linux-ext4 - RE: ext4 dbench performance with CONFIG_PREEMPT

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Date:	Fri, 9 Apr 2010 17:48:23 -0600
From:	"Chen, Tim C" <tim.c.chen@...el.com>
To:	"tytso@....edu" <tytso@....edu>, Andi Kleen <andi@...stfloor.org>
CC:	john stultz <johnstul@...ibm.com>,
	"linux-ext4@...r.kernel.org" <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

 

>tytso@....edu wrote 
>
>Yeah, I'm very much aware of that.  What worries me is that locking
>problems in the jbd2 layer could be very hard to debug, so we need to
>make sure we have some really good testing as we make any changes.
>
>Not taking the j_state_lock spinlock in jbd2_stop_lock() was relatively
>easy to prove to be safe, but I'm really worried about
>start_this_handle() the locking around that is going to be subtle, and
>it's not just the specific fields in the transaction and journal
>handle.
>
>And even with the jbd2_stop_lock() change, I'd really prefer some
>pretty exhaustive testing, including power fail testing, just to make
>sure we're in practice when/if we make more subtle or more invasive
>changes to the jbd2 layer...
>
>So I'm mot waving the red flag, but the yellow flag (as they would say
>in auto racing circles).
>

Your patch did remove the contention on the j_state_lock for dbench
in my testing with 64 threads.  The contention point now
moves dcache_lock, which is also another tricky bottleneck.

In our other testing with FFSB that creates/rename/remove a lot of directories,
we found that journal->j_revoke_lock was also heavily contended.

Tim--
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