boundary to work properly with large
dnodes.

* Separate the dmu_hold_impl() code for the FREE case from the code for
the ALLOCATED case to make it easier to read.

* Fully populate the dnode handle array immediately after reading a
block of the metadnode from disk.  Subsequently the dnode handle array
provides enough information to determine which dnode slots are in use
and which are free.

* Add several kstats to allow the behavior of the code to be examined.

* Verify dnode packing in large_dnode_008_pos.ksh.  Since the test is
purely creates, it should leave very few holes in the metadnode.

* Add test large_dnode_009_pos.ksh, which performs concurrent creates
and deletes, to complement existing test which does only creates.

With the above fixes, there is very little contention in a test of about
200,000 racing dnode allocations produced by tests 'large_dnode_008_pos'
and 'large_dnode_009_pos'.

name                            type data
dnode_hold_dbuf_hold            4    0
dnode_hold_dbuf_read            4    0
dnode_hold_alloc_hits           4    3804690
dnode_hold_alloc_misses         4    216
dnode_hold_alloc_interior       4    3
dnode_hold_alloc_lock_retry     4    0
dnode_hold_alloc_lock_misses    4    0
dnode_hold_alloc_type_none      4    0
dnode_hold_free_hits            4    203105
dnode_hold_free_misses          4    4
dnode_hold_free_lock_misses     4    0
dnode_hold_free_lock_retry      4    0
dnode_hold_free_overflow        4    0
dnode_hold_free_refcount        4    57
dnode_hold_free_txg             4    0
dnode_allocate                  4    203154
dnode_reallocate                4    0
dnode_buf_evict                 4    23918
dnode_alloc_next_chunk          4    4887
dnode_alloc_race                4    0
dnode_alloc_next_block          4    18

The performance is slightly improved for concurrent creates with
16+ threads, and unchanged for low thread counts.

Signed-off-by: Brian Behlendorf <[email protected]>
Signed-off-by: Olaf Faaland <[email protected]>

1 files changed, 29 insertions, 7 deletions

diff --git a/cmd/zdb/zdb.c b/cmd/zdb/zdb.c
index 21f8ea87c..1097501e8 100644
--- a/cmd/zdb/zdb.c
+++ b/cmd/zdb/zdb.c
@@ -1933,7 +1933,8 @@ static object_viewer_t *object_viewer[DMU_OT_NUMTYPES + 1] = {
 };
 
 static void
-dump_object(objset_t *os, uint64_t object, int verbosity, int *print_header)
+dump_object(objset_t *os, uint64_t object, int verbosity, int *print_header,
+    uint64_t *dnode_slots_used)
 {
 	dmu_buf_t *db = NULL;
 	dmu_object_info_t doi;
@@ -1965,6 +1966,9 @@ dump_object(objset_t *os, uint64_t object, int verbosity, int *print_header)
 	}
 	dmu_object_info_from_dnode(dn, &doi);
 
+	if (dnode_slots_used)
+		*dnode_slots_used = doi.doi_dnodesize / DNODE_MIN_SIZE;
+
 	zdb_nicenum(doi.doi_metadata_block_size, iblk);
 	zdb_nicenum(doi.doi_data_block_size, dblk);
 	zdb_nicenum(doi.doi_max_offset, lsize);
@@ -2072,6 +2076,9 @@ dump_dir(objset_t *os)
 	int verbosity = dump_opt['d'];
 	int print_header = 1;
 	int i, error;
+	uint64_t total_slots_used = 0;
+	uint64_t max_slot_used = 0;
+	uint64_t dnode_slots;
 
 	dsl_pool_config_enter(dmu_objset_pool(os), FTAG);
 	dmu_objset_fast_stat(os, &dds);
@@ -2112,7 +2119,7 @@ dump_dir(objset_t *os)
 	if (zopt_objects != 0) {
 		for (i = 0; i < zopt_objects; i++)
 			dump_object(os, zopt_object[i], verbosity,
-			    &print_header);
+			    &print_header, NULL);
 		(void) printf("\n");
 		return;
 	}
@@ -2129,24 +2136,39 @@ dump_dir(objset_t *os)
 	if (BP_IS_HOLE(os->os_rootbp))
 		return;
 
-	dump_object(os, 0, verbosity, &print_header);
+	dump_object(os, 0, verbosity, &print_header, NULL);
 	object_count = 0;
 	if (DMU_USERUSED_DNODE(os) != NULL &&
 	    DMU_USERUSED_DNODE(os)->dn_type != 0) {
-		dump_object(os, DMU_USERUSED_OBJECT, verbosity, &print_header);
-		dump_object(os, DMU_GROUPUSED_OBJECT, verbosity, &print_header);
+		dump_object(os, DMU_USERUSED_OBJECT, verbosity, &print_header,
+		    NULL);
+		dump_object(os, DMU_GROUPUSED_OBJECT, verbosity, &print_header,
+		    NULL);
 	}
 
 	object = 0;
 	while ((error = dmu_object_next(os, &object, B_FALSE, 0)) == 0) {
-		dump_object(os, object, verbosity, &print_header);
+		dump_object(os, object, verbosity, &print_header, &dnode_slots);
 		object_count++;
+		total_slots_used += dnode_slots;
+		max_slot_used = object + dnode_slots - 1;
 	}
 
 	ASSERT3U(object_count, ==, usedobjs);
 
 	(void) printf("\n");
 
+	(void) printf("    Dnode slots:\n");
+	(void) printf("\tTotal used:    %10llu\n",
+	    (u_longlong_t)total_slots_used);
+	(void) printf("\tMax used:      %10llu\n",
+	    (u_longlong_t)max_slot_used);
+	(void) printf("\tPercent empty: %10lf\n",
+	    (double)(max_slot_used - total_slots_used)*100 /
+	    (double)max_slot_used);
+
+	(void) printf("\n");
+
 	if (error != ESRCH) {
 		(void) fprintf(stderr, "dmu_object_next() = %d\n", error);
 		abort();
@@ -2610,7 +2632,7 @@ dump_path_impl(objset_t *os, uint64_t obj, char *name)
 			return (dump_path_impl(os, child_obj, s + 1));
 		/*FALLTHROUGH*/
 	case DMU_OT_PLAIN_FILE_CONTENTS:
-		dump_object(os, child_obj, dump_opt['v'], &header);
+		dump_object(os, child_obj, dump_opt['v'], &header, NULL);
 		return (0);
 	default:
 		(void) fprintf(stderr, "object %llu has non-file/directory "