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

[Shailabh Nagar <nagar@watson.ibm.com>]

Jay Lan 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.
> 
> 
> 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
> 

Hmmm. Looks like do_posix_clock_monotonic_gettime will suffice for this patch.

Wonder why the clock parameter to do_posix_clock_monotonic_get is needed ?
Doesn't seem to be used.

Any possibility of these set of functions changing their behaviour ?

-- Shailabh







>>
>> 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
>>
> 
>