linux-kernel - Re: [PATCH [RT] 08/14] add a loop counter based timeout mechanism

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Date:	Sat, 23 Feb 2008 08:32:11 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andi Kleen <ak@...e.de>
Cc:	gregory.haskins@...il.com,
	Sven-Thorsten Dietrich <sdietrich@...ell.com>,
	"Bill Huey (hui)" <bill.huey@...il.com>,
	Gregory Haskins <ghaskins@...ell.com>, mingo@...e.hu,
	a.p.zijlstra@...llo.nl, tglx@...utronix.de, rostedt@...dmis.org,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	kevin@...man.org, cminyard@...sta.com, dsingleton@...sta.com,
	dwalker@...sta.com, npiggin@...e.de, dsaxena@...xity.net,
	gregkh@...e.de, pmorreale@...ell.com, mkohari@...ell.com
Subject: Re: [PATCH [RT] 08/14] add a loop counter based timeout mechanism

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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