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

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Date:	Mon, 25 Feb 2008 15:52:06 -0800
From:	Sven-Thorsten Dietrich <sdietrich@...ell.com>
To:	Andi Kleen <ak@...e.de>
Cc:	gregory.haskins@...il.com, paulmck@...ux.vnet.ibm.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, 2008-02-23 at 13:31 +0100, Andi Kleen wrote:
> > *) compute the context-switch pair time average for the system.  This is 
> > your time threshold (CSt).
> 

Hi Andi,

> 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.
> 
> >
> > *) 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.
> 

I agree, this does not make any sense until you get to very large SMP
systems - and only iff it can be demonstrated, that the overhead
associated with lock statistics and including some notion of the context
switch overhead still comes out with a net gain.

There is some notion of task-routing in the RT scheduler already, and
this is quite a clever little algorithm. 

I see no reason, why the scheduler should not eventually take directly
into account (when balancing), the quantifiable context-switch and CPU
overhead of moving a task to a distant processor.

In fact, for a deadline-based scheduler working on high-frequency tasks,
given that the times can switch so radically, this is would be
requiredOnve context-switch-overhead data is available to the scheduler,
there is no particular reason why adaptive locking could not also
utilize that data.

Sven

> -Andi
> --
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