tdb2: use expansion heuristics from tdb1

This reduces the amount of expansion we do.

Before:
./speed 1000000
Adding 1000000 records:  23210 ns (59193360 bytes)
Finding 1000000 records:  2387 ns (59193360 bytes)
Traversing 1000000 records:  2150 ns (59193360 bytes)
Deleting 1000000 records:  13392 ns (59193360 bytes)
Re-adding 1000000 records:  11546 ns (59193360 bytes)
Appending 1000000 records:  29327 ns (91193360 bytes)
Churning 1000000 records:  33026 ns (91193360 bytes)

After:
$ ./speed 1000000
Adding 1000000 records:  17480 ns (61472904 bytes)
Finding 1000000 records:  2431 ns (61472904 bytes)
Traversing 1000000 records:  2194 ns (61472904 bytes)
Deleting 1000000 records:  10948 ns (61472904 bytes)
Re-adding 1000000 records:  11247 ns (61472904 bytes)
Appending 1000000 records:  21826 ns (96051424 bytes)
Churning 1000000 records:  27242 ns (96051424 bytes)

ccan/tdb2/free.c

@@ -566,6 +566,14 @@ static int tdb_expand(struct tdb_context *tdb, tdb_len_t size)
 		return -1;
 	}
 
+	/* always make room for at least 100 more records, and at
+           least 25% more space. */
+	if (size * TDB_EXTENSION_FACTOR > tdb->map_size / 4)
+		wanted = size * TDB_EXTENSION_FACTOR;
+	else
+		wanted = tdb->map_size / 4;
+	wanted = adjust_size(0, wanted);
+
 	/* Only one person can expand file at a time. */
 	if (tdb_lock_expand(tdb, F_WRLCK) != 0)
 		return -1;
@@ -578,7 +586,7 @@ static int tdb_expand(struct tdb_context *tdb, tdb_len_t size)
 		return 0;
 	}
 
-	if (tdb->methods->expand_file(tdb, wanted*TDB_EXTENSION_FACTOR) == -1) {
+	if (tdb->methods->expand_file(tdb, wanted) == -1) {
 		tdb_unlock_expand(tdb, F_WRLCK);
 		return -1;
 	}
@@ -586,7 +594,7 @@ static int tdb_expand(struct tdb_context *tdb, tdb_len_t size)
 	/* We need to drop this lock before adding free record. */
 	tdb_unlock_expand(tdb, F_WRLCK);
 
-	return add_free_record(tdb, old_size, wanted * TDB_EXTENSION_FACTOR);
+	return add_free_record(tdb, old_size, wanted);
 }
 
 /* This won't fail: it will expand the database if it has to. */

ccan/tdb2/private.h

@@ -92,8 +92,8 @@ typedef uint64_t tdb_off_t;
 /* And 8 entries in each group, ie 8 groups per sublevel. */
 #define TDB_HASH_GROUP_BITS 3
 
-/* Extend file by least 32 times larger than needed. */
-#define TDB_EXTENSION_FACTOR 32
+/* Extend file by least 100 times larger than needed. */
+#define TDB_EXTENSION_FACTOR 100
 
 /* We steal bits from the offsets to store hash info. */
 #define TDB_OFF_HASH_GROUP_MASK ((1ULL << TDB_HASH_GROUP_BITS) - 1)