Messages in this thread | | | Date | Thu, 15 Apr 2010 12:04:15 +0200 | From | Jens Axboe <> | Subject | Re: [PATCH] Fix regression in O_DIRECT|O_SYNC writes to block devices |
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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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