linux-ext4 - Re: ext4 dbench performance with CONFIG_PREEMPT

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Message-ID: <20100408211054.GB1849@thunk.org>
Date:	Thu, 8 Apr 2010 17:10:54 -0400
From:	tytso@....edu
To:	john stultz <johnstul@...ibm.com>
Cc:	linux-ext4@...r.kernel.org, Mingming Cao <cmm@...ibm.com>,
	keith maanthey <kmannth@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Darren Hart <dvhltc@...ibm.com>
Subject: Re: ext4 dbench performance with CONFIG_PREEMPT_RT

On Thu, Apr 08, 2010 at 01:41:57PM -0700, john stultz wrote:
> 
> 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.

You might want to ask djwong to play with it with his nice big
machine.  (We don't need a big file system, but we want as many CPU's
as possible, and to use his "mailserver" workload to really stress the
journal.  I'd recommend using barrier=0 for additional journal
lock-level stress testing, and then try some forced sysrq-b reboots
and then make sure that the filesystem is consistent after the journal
replay.)

I've since done basic two-CPU testing using xfstests under KVM, but
that's really not going to test any locking issues.

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

That's going to be much harder.  We're going to have to take
j_state_lock at some point inside start_this_handle.  We might be able
to decrease the amount of code which is run while the spinlock is
taken, but I very much doubt it's possible to eliminate that spinlock
entirely.

Do you have detailed lockstat information showing the hold-time and
wait-time of j_lock_stat (especially in start_this_handle)?

						- Ted
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