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

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Date:	Fri, 22 Feb 2008 17:03:27 -0500
From:	Gregory Haskins <gregory.haskins.ml@...il.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	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

Paul E. McKenney wrote:
> Governing the timeout by context-switch overhead sounds even better to me.
> Really easy to calibrate, and short critical sections are of much shorter
> duration than are a context-switch pair.

Yeah, fully agree.  This is on my research "todo" list.  My theory is 
that the ultimate adaptive-timeout algorithm here would essentially be 
the following:

*) compute the context-switch pair time average for the system.  This is 
your time threshold (CSt).

*) For each lock, maintain an average hold-time (AHt) statistic (I am 
assuming this can be done cheaply...perhaps not).

The adaptive code would work as follows:

if (AHt > CSt) /* dont even bother if the average is greater than CSt */
    timeout = 0;
else
    timeout = AHt;

if (adaptive_wait(timeout))
    sleep();

Anyone have some good ideas on how to compute CSt?  I was thinking you 
could create two kthreads that message one another (measuring round-trip 
time) for some number (say 100) to get an average.  You could probably 
just approximate it with flushing workqueue jobs.

-Greg

> 
> 							Thanx, Paul
> 
>> Sven
>>
>>> 							Thanx, Paul
>>> -
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