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

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Date:	Sat, 23 Feb 2008 13:31:00 +0100
From:	Andi Kleen <ak@...e.de>
To:	gregory.haskins@...il.com
Cc:	paulmck@...ux.vnet.ibm.com,
	Sven-Thorsten Dietrich <sdietrich@...ell.com>,
	"Bill Huey (hui)" <bill.huey@...il.com>, Andi Kleen <ak@...e.de>,
	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

> *) 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.

>
> *) 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.

-Andi
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