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authorJeremy Allison <jra@samba.org>2005-09-01 00:36:46 +0000
committerGerald (Jerry) Carter <jerry@samba.org>2007-10-10 11:03:26 -0500
commit76a85f222f52bb794f4ce4be2a067042056b36e4 (patch)
treeee9587678e33aa3bccbf225e740aedcb7b725dd9 /source3/tdb
parent2f626e7b5ad2d921490723e32863bd423e8272cd (diff)
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r9852: Add tridge's Samba4 tdb optimisations.
Jeremy. (This used to be commit cfe5a7e5f8eccecf8876c36ddeb0f6e91eb6aac4)
Diffstat (limited to 'source3/tdb')
-rw-r--r--source3/tdb/tdb.c48
1 files changed, 48 insertions, 0 deletions
diff --git a/source3/tdb/tdb.c b/source3/tdb/tdb.c
index d68df03776..908657b355 100644
--- a/source3/tdb/tdb.c
+++ b/source3/tdb/tdb.c
@@ -1248,6 +1248,54 @@ static int tdb_next_lock(TDB_CONTEXT *tdb, struct tdb_traverse_lock *tlock,
/* Lock each chain from the start one. */
for (; tlock->hash < tdb->header.hash_size; tlock->hash++) {
+
+ /* this is an optimisation for the common case where
+ the hash chain is empty, which is particularly
+ common for the use of tdb with ldb, where large
+ hashes are used. In that case we spend most of our
+ time in tdb_brlock(), locking empty hash chains.
+
+ To avoid this, we do an unlocked pre-check to see
+ if the hash chain is empty before starting to look
+ inside it. If it is empty then we can avoid that
+ hash chain. If it isn't empty then we can't believe
+ the value we get back, as we read it without a
+ lock, so instead we get the lock and re-fetch the
+ value below.
+
+ Notice that not doing this optimisation on the
+ first hash chain is critical. We must guarantee
+ that we have done at least one fcntl lock at the
+ start of a search to guarantee that memory is
+ coherent on SMP systems. If records are added by
+ others during the search then thats OK, and we
+ could possibly miss those with this trick, but we
+ could miss them anyway without this trick, so the
+ semantics don't change.
+
+ With a non-indexed ldb search this trick gains us a
+ factor of around 80 in speed on a linux 2.6.x
+ system (testing using ldbtest).
+ */
+ if (!tlock->off && tlock->hash != 0) {
+ u32 off;
+ if (tdb->map_ptr) {
+ for (;tlock->hash < tdb->header.hash_size;tlock->hash++) {
+ if (0 != *(u32 *)(TDB_HASH_TOP(tlock->hash) + (unsigned char *)tdb->map_ptr)) {
+ break;
+ }
+ }
+ if (tlock->hash == tdb->header.hash_size) {
+ continue;
+ }
+ } else {
+ if (ofs_read(tdb, TDB_HASH_TOP(tlock->hash), &off) == 0 &&
+ off == 0) {
+ continue;
+ }
+ }
+ }
+
if (tdb_lock(tdb, tlock->hash, F_WRLCK) == -1)
return -1;