Re: time: Should real time usage account discontinous jumps?

[David C Niemi]

I presume the point here is about the wall clock time, not actual CPU time 
usage, which is measured completely separately and for which 
CLOCK_MONOTONIC* are not in the picture.

I can only think of 3 fairly special cases where a difference in 
CLOCK_MONOTONIC_RAW differs meaningfully from a difference in the system 
time:

1) a leap second occurs

2) the hardware clock is inaccurate and NTP is making frequent small 
adjustments

3) the system time was set wrong and NTP is adjusting clock speed to get 
it to the correct time

Case 1 is very infrequent, and unlikely to be seen while you are using 
time other than on a very long-running task; and if it is a long-running 
task the error in the hardware clock could well exceed 1 second.  So it's 
about a wash either way.

Case 2 you are much better off with the system time than the hardware 
time, because the hardware time is far less accurate.

Case 3 you are temporarily better off with hardware time, but this is a 
rather special case.

So on the whole I don't see CLOCK_MONOTONIC_RAW being a more accurate 
source, especially not in the long run.  In addition, CLOCK_MONOTONIC_RAW 
is Linux-specific, and portability and simplicity are worthwhile goals in 
themselves when not faced with a clearly advantageous alternative.

David C Niemi

On Wed, 25 Sep 2013, Petr Pisar wrote:
> Hello,
>
> The GNU time as well as bash built-in compute real time process usage as
> a simple difference between two real time points. If there was a time
> adjustement in between (by NTP or manual), the meassured value would be
> affected.
>
> I have found any hint nowhere if this is intended behavior or if one should
> meassure some kind of monotic time line.
>
> Is there any general agreement? My personal opinion is that monotonic
> clock_gettime(CLOCK_MONOTONIC_RAW) is the best option.
>
> -- Petr

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| David C Niemi  (Reston, VA, USA)  niemi at tuxers dot net |
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