> code:
>                 struct timeval tvStart,tvEnd;
>                 double linStart = 0,linEnd = 0,lTime = 0;

>                 gettimeofday (&tvStart,NULL);

> > I hope someone can help me with negative time measurements > I am
> > getting.
> > Surely i'm doing something silly but cannot find it.

> > code:
> >                 struct timeval tvStart,tvEnd;
> >                 double linStart = 0,linEnd = 0,lTime = 0;

> >                 gettimeofday (&tvStart,NULL);

> You are using the wrong function. The 'gettimeofday' function gets the
> system's best guess at wall time. This *can* go backwards. What you
> want is 'clock_gettime(CLOCK_MONOTONIC)' which measures the elaspsed
> time from an arbitrary reference time. (Usually the system uptime, but
> that's not guaranteed.)

> DS

> > As I know, the Linux kernel read the system RTC(wall time) and then
> > maintain the Wall time through OS timer, but why the gettimeofday not
> > accurate?

> What makes you think it's not accurate?

> DS

> > > What makes you think it's not accurate?
> > You said that it can go backwards. If it will go back of the
> > time, is
> > it accurate. Oh, yes, it is wrong.

> The purpose of 'gettimeofday' is to get the system's best guess at
> actual wall clock time. This can go backwards.

> For example, suppose you check the time on a very accurate clock. It
> reads "10:03". About 40 minutes pass and I ask you what time it is.
> You say, "my best guess is 10:43". Then you look at the clock again
> and see it is 10:31. I ask you what time it is about a minute later
> and you say, "my best guess is 10:32".

> You can ask for a time that is guaranteed monotonic or you can ask for
> the best guess of wall time, but there is no way to get both at the
> same time. The function you are using requests the system's best guess
> at actual wall clock time, and it can be working perfectly and still
> go backwards.

> DS

> > I have some questions after I dive into the kernel source, as you
> > said, the kernel just estimate the time when you call the
> > gettimeofday(). I remember this *guess* is done by accumulating each
> > jiffy that created by OS timer.

> Correct.

> > But I don't find the code that kernel read the RTC(wall time) to
> > calibrate the guessed value.

> It tracks wall time itself in most hardware configurations you are
> likely to encounter. If the TSC is usable, it will periodically work
> out calibration values to convert the TSC to wall time.

> If you need/want monotonic time, there are functions for that. But
> 'gettimeofday' *specifically* gives up monotonicity in exchange for a
> better estimate of wall time.

> There are plenty of reasons it guesses backwards. The kernel usually
> has at least two time sources, one that indicates time boundaries and
> one that indicates time passage. If the source of time passage speeds
> up a bit, the next boundary may result in the time going a bit
> backwards.

> For example, suppose someone tells you the time periodically (this is
> like the timer interrupt). You also calibrate your own clock (this is
> like the TSC). If the TSC speeds up a bit, you might think it's 10:01
> when you're told it's 10:00. You'll immediately slow your own clock
> down to compensate, but at that instant, your best guess of wall time
> will go backwards.

> Simply put, having monotonicity (never going backwards) and making the
> best possible guess of wall clock time are *conflicting* requirements.
> You specifically used the function that trades one for the other.

> DS