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

[Jay Lan <jlan@engr.sgi.com>]

john stultz wrote:
> 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.

How is this proposed function different from
do_posix_clock_monotonic_gettime()?
It calls getnstimeofday(), it also adjusts with wall_to_monotinic.

It seems to me we just need to EXPORT_SYMBOL_GPL the
do_posix_clock_monotonic_gettime()?

Thanks,
  - jay

> 
> 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
>