[PATCH 16/49] ext4: Don't avoid using BLOCK_UNINIT block groups in mballoc

Theodore Ts'o
2009-06-08T19:34:20Z

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>
---
fs/ext4/mballoc.c | 9 +--------
1 files changed, 1 insertions(+), 8 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index c3af9e6..dbd47ea 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -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)
--
1.6.3.2.1.gb9f7d.dirty