ext4: optimize ext4_find_delalloc_range() in nodelalloc mode
We found performance regression when using bigalloc with "nodelalloc" (1MB cluster size): 1. mke2fs -C 1048576 -O ^has_journal,bigalloc /dev/sda 2. mount -o nodelalloc /dev/sda /test/ 3. time dd if=/dev/zero of=/test/io bs=1048576 count=1024 The "dd" will cost about 2 seconds to finish, but if we mke2fs without "bigalloc", "dd" will only cost less than 1 second. The reason is: when using ext4 with "nodelalloc", it will call ext4_find_delalloc_cluster() nearly everytime it call ext4_ext_map_blocks(), and ext4_find_delalloc_range() will also scan all pages in cluster because no buffer is "delayed". A cluster has 256 pages (1MB cluster), so it will scan 256 * 256k pags when creating a 1G file. That severely hurts the performance. Therefore, we return immediately from ext4_find_delalloc_range() in nodelalloc mode, since by definition there can't be any delalloc pages. Signed-off-by: Robin Dong <sanbai@taobao.com> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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@ -3281,6 +3281,9 @@ static int ext4_find_delalloc_range(struct inode *inode,
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ext4_lblk_t i, pg_lblk;
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pgoff_t index;
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if (!test_opt(inode->i_sb, DELALLOC))
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return 0;
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/* reverse search wont work if fs block size is less than page size */
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if (inode->i_blkbits < PAGE_CACHE_SHIFT)
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search_hint_reverse = 0;
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