Re: 2.6.35 performance results

[Chris Mason <chris.mason@oracle.com>]

On Fri, Aug 06, 2010 at 01:44:11PM -0500, Steven Pratt wrote:
> Here is the latest set of performance runs from the 2.6.35-rc5 tree.
> Included is a refresh of all the other filesystems with some changes
> for barriers on and off since this has been somewhat of a hot topic
> recently.
> 
> New data linked in to the history graphs here:
> http://btrfs.boxacle.net/repository/raid/history/History.html
> 
> From a BTRFS performance perspective, we took a major regression on
> write heavy workloads. As much as a 10x hit!  The problems seems to
> be due to this changeset:
> http://git.kernel.org/?p=linux/kernel/git/mason/btrfs-unstable.git;a=commit;h=5da9d01b66458b180a6bee0e637a1d0a3effc622

Ouch!  The problem is we're not being aggressive enough about
allocating chunks for data, which makes the flusher come in and start
data IO.

Thanks a lot for finding the regression, my machine definitely didn't
show this.

I'll reproduce and fix it up.

-chris

> Btrfs: Shrink delay allocated space in a synchronized
> 
> Shrink delayed allocation space in a synchronized manner is more
> controllable than flushing all delay allocated space in an async
> thread.
> 
> This changeset introduced "btrfs_start_one_delalloc_inode" in
> http://git.kernel.org/?p=linux/kernel/git/mason/btrfs-unstable.git;a=commitdiff;h=5da9d01b66458b180a6bee0e637a1d0a3effc622
> 
> In heavy write workloads this new function is now dominating the profiles:
> 
> samples  %        app name                 symbol name
> 8914973  65.1261  btrfs.ko                 btrfs_start_one_delalloc_inode
> 1024841   7.4867  vmlinux-2.6.35-rc5-autokern1 rb_get_reader_page
> 716046    5.2309  vmlinux-2.6.35-rc5-autokern1 ring_buffer_consume
> 315354    2.3037  oprofile.ko              add_event_entry
> 202484    1.4792  vmlinux-2.6.35-rc5-autokern1 write_inode_now
> 195018    1.4247  btrfs.ko                 btrfs_tree_lock
> 
> 
> Appears to be major contention on the spin lock, as this gets worse
> with more threads.  This needs to be redone.
> 
> 
> Steve
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