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

[Gregory Haskins <gregory.haskins.ml@gmail.com>]

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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