This is the mail archive of the mailing list for the systemtap project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

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

Frank Ch. Eigler wrote:
Li Guanglei <> 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.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]