[PATCH v3 1/5] libext2fs: optimize ext2fs_convert_subcluster_bitmap()

[Li Dongyang <dongyangli@ddn.com>]

For a bigalloc filesystem, converting the block bitmap from blocks
to chunks in ext2fs_convert_subcluster_bitmap() can take a long time
when the device is huge, because we test the bitmap
bit-by-bit using ext2fs_test_block_bitmap2().
Use ext2fs_find_first_set_block_bitmap2() which is more efficient
for mke2fs when the fs is mostly empty.

e2fsck can also benefit from this during pass1 block scanning.

Time taken for "mke2fs -O bigalloc,extent -C 131072 -b 4096" on a 1PB
device:

without patch:
real    27m49.457s
user    21m36.474s
sys     6m9.514s

with patch:
real    6m31.908s
user    0m1.806s
sys    6m29.697s

Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
---
 lib/ext2fs/gen_bitmap64.c | 20 +++++++-------------
 1 file changed, 7 insertions(+), 13 deletions(-)

diff --git a/lib/ext2fs/gen_bitmap64.c b/lib/ext2fs/gen_bitmap64.c
index 6e4d8b71..f1dd1891 100644
--- a/lib/ext2fs/gen_bitmap64.c
+++ b/lib/ext2fs/gen_bitmap64.c
@@ -799,8 +799,7 @@ errcode_t ext2fs_convert_subcluster_bitmap(ext2_filsys fs,
 	ext2fs_generic_bitmap_64 bmap, cmap;
 	ext2fs_block_bitmap	gen_bmap = *bitmap, gen_cmap;
 	errcode_t		retval;
-	blk64_t			i, b_end, c_end;
-	int			n, ratio;
+	blk64_t			i, next, b_end, c_end;
 
 	bmap = (ext2fs_generic_bitmap_64) gen_bmap;
 	if (fs->cluster_ratio_bits == ext2fs_get_bitmap_granularity(gen_bmap))
@@ -817,18 +816,13 @@ errcode_t ext2fs_convert_subcluster_bitmap(ext2_filsys fs,
 	bmap->end = bmap->real_end;
 	c_end = cmap->end;
 	cmap->end = cmap->real_end;
-	n = 0;
-	ratio = 1 << fs->cluster_ratio_bits;
 	while (i < bmap->real_end) {
-		if (ext2fs_test_block_bitmap2(gen_bmap, i)) {
-			ext2fs_mark_block_bitmap2(gen_cmap, i);
-			i += ratio - n;
-			n = 0;
-			continue;
-		}
-		i++; n++;
-		if (n >= ratio)
-			n = 0;
+		retval = ext2fs_find_first_set_block_bitmap2(gen_bmap,
+						i, bmap->real_end, &next);
+		if (retval)
+			break;
+		ext2fs_mark_block_bitmap2(gen_cmap, next);
+		i = EXT2FS_C2B(fs, EXT2FS_B2C(fs, next) + 1);
 	}
 	bmap->end = b_end;
 	cmap->end = c_end;
-- 
2.24.0