Re: using get_cycles() to get the real elapsed time

["Jose R. Santos" <jrs at us dot ibm dot com>]

Frank Ch. Eigler wrote:
> Li Guanglei <guanglei@cn.ibm.com> writes:
>
> > [...]
> > So we need deal with different Arches to get the elapsed time by
> > using get_cycles()
>
> Yes, eventually recreating the complexity of gettimeofday().
>   

This we can offload to user space as well.  I order for this to work, we 
would need to specify the type of processors being used somewhere on the 
trace header output.

> > BTW, get_cycles() will return 32-bit value for some architectures
> > e.g ppc32, although the major architectures such as ppc64, x86,
> > x86_64 will all return a 64-bit value. [...]
>
> It's even worse than that: I gather some machines return a putatively
> 64-bit value, but the number within overflows after a few bits past 40.
> So any user of get_cycles() must correct at least for
> - change in frequency
> - counter overflow
> - cross-CPU drift
>   

This are some of the reasons we decided to use gettimeofday() in the 
first place.  Counter overflow was something that I wanted to wait for 
perfmon2 which dadresses this.  The problem with perfmon2 is that is 
still unknown when it will be supported by SystemTap and perfmon2 still 
limited CPU support.

Hopefully we will get access to system capable of doing  more that 150k 
IO/sec.  The performance data from this system will be able to tell us 
what the performance impact of gettimeofday() is and if we need to 
concentrate our efforts in other tasks first.

-JRS