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From: Jens Axboe on 15 Apr 2010 06:10
On Thu, Apr 15 2010, Anton Blanchard wrote:
>
> 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.

Looks good to me, I see Jan already made a few style suggestions.

--
Jens Axboe

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Prev: [tip:sched/core] sched: Fix UP update_avg() build warning
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