direct-io: only inc/dec inode->i_dio_count for file systems
do_blockdev_direct_IO() increments and decrements the inode
->i_dio_count for each IO operation. It does this to protect against
truncate of a file. Block devices don't need this sort of protection.
For a capable multiqueue setup, this atomic int is the only shared
state between applications accessing the device for O_DIRECT, and it
presents a scaling wall for that. In my testing, as much as 30% of
system time is spent incrementing and decrementing this value. A mixed
read/write workload improved from ~2.5M IOPS to ~9.6M IOPS, with
better latencies too. Before:
clat percentiles (usec):
| 1.00th=[ 33], 5.00th=[ 34], 10.00th=[ 34], 20.00th=[ 34],
| 30.00th=[ 34], 40.00th=[ 34], 50.00th=[ 35], 60.00th=[ 35],
| 70.00th=[ 35], 80.00th=[ 35], 90.00th=[ 37], 95.00th=[ 80],
| 99.00th=[ 98], 99.50th=[ 151], 99.90th=[ 155], 99.95th=[ 155],
| 99.99th=[ 165]
After:
clat percentiles (usec):
| 1.00th=[ 95], 5.00th=[ 108], 10.00th=[ 129], 20.00th=[ 149],
| 30.00th=[ 155], 40.00th=[ 161], 50.00th=[ 167], 60.00th=[ 171],
| 70.00th=[ 177], 80.00th=[ 185], 90.00th=[ 201], 95.00th=[ 270],
| 99.00th=[ 390], 99.50th=[ 398], 99.90th=[ 418], 99.95th=[ 422],
| 99.99th=[ 438]
In other setups, Robert Elliott reported seeing good performance
improvements:
https://lkml.org/lkml/2015/4/3/557
The more applications accessing the device, the worse it gets.
Add a new direct-io flags, DIO_SKIP_DIO_COUNT, which tells
do_blockdev_direct_IO() that it need not worry about incrementing
or decrementing the inode i_dio_count for this caller.
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Elliott, Robert (Server Storage) <elliott@hp.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/nfs/direct.c b/fs/nfs/direct.c
index ed0e603..b2cbc3a 100644
--- a/fs/nfs/direct.c
+++ b/fs/nfs/direct.c
@@ -386,7 +386,7 @@
if (write)
nfs_zap_mapping(inode, inode->i_mapping);
- inode_dio_done(inode);
+ inode_dio_end(inode);
if (dreq->iocb) {
long res = (long) dreq->error;
@@ -486,7 +486,7 @@
&nfs_direct_read_completion_ops);
get_dreq(dreq);
desc.pg_dreq = dreq;
- atomic_inc(&inode->i_dio_count);
+ inode_dio_begin(inode);
while (iov_iter_count(iter)) {
struct page **pagevec;
@@ -538,7 +538,7 @@
* generic layer handle the completion.
*/
if (requested_bytes == 0) {
- inode_dio_done(inode);
+ inode_dio_end(inode);
nfs_direct_req_release(dreq);
return result < 0 ? result : -EIO;
}
@@ -872,7 +872,7 @@
&nfs_direct_write_completion_ops);
desc.pg_dreq = dreq;
get_dreq(dreq);
- atomic_inc(&inode->i_dio_count);
+ inode_dio_begin(inode);
NFS_I(inode)->write_io += iov_iter_count(iter);
while (iov_iter_count(iter)) {
@@ -928,7 +928,7 @@
* generic layer handle the completion.
*/
if (requested_bytes == 0) {
- inode_dio_done(inode);
+ inode_dio_end(inode);
nfs_direct_req_release(dreq);
return result < 0 ? result : -EIO;
}