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direct-io: only inc/dec inode->i_dio_count for file systems

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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>
This commit is contained in:
Jens Axboe
2015-04-15 17:05:48 -06:00
committed by Al Viro
parent 8e3c500594
commit fe0f07d08e
9 changed files with 50 additions and 33 deletions
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29
include/linux/fs.h
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@@ -2635,6 +2635,9 @@ enum {
/* filesystem can handle aio writes beyond i_size */
DIO_ASYNC_EXTEND = 0x04,
/* inode/fs/bdev does not need truncate protection */
DIO_SKIP_DIO_COUNT = 0x08,
};
void dio_end_io(struct bio *bio, int error);
@@ -2657,7 +2660,31 @@ static inline ssize_t blockdev_direct_IO(struct kiocb *iocb,
#endif
void inode_dio_wait(struct inode *inode);
void inode_dio_done(struct inode *inode);
/*
* inode_dio_begin - signal start of a direct I/O requests
* @inode: inode the direct I/O happens on
*
* This is called once we've finished processing a direct I/O request,
* and is used to wake up callers waiting for direct I/O to be quiesced.
*/
static inline void inode_dio_begin(struct inode *inode)
{
atomic_inc(&inode->i_dio_count);
}
/*
* inode_dio_end - signal finish of a direct I/O requests
* @inode: inode the direct I/O happens on
*
* This is called once we've finished processing a direct I/O request,
* and is used to wake up callers waiting for direct I/O to be quiesced.
*/
static inline void inode_dio_end(struct inode *inode)
{
if (atomic_dec_and_test(&inode->i_dio_count))
wake_up_bit(&inode->i_state, __I_DIO_WAKEUP);
}
extern void inode_set_flags(struct inode *inode, unsigned int flags,
unsigned int mask);