Commit 75507efb authored May 01, 2009 by Theodore Ts'o

ext4: Don't avoid using BLOCK_UNINIT block groups in mballoc



By avoiding the use of not-yet-used block groups (i.e., block groups
with the BLOCK_UNINIT flag), mballoc had a tendency to create large
files with large non-contiguous gaps.  In addition avoiding the use of
new block groups had a tendency to push regular file data into the
first block group in a flex_bg group, which slows down the speed of
e2fsck pass 2, since it has a tendency to seek much more.  For
example:

               Before Patch                       After Patch
              Time in seconds                   Time in seconds
            Real /  User/  Sys   MB/s      Real /  User/  Sys    MB/s
Pass 1      8.52 / 2.21 / 0.46  20.43      8.84 / 4.97 / 1.11   19.68
Pass 2     21.16 / 1.02 / 1.86  11.30      6.54 / 1.77 / 1.78   36.39
Pass 3      0.01 / 0.00 / 0.00 139.00      0.01 / 0.01 / 0.00  128.90
Pass 4      0.16 / 0.15 / 0.00   0.00      0.17 / 0.17 / 0.00    0.00
Pass 5      2.52 / 1.99 / 0.09   0.79      2.31 / 1.78 / 0.06    0.86
Total      32.40 / 5.11 / 2.49  12.81     17.99 / 8.75 / 2.98   23.01

This was on a sample 80 gig root filesystem which was approximately
50% full.  Note the improved e2fsck pass 2 performance, by over a
factor of 3, due to a decreased number of seeks.  (The total amount of
I/O in pass 2 was unchanged; the layout of the directory blocks was
simply much better from e2fsck's's perspective.)

Other changes as a result of this patch on this sample filesystem:

                             Before Patch    After Patch
# of non-contig files           762             779
# of non-contig directories     571             570
# of BLOCK_UNINIT bg's          307             293
# of INODE_UNINIT bg's          503             503

Out of 640 block groups, of which 333 were in use, this patch caused
an extra 14 block groups to be utilized.  The number of non-contiguous
files did go up slightly, but when measured against the 99.9% of the
files (603,154) which were contiguously allocated, this is pretty
insignificant.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Andreas Dilger <adilger@sun.com>

parent 32ed5058

fs/ext4/mballoc.c

+1 −8

Original line number	Diff line number	Diff line
		@@ -1728,7 +1728,6 @@ static int ext4_mb_good_group(struct ext4_allocation_context *ac,
		unsigned free, fragments;
		unsigned i, bits;
		int flex_size = ext4_flex_bg_size(EXT4_SB(ac->ac_sb));
		struct ext4_group_desc *desc;
		struct ext4_group_info *grp = ext4_get_group_info(ac->ac_sb, group);

		BUG_ON(cr < 0 \|\| cr >= 4);
		@@ -1744,10 +1743,6 @@ static int ext4_mb_good_group(struct ext4_allocation_context *ac,
		switch (cr) {
		case 0:
		BUG_ON(ac->ac_2order == 0);
		/* If this group is uninitialized, skip it initially */
		desc = ext4_get_group_desc(ac->ac_sb, group, NULL);
		if (desc->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT))
		return 0;

		/* Avoid using the first bg of a flexgroup for data files */
		if ((ac->ac_flags & EXT4_MB_HINT_DATA) &&
		@@ -2067,9 +2062,7 @@ repeat:

		ac->ac_groups_scanned++;
		desc = ext4_get_group_desc(sb, group, NULL);
		if (cr == 0 \|\| (desc->bg_flags &
		cpu_to_le16(EXT4_BG_BLOCK_UNINIT) &&
		ac->ac_2order != 0))
		if (cr == 0)
		ext4_mb_simple_scan_group(ac, &e4b);
		else if (cr == 1 &&
		ac->ac_g_ex.fe_len == sbi->s_stripe)

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