fs/block_dev.c: fix performance regression in O_DIRECT|O_SYNC writes to block devices
We are seeing a large regression in database performance on recent
kernels. The database opens a block device with O_DIRECT|O_SYNC and a
number of threads write to different regions of the file at the same time.
A simple test case is below. I haven't defined DEVICE since getting it
wrong will destroy your data :) On an 3 disk LVM with a 64k chunk size we
see about 17MB/sec and only a few threads in IO wait:
procs -----io---- -system-- -----cpu------
r b bi bo in cs us sy id wa st
0 3 0 16170 656 2259 0 0 86 14 0
0 2 0 16704 695 2408 0 0 92 8 0
0 2 0 17308 744 2653 0 0 86 14 0
0 2 0 17933 759 2777 0 0 89 10 0
Most threads are blocking in vfs_fsync_range, which has:
mutex_lock(&mapping->host->i_mutex);
err = fop->fsync(file, dentry, datasync);
if (!ret)
ret = err;
mutex_unlock(&mapping->host->i_mutex);
commit 148f948ba877f4d3cdef036b1ff6d9f68986706a (vfs: Introduce new
helpers for syncing after writing to O_SYNC file or IS_SYNC inode) offers
some explanation of what is going on:
Use these new helpers for syncing from generic VFS functions. This makes
O_SYNC writes to block devices acquire i_mutex for syncing. If we really
care about this, we can make block_fsync() drop the i_mutex and reacquire
it before it returns.
Thanks Jan for such a good commit message! As well as dropping i_mutex,
Christoph suggests we should remove the call to sync_blockdev():
> sync_blockdev is an overcomplicated alias for filemap_write_and_wait on
> the block device inode, which is exactly what we did just before calling
> into ->fsync
The patch below incorporates both suggestions. With it the testcase improves
from 17MB/s to 68M/sec:
procs -----io---- -system-- -----cpu------
r b bi bo in cs us sy id wa st
0 7 0 65536 1000 3878 0 0 70 30 0
0 34 0 69632 1016 3921 0 1 46 53 0
0 57 0 69632 1000 3921 0 0 55 45 0
0 53 0 69640 754 4111 0 0 81 19 0
Testcase:
#define _GNU_SOURCE
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#define NR_THREADS 64
#define BUFSIZE (64 * 1024)
#define DEVICE "/dev/mapper/XXXXXX"
#define ALIGN(VAL, SIZE) (((VAL)+(SIZE)-1) & ~((SIZE)-1))
static int fd;
static void *doit(void *arg)
{
unsigned long offset = (long)arg;
char *b, *buf;
b = malloc(BUFSIZE + 1024);
buf = (char *)ALIGN((unsigned long)b, 1024);
memset(buf, 0, BUFSIZE);
while (1)
pwrite(fd, buf, BUFSIZE, offset);
}
int main(int argc, char *argv[])
{
int flags = O_RDWR|O_DIRECT;
int i;
unsigned long offset = 0;
if (argc > 1 && !strcmp(argv[1], "O_SYNC"))
flags |= O_SYNC;
fd = open(DEVICE, flags);
if (fd == -1) {
perror("open");
exit(1);
}
for (i = 0; i < NR_THREADS-1; i++) {
pthread_t tid;
pthread_create(&tid, NULL, doit, (void *)offset);
offset += BUFSIZE;
}
doit((void *)offset);
return 0;
}
Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Jan Kara <jack@suse.cz>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/block_dev.c b/fs/block_dev.c
index 2a6d019..6dcee88 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -406,16 +406,23 @@
int blkdev_fsync(struct file *filp, struct dentry *dentry, int datasync)
{
- struct block_device *bdev = I_BDEV(filp->f_mapping->host);
+ struct inode *bd_inode = filp->f_mapping->host;
+ struct block_device *bdev = I_BDEV(bd_inode);
int error;
- error = sync_blockdev(bdev);
- if (error)
- return error;
-
+ /*
+ * There is no need to serialise calls to blkdev_issue_flush with
+ * i_mutex and doing so causes performance issues with concurrent
+ * O_SYNC writers to a block device.
+ */
+ mutex_unlock(&bd_inode->i_mutex);
+
error = blkdev_issue_flush(bdev, NULL);
if (error == -EOPNOTSUPP)
error = 0;
+
+ mutex_lock(&bd_inode->i_mutex);
+
return error;
}
EXPORT_SYMBOL(blkdev_fsync);
