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Date: Fri, 22 Feb 2008 17:03:27 -0500
From: Gregory Haskins <gregory.haskins.ml@gmail.com>
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To: paulmck@linux.vnet.ibm.com
CC: Sven-Thorsten Dietrich <sdietrich@novell.com>,
       "Bill Huey (hui)" <bill.huey@gmail.com>, Andi Kleen <ak@suse.de>,
       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,
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       gregkh@suse.de, pmorreale@novell.com, mkohari@novell.com
Subject: Re: [PATCH [RT] 08/14] add a loop counter based timeout mechanism
References: <20080221152504.4804.8724.stgit@novell1.haskins.net> <20080221152707.4804.59177.stgit@novell1.haskins.net> <200802211741.10299.ak@suse.de> <20080222190814.GD11213@linux.vnet.ibm.com> <9810cff90802221119j23818e74g2721512a693a0a01@mail.gmail.com> <9810cff90802221121s216f69f4k4a5f39eaaf11dd7f@mail.gmail.com> <20080222194341.GE11213@linux.vnet.ibm.com> <1203710145.4772.107.camel@sven.thebigcorporation.com> <20080222202316.GF11213@linux.vnet.ibm.com>
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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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