Date: Sat, 23 Feb 2008 08:32:11 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Andi Kleen <ak@suse.de>
Cc: gregory.haskins@gmail.com, Sven-Thorsten Dietrich <sdietrich@novell.com>,
       "Bill Huey (hui)" <bill.huey@gmail.com>,
       Gregory Haskins <ghaskins@novell.com>, mingo@elte.hu,
       a.p.zijlstra@chello.nl, tglx@linutronix.de, rostedt@goodmis.org,
       linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org,
       kevin@hilman.org, cminyard@mvista.com, dsingleton@mvista.com,
       dwalker@mvista.com, npiggin@suse.de, dsaxena@plexity.net,
       gregkh@suse.de, pmorreale@novell.com, mkohari@novell.com
Subject: Re: [PATCH [RT] 08/14] add a loop counter based timeout mechanism
Message-ID: <20080223163211.GB12351@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
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On Sat, Feb 23, 2008 at 01:31:00PM +0100, Andi Kleen wrote:
> > *) compute the context-switch pair time average for the system.  This is 
> > your time threshold (CSt).
> 
> This is not a uniform time. Consider the difference between
> context switch on the same hyperthread, context switch between cores
> on a die, context switch between sockets, context switch between
> distant numa nodes. You could have several orders of magnitude
> between all those.

Wouldn't the common case for blocking on a lock with a short hold
time be the minimal context-switch pair on the same CPU?

> > *) For each lock, maintain an average hold-time (AHt) statistic (I am 
> > assuming this can be done cheaply...perhaps not).
> 
> That would assume that the hold times are very uniform. But what happens
> when you e.g. have a workload where 50% of the lock aquisitions are short 
> and 30% are long? 
> 
> I'm a little sceptical of such "too clever" algorithms.

Me too.  That said, I cannot resist pointing out that the measurement
of interest would be the fraction of lock-hold times that are shorter
than the context-switch time.  ;-)

							Thanx, Paul
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