Re: [PATCH] optimize writer path in time_interpolator_get_counter()

[Alex Williamson <alex.williamson@hp.com>]

On Wed, 2005-08-03 at 09:10 -0700, Christoph Lameter wrote:
> On Wed, 3 Aug 2005, Alex Williamson wrote:
> 
> > be a reasonable performance vs absolute accuracy trade-off.  What
> > happens to your worst case time if you (just for a test) hard code a
> > min_delta of something around 20-50?  There could be some kind of
> 
> Think about a threaded process that gets time on multiple processors 
> and then compares the times. This means that the time value obtained later 
> on one thread may indicate a time earlier than that obtained on another 
> thread. An essential requirement for time values is that they are 
> monotonically increasing. You are changing that basic nature.

   Ok, I can see the scenario where that could produce jitter.  However,
that implies than any exit through that path could produce jitter as it
is.  For instance:

CPU0				CPU1
read lcycle
read itc
				read lcycle
				read itc
cmpxchg
				oops, lcycle is stale

So the window already exists for this...

	Alex

-- 
Alex Williamson                             HP Linux & Open Source Lab