Back to Blog
Utime vs time6/11/2023 POSIX 7 describes CLOCK_MONOTONIC as an optional extension. This looks like a very good clock, but unfortunately it isn't yet available on OS X. Thus, any wrap-around would occur after many dozen years of uptime. I believe the 'seconds' and 'nanoseconds' are stored separately, each in 32-bit counters. ) provides nanosecond resolution, is monotonic. man clock says that since glibc 2.18 it is implemented with clock_gettime(CLOCK_PROCESS_CPUTIME_ID. This clock wraps around once it tops out (this typically happens after ~2^32 ticks, which is not very long for a 1 MHz clock). The precision on my system is indeed 1 µs. At one time this was supposed to be the CPU time in cycles, but modern standards like POSIX require CLOCKS_PER_SEC to be 1000000, giving a maximum possible precision of 1 µs. clock() seems to return the sum of user and system time.time() returns the wall-clock time from the OS, with precision in seconds.Here's what I've found so far for clocks in Linux and OS X: It's only really good for what the name says, a clock on the wall (or desktop). None of these things are good if you're using the time to schedule events or to benchmark performance. Is the specific function obsolete, non standard, etc.?īefore starting the list, I'd like to point out that wall-clock time is rarely the right time to use, whereas it changes with time zone changes, daylight savings time changes, or if the wall clock is synchronized by NTP.How do the above vary between implementations?.Is the clock monotonic, or will it change with changes in the system time (via NTP, time zone, daylight savings time, by the user, etc.)?.After how much time does the clock wrap around? Or is there some mechanism to avoid this?.What is the precision of the clock? (s, ms, µs, or faster?).What time is measured by the clock? (real, user, system, or, hopefully not, wall-clock?).For starts let's ask what the relevant properties are that we're looking for. Compiling a list of clock functions together with their properties would answer the question properly. There are also a lot of half-answers floating around. The problem is that there are several different time functions available in C and C , and some of them vary in behavior between implementations. What about clock_gettime, the API is in nanosecond, does it means that it's able to be so accurate if underlying hardware allows it? What about monotonicity? What about clock, the man page speak about "approximation", what does it mean? As a consequence its accuracy is usually the microsecond (can't be more because of the API) on recent hardware. Then the gettimeofday function seems to use the most accurate underlying hardware available. It seems that getrusage uses only the kernel tick (usually 1ms long) and as a consequence can't be more accurate than the ms. ![]() ![]() Which hardware or software mechanism does these functions use. Then the second question regards the implementation of these functions and as a consequence, their accuracy. getrusage and clock return process values. ![]() gettimeofday returns wall-clock value, clock_gettime returns wall-clock value or process or threads values depending on the Clock parameter passed to it. Among the timing functions, time, clock getrusage, clock_gettime, gettimeofday and timespec_get, I want to understand clearly how they are implemented and what are their return values in order to know in which situation I have to use them.įirst we need to classify functions returning wall-clock values compare to functions returning process or threads values. Slope = 3 m − 1 m 4 s − 0 s (Pick two points and plug the x values into the numerator and the t values into the denominator.) \text distance = 9 m start text, d, i, s, t, a, n, c, e, end text, equals, 9, start text, space, m, end text.
0 Comments
Read More
Leave a Reply. |