[PATCH] Btrfs: use asynchronous submit for large DIO io in single profile

[Liu Bo <bo.li.liu@oracle.com>]

Commit 1ae399382512 ("Btrfs: do not use async submit for small DIO io's")
benefits small DIO io's.

However, if we're owning the SINGLE profile, this also affects large DIO io's
since in that case, map_length is (chunk_length - bio's offset_in_chunk),
it's farily large so that it's very likely to be larger than a large bio's
size, which avoids asynchronous submit.
For instance, if we have a 512k bio, the efforts of calculating (512k/4k=128)
checksums will be taken by the DIO task.

Test results with fio (tested on a hard disk, not tested on ssd, 4cpu, 8g memory)

bs	async	sync		async	sync
	bw	bw(KB/S)	iops	iop
4k	115312	115480		28827.6	28869.6
8k	114381	115586		14297.4	14447.6
16k	115393	116290		7211.4	7267.6
32k	114268	116589		3570.4	3643
64k	115421	113417		1803	1771.8	<-----ASYNC wins here
128k	115545	112585		902	879
256k	115178	111521		449.2	435
512k	115874	111620		226	217.6

This adds a limit 'BTRFS_STRIPE_LEN(64k)' to decide if it's small enough to avoid
asynchronous submit.

Still, in this case we don't need to split the bio and can submit it directly.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
---
The job in the test,

[global]
rw=write
ioengine=libaio
direct=1
iodepth=64
iodepth_batch=64
iodepth_batch_complete=64
iodepth_low=64
bs=4k
size=8g
sync=0
group_reporting
fallocate=posix
invalidate=1
runtime=30

[dio]
directory=/mnt/btrfs
filename=foobar

 fs/btrfs/inode.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index e687bb0..c640d7e 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -7792,6 +7792,7 @@ static int btrfs_submit_direct_hook(int rw, struct btrfs_dio_private *dip,
 	int nr_pages = 0;
 	int ret;
 	int async_submit = 0;
+	u64 alloc_profile;
 
 	map_length = orig_bio->bi_iter.bi_size;
 	ret = btrfs_map_block(root->fs_info, rw, start_sector << 9,
@@ -7799,15 +7800,26 @@ static int btrfs_submit_direct_hook(int rw, struct btrfs_dio_private *dip,
 	if (ret)
 		return -EIO;
 
+	alloc_profile = btrfs_get_alloc_profile(root, 1);
+
 	if (map_length >= orig_bio->bi_iter.bi_size) {
 		bio = orig_bio;
 		dip->flags |= BTRFS_DIO_ORIG_BIO_SUBMITTED;
+
+		/*
+		 * In the case of 'single' profile, the above check is very
+		 * likely to be true as map_length is (chunk_length - offset),
+		 * so checking BTRFS_STRIPE_LEN here.
+		 */
+		if ((alloc_profile & BTRFS_BLOCK_GROUP_PROFILE_MASK) == 0 &&
+		    orig_bio->bi_iter.bi_size >= BTRFS_STRIPE_LEN)
+			async_submit = 1;
+
 		goto submit;
 	}
 
 	/* async crcs make it difficult to collect full stripe writes. */
-	if (btrfs_get_alloc_profile(root, 1) &
-	    (BTRFS_BLOCK_GROUP_RAID5 | BTRFS_BLOCK_GROUP_RAID6))
+	if (alloc_profile & (BTRFS_BLOCK_GROUP_RAID5 | BTRFS_BLOCK_GROUP_RAID6))
 		async_submit = 0;
 	else
 		async_submit = 1;
-- 
1.8.1.4