Hey! > Le 10 oct. 2018 à 10:10, Bruno Haible <br...@clisp.org> a écrit : > > Hi Akim, > >> With respect to precision: > >> - times returns a clock_t, in system dependent clock ticks. >> Resolution is unclear, but I think I understood from various >> place that it’s typically 1µs. > > No, you can't assume that. You need to measure it. Here's what I get > on today's mainstream architecture (Linux 4.x, x86_64 CPU from Intel): > > $ ./measure 1000 > tms_utime: 0.000000 > monotonic: 0.000015
You’re right, I stayed at the level of the theoretical precision rather than in practice. > So, very obviously, the resolution of times() is [10] milliseconds (!). […] > So, the resolution of getrusage() is 4 ms. Slightly better than times(), I’m surprised by this. I would have bet both shared the same basic implementation. > bit still orders of magnitude worse than CLOCK_MONOTONIC. >> With respect to what you called multiplicity (i.e., time spent >> in subprocesses or threads): >> >> - getrusage offers two choices: >> - RUSAGE_SELF: the calling process and its threads. >> - RUSAGE_CHILDREN: main and subprocesses that have been wait’ed. >> - Linux offers RUSAGE_THREAD, which is just about the current >> thread. I don’t think we care about this. >> (https://linux.die.net/man/2/getrusage) > > Good. Do you want 'timevar' to support RUSAGE_SELF and RUSAGE_CHILDREN, > at the user's choice, or do you want to pick one for the user? > > In summary, I think the user needs to have the choice: > - For low-precision timings, offer getrusage with RUSAGE_CHILDREN or > optionally RUSAGE_SELF as an alternative. > - For high-precision timings, offer CLOCK_MONOTONIC and don't call > neither getrusage nor times (because the point of high-precision > timings is to have minimal system call overhead). > > And this choice needs to be made at runtime. It would be silly to have > to recompile a program just to measure longer vs. shorter tasks. You lost me here. CLOCK_MONOTONIC is about wall time, the ‘real’ time (sorry Albert, I didn’t mean that), not time spend on the behalf of the process. Maybe there’s an API I missed? I think that at this stage we want usr/sys via getrusage and wall via CLOCK_MONOTONIC. The choice between proper time and self+children seems a reasonable option to offer, but if we are to keep just one, I’d stick with the one we have currently: including subprocesses.
