libext2fs: optimize rb_test_bit

Optimize testing for a bit in an rbtree-based bitmap for the case
where the calling application is scanning through the bitmap
sequentially.  Previously, we did this for a set of bits which were
inside an allocated extent, but we did not optimize the case where
there was a large number of bits after an allocated extents which were
not in use.

             1111111111111110000000000000000000
             ^ optimized    ^not optimized

In my tests of a roughly half-filled file system, the run time of
e2freefrag was halved, and the cpu time spent in userspace was during
e2fsck's pass 5 was reduced by a factor of 30%.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>

diff --git a/lib/ext2fs/blkmap64_rb.c b/lib/ext2fs/blkmap64_rb.c
index a83f8ac..c9006f8 100644 (file)
--- a/lib/ext2fs/blkmap64_rb.c
+++ b/lib/ext2fs/blkmap64_rb.c
@@ -314,8 +314,8 @@ static errcode_t rb_resize_bmap(ext2fs_generic_bitmap bmap,
 inline static int
 rb_test_bit(struct ext2fs_rb_private *bp, __u64 bit)
 {
-       struct bmap_rb_extent *rcursor;
-       struct rb_node *parent = NULL;
+       struct bmap_rb_extent *rcursor, *next_ext;
+       struct rb_node *parent = NULL, *next;
        struct rb_node **n = &bp->root.rb_node;
        struct bmap_rb_extent *ext;
 
@@ -330,6 +330,18 @@ rb_test_bit(struct ext2fs_rb_private *bp, __u64 bit)
                return 1;
        }
 
+       next = ext2fs_rb_next(&rcursor->node);
+       if (next) {
+               next_ext = ext2fs_rb_entry(next, struct bmap_rb_extent, node);
+               if ((bit >= rcursor->start + rcursor->count) &&
+                   (bit < next_ext->start)) {
+#ifdef BMAP_STATS_OPS
+                       bp->test_hit++;
+#endif
+                       return 0;
+               }
+       }
+
        rcursor = *bp->wcursor;
        if (!rcursor)
                goto search_tree;