lkml.org 
[lkml]   [2010]   [Apr]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[PATCH] Fix 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 to anything 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! The patch below drops the i_mutex
in blkdev_fsync as suggested. 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

I'd appreciate any comments from the I/O guys on if this is the right approach.


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>
---

Index: linux-2.6/fs/block_dev.c
===================================================================
--- linux-2.6.orig/fs/block_dev.c 2010-04-14 12:55:50.000000000 +1000
+++ linux-2.6/fs/block_dev.c 2010-04-14 13:17:45.000000000 +1000
@@ -406,16 +406,24 @@ static loff_t block_llseek(struct file *

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;

+ mutex_unlock(&bd_inode->i_mutex);
+
error = sync_blockdev(bdev);
- if (error)
+ if (error) {
+ mutex_lock(&bd_inode->i_mutex);
return error;
+ }

error = blkdev_issue_flush(bdev, NULL);
if (error == -EOPNOTSUPP)
error = 0;
+
+ mutex_lock(&bd_inode->i_mutex);
+
return error;
}
EXPORT_SYMBOL(blkdev_fsync);

\
 
 \ /
  Last update: 2010-04-15 06:45    [W:0.100 / U:0.284 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site