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

[George Anzinger <george@mvista.com>]

john stultz wrote:
> On Mon, 2005-12-12 at 20:00 +0000, Shailabh Nagar wrote:
> 
>>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.
>>>
>>>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, that clarifies. I guess the other underlying concern here would be whether these
>>improvements (in resolution and reliability) should be going into getnstimeofday()
>>itself (rather than creating a new func for the same) ? Or is it better to leave
>>getnstimeofday as it is ?
> 
> 
> No, getnstimeofday() is very much needed to get a nanosecond grained
> wall-time clock, so a new function is needed for the monotonic clock.
> 
> In my timeofday re-work I have used the name "get_monotonic_clock()" and
> "get_monotonic_clock_ts()" for basically the same functionality
> (providing a ktime and a timespec respectively). You might consider
> naming it as such, but resolving these naming collisions shouldn't be
> too difficult either way.

Indeed.  Lets use a name with "monotonic" in it, please.  And, 
possibly not "clock".  How about get_nsmonotonic_time() or some such?


-- 
George Anzinger   george@mvista.com
HRT (High-res-timers):  http://sourceforge.net/projects/high-res-timers/