Re: ext4 dbench performance with CONFIG_PREEMPT_RT

[john stultz <johnstul@us.ibm.com>]

On Wed, 2010-04-07 at 23:46 -0400, tytso@mit.edu wrote:
> On Wed, Apr 07, 2010 at 04:21:18PM -0700, john stultz wrote:
> > Further using lockstat I was able to isolate it the contention down to
> > the journal j_state_lock, and then adding some lock owner tracking, I
> > was able to see that the lock owners were almost always in
> > start_this_handle, and jbd2_journal_stop when we saw contention (with
> > the freq breakdown being about 55% in jbd2_journal_stop and 45% in
> > start_this_handle).
> 
> Hmm....  I've taken a very close look at jbd2_journal_stop(), and I
> don't think we need to take j_state_lock() at all except if we need to
> call jbd2_log_start_commit().  t_outstanding_credits,
> h_buffer_credits, and t_updates are all documented (and verified by
> me) to be protected by the t_handle_lock spinlock.
> 
> So I ***think*** the following might be safe.  WARNING!  WARNING!!  No
> real testing done on this patch, other than "it compiles!  ship it!!".
> 
> I'll let other people review it, and maybe you could give this a run
> and see what happens with this patch?


So this patch seems to match the performance and has similar perf log
output to what I was getting with my hack. 

Very very cool!

I'll continue to play with your patch and see if I can con some some
folks with more interesting storage setups to do some testing as well.

Any thoughts for ways to rework the state_lock in start_this_handle?
(Now that its at the top of the contention logs? :)

thanks so much!
-john