[PATCH] ext4: mballoc: fix mb_normalize_request algorithm for 1KB block size filesystems

[Valerie Clement <valerie.clement@bull.net>]

mballoc: fix mb_normalize_request algorithm for 1KB block size filesystems

From: Valerie Clement <valerie.clement@bull.net>

In case of inode preallocation, the number of blocks to allocate depends
on the file size and it is calculated in ext4_mb_normalize_group_request().
Each group in the filesystem is then checked to find one that can be used
for allocation; this is done in ext4_mb_good_group().

When a file bigger than 4MB is created, the requested number of blocks to
preallocate, calculated by ext4_mb_normalize_group_request is 4096.
However for a filesystem with 1KB block size, the maximum size of the
block buddies used by the multiblock allocator is 2048, so none of
groups in the filesystem satisfies the search criteria in
ext4_mb_good_group(). Scanning all the filesystem groups impacts
performance.

The following numbers show that:
- on an ext4 FS with 1KB block size mounted with nodelalloc option:
# dd if=/dev/zero of=/mnt/test/foo bs=8k count=1k conv=fsync
1024+0 records in
1024+0 records out
8388608 bytes (8.4 MB) copied, 35.5091 seconds, 236 kB/s

- on an ext4 FS with 1KB block size mounted with nodelalloc and nomballoc
options:
# dd if=/dev/zero of=/mnt/test/foo bs=8k count=1k conv=fsync
1024+0 records in
1024+0 records out
8388608 bytes (8.4 MB) copied, 0.233754 seconds, 35.9 MB/s

In the two cases, dd is done after creating the FS with -b1024 option,
mounting the FS with the options specified before and flushing all caches
using echo 3 > /proc/sys/vm/drop_caches.
The partition size is 70GB.
I did the same test on a 1TB partition, it took several minutes to write
8MB!

This patch modifies the algorithm in ext4_mb_normalize_group_request to
calculate the number of blocks to allocate by taking into account the
maximum size of free blocks chunks handled by the multiblock allocator.

It has also been tested for filesystems with 2KB and 4KB block sizes to
ensure that those cases don't regress.

Signed-off-by: Valerie Clement <valerie.clement@bull.net>

---

 mballoc.c |   19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

Index: linux-2.6.25/fs/ext4/mballoc.c
===================================================================
--- linux-2.6.25.orig/fs/ext4/mballoc.c	2008-04-25 16:19:32.000000000 +0200
+++ linux-2.6.25/fs/ext4/mballoc.c	2008-04-25 16:49:34.000000000 +0200
@@ -2905,12 +2905,11 @@ ext4_mb_normalize_request(struct ext4_al
 	if (size < i_size_read(ac->ac_inode))
 		size = i_size_read(ac->ac_inode);
 
-	/* max available blocks in a free group */
-	max = EXT4_BLOCKS_PER_GROUP(ac->ac_sb) - 1 - 1 -
-				EXT4_SB(ac->ac_sb)->s_itb_per_group;
+	/* max size of free chunks */
+	max = 2 << bsbits;
 
-#define NRL_CHECK_SIZE(req, size, max,bits)	\
-		(req <= (size) || max <= ((size) >> bits))
+#define NRL_CHECK_SIZE(req, size, max, chunk_size)	\
+		(req <= (size) || max <= (chunk_size))
 
 	/* first, try to predict filesize */
 	/* XXX: should this table be tunable? */
@@ -2929,16 +2928,16 @@ ext4_mb_normalize_request(struct ext4_al
 		size = 512 * 1024;
 	} else if (size <= 1024 * 1024) {
 		size = 1024 * 1024;
-	} else if (NRL_CHECK_SIZE(size, 4 * 1024 * 1024, max, bsbits)) {
+	} else if (NRL_CHECK_SIZE(size, 4 * 1024 * 1024, max, 2 * 1024)) {
 		start_off = ((loff_t)ac->ac_o_ex.fe_logical >>
-						(20 - bsbits)) << 20;
-		size = 1024 * 1024;
-	} else if (NRL_CHECK_SIZE(size, 8 * 1024 * 1024, max, bsbits)) {
+						(21 - bsbits)) << 21;
+		size = 2* 1024 * 1024;
+	} else if (NRL_CHECK_SIZE(size, 8 * 1024 * 1024, max, 4 * 1024)) {
 		start_off = ((loff_t)ac->ac_o_ex.fe_logical >>
 							(22 - bsbits)) << 22;
 		size = 4 * 1024 * 1024;
 	} else if (NRL_CHECK_SIZE(ac->ac_o_ex.fe_len,
-					(8<<20)>>bsbits, max, bsbits)) {
+					(8<<20)>>bsbits, max, 8 * 1024)) {
 		start_off = ((loff_t)ac->ac_o_ex.fe_logical >>
 							(23 - bsbits)) << 23;
 		size = 8 * 1024 * 1024;