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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, Feb 23, 2008 at 01:31:00PM +0100, Andi Kleen wrote:
> > *) 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.

Wouldn't the common case for blocking on a lock with a short hold
time be the minimal context-switch pair on the same CPU?

> > *) 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.

Me too.  That said, I cannot resist pointing out that the measurement
of interest would be the fraction of lock-hold times that are shorter
than the context-switch time.  ;-)

							Thanx, Paul