Re: [Lse-tech] [RFC][Patch 1/5] nanosecond timestamps and diffs

[john stultz <johnstul@us.ibm.com>]

On Mon, 2005-12-12 at 19:31 +0000, Shailabh Nagar wrote:
> Christoph Lameter wrote:
> > On Wed, 7 Dec 2005, Shailabh Nagar wrote:
> > 
> > 
> >>+void getnstimestamp(struct timespec *ts)
> > 
> > 
> > There is already getnstimeofday in the kernel.
> > 
> 
> Yes, and that function is being used within the getnstimestamp() being proposed.
> However, John Stultz had advised that getnstimeofday could get affected by calls to
> settimeofday and had recommended adjusting the getnstimeofday value with wall_to_monotonic.
> 
> John, could you elaborate ?

I think you pretty well have it covered. 

getnstimeofday + wall_to_monotonic should be higher-res and more
reliable (then TSC based sched_clock(), for example) for getting a
timestamp.

There may be performance concerns as you have to access the clock
hardware in getnstimeofday(), but there really is no other way for
reliable finely grained monotonically increasing timestamps.

thanks
-john