[PATCH 5/5] xfs: reverse search directory freespace indexes

Dave Chinner
2019-08-29T06:30:52Z

From: Dave Chinner <dchinner@redhat.com>

When a directory is growing rapidly, new blocks tend to get added at
the end of the directory. These end up at the end of the freespace
index, and when the directory gets large finding these new
freespaces gets expensive. The code does a linear search across the
frespace index from the first block in the directory to the last,
hence meaning the newly added space is the last index searched.

Instead, do a reverse order index search, starting from the last
block and index in the freespace index. This makes most lookups for
free space on rapidly growing directories O(1) instead of O(N), but
should not have any impact on random insert workloads because the
average search length is the same regardless of which end of the
array we start at.

The result is a major improvement in large directory grow rates:

create time(sec) / rate (files/s)
File count vanilla Prev commit Patched
10k 0.41 / 24.3k 0.42 / 23.8k 0.41 / 24.3k
20k 0.74 / 27.0k 0.76 / 26.3k 0.75 / 26.7k
100k 3.81 / 26.4k 3.47 / 28.8k 3.27 / 30.6k
200k 8.58 / 23.3k 7.19 / 27.8k 6.71 / 29.8k
1M 85.69 / 11.7k 48.53 / 20.6k 37.67 / 26.5k
2M 280.31 / 7.1k 130.14 / 15.3k 79.55 / 25.2k
10M 3913.26 / 2.5k 552.89 / 18.1k

Signed-Off-By: Dave Chinner <dchinner@redhat.com>
---
fs/xfs/libxfs/xfs_dir2_node.c | 20 +++++++++++---------
1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_dir2_node.c b/fs/xfs/libxfs/xfs_dir2_node.c
index c6a1d1cc638f..d8abbcbde055 100644
--- a/fs/xfs/libxfs/xfs_dir2_node.c
+++ b/fs/xfs/libxfs/xfs_dir2_node.c
@@ -1746,6 +1746,7 @@ xfs_dir2_node_find_freeblk(
struct xfs_inode *dp = args->dp;
struct xfs_trans *tp = args->trans;
struct xfs_buf *fbp = NULL;
+ xfs_dir2_db_t firstfbno;
xfs_dir2_db_t lastfbno;
xfs_dir2_db_t ifbno = -1;
xfs_dir2_db_t dbno = -1;
@@ -1781,6 +1782,9 @@ xfs_dir2_node_find_freeblk(
*/
ifbno = fblk->blkno;
fbno = ifbno;
+ xfs_trans_brelse(tp, fbp);
+ fbp = NULL;
+ fblk->bp = NULL;
}

/*
@@ -1792,16 +1796,15 @@ xfs_dir2_node_find_freeblk(
if (error)
return error;
lastfbno = xfs_dir2_da_to_db(args->geo, (xfs_dablk_t)fo);
-
- /* If we haven't get a search start block, set it now */
- if (fbno == -1)
- fbno = xfs_dir2_byte_to_db(args->geo, XFS_DIR2_FREE_OFFSET);
+ firstfbno = xfs_dir2_byte_to_db(args->geo, XFS_DIR2_FREE_OFFSET);

/*
* While we haven't identified a data block, search the freeblock
- * data for a data block with enough free space in it.
+ * data for a good data block. Do a reverse order search, as growing
+ * directories will put new blocks with free space at the end of the
+ * free space index.
*/
- for ( ; fbno < lastfbno; fbno++) {
+ for (fbno = lastfbno - 1; fbno >= firstfbno; fbno--) {
/* If it's ifbno we already looked at it. */
if (fbno == ifbno)
continue;
@@ -1819,19 +1822,18 @@ xfs_dir2_node_find_freeblk(
if (!fbp)
continue;

- findex = 0;
free = fbp->b_addr;
bests = dp->d_ops->free_bests_p(free);
dp->d_ops->free_hdr_from_disk(&freehdr, free);

/* Scan the free entry array for a large enough free space. */
- do {
+ for (findex = freehdr.nvalid - 1; findex >= 0; findex--) {
if (be16_to_cpu(bests[findex]) != NULLDATAOFF &&
be16_to_cpu(bests[findex]) >= length) {
dbno = freehdr.firstdb + findex;
goto found_block;
}
- } while (++findex < freehdr.nvalid);
+ };

/* Didn't find free space, go on to next free block */
xfs_trans_brelse(tp, fbp);
--
2.23.0.rc1