MPI_WTIME(3)

💖 MPI_Wtime — the "how long did that even take" girlie 💖

Okay bestie, this is the MPI_WTIME(3) page from the Open MPI manual, and honestly? It's the glow-up your timing code has been waiting for. Sit down, grab a strawberry 🍓, and let's get into it.

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✨ NAME ✨

MPI_Wtime — returns an elapsed time on the calling processor. That's it, that's the whole vibe: it tells you how much time has gone by on your little corner of the machine. ⏱️

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🎀 SYNTAX 🎀

💅 C Syntax

#include <mpi.h>

double MPI_Wtime()

🦋 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
DOUBLE PRECISION MPI_WTIME()

🌸 Fortran 2008 Syntax

USE mpi_f08
DOUBLE PRECISION MPI_WTIME()

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⭐ RETURN VALUE ⭐

A time in seconds since an arbitrary time in the past. Yes, arbitrary. Like, don't ask when the clock started, it's not that kind of relationship. 🙈

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🌈 DESCRIPTION 🌈

MPI_Wtime hands you back a floating-point number of seconds representing elapsed wall-clock time since some time in the past. Main character clock energy, no cap. 👑

The "time in the past" is guaranteed not to change during the life of the process — so your starting point stays the same the whole time, slay. ✨ The user (that's you, bestie) is responsible for converting big piles of seconds into other units if you prefer something else. Seconds to minutes? That's on you, not on MPI.

This function is literally so portable — it returns seconds, not "ticks" — it allows high resolution, and it carries no unnecessary baggage. It's the low-maintenance friend of the timing world. 🫶 One would use it like this:

{
   double starttime, endtime;
   starttime = MPI_Wtime();
    ....  stuff to be timed  ...
   endtime   = MPI_Wtime();
   printf("That took %f seconds\n",endtime-starttime);
}

Look at that. Grab a start, do the thing, grab an end, subtract. Iconic behavior. 🎀

Heads up though: the times returned are local to the node that called them. There is no requirement that different nodes return the "same" time. So don't be out here comparing clocks across nodes like they're in the same group chat — they are not. 💀

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🔥 NOTES 🔥

• The boolean variable MPI_WTIME_IS_GLOBAL — a predefined attribute key that indicates whether clocks are synchronized — does not have a valid value in Open MPI, because the clocks are not guaranteed to be synchronized. So that whole check? Yeah, it's not giving what you want it to give. 🙈
• This function is intended to be a high-resolution, elapsed (or wall) clock. See MPI_Wtick to determine the resolution of MPI_Wtime. Go ask its sibling, iykyk. 💜
• On POSIX platforms, this function may utilize a timer that is cheaper to invoke than the gettimeofday() system call, but will fall back to gettimeofday() if a cheap high-resolution timer is not available. The ompi_info(1) command can be consulted to see if Open MPI supports a native high-resolution timer on your platform — look at the value for "MPI_Wtime support" (or "options:mpi-wtime" when viewing the parsable output). If that value is "native", a method that is likely to be cheaper than gettimeofday() will be used to obtain the time when MPI_Wtime is invoked. ✨
• For example, on platforms that support it, the clock_gettime() function will be used to obtain a monotonic clock value with whatever precision is supported on that platform (e.g., nanoseconds). We love a platform-specific flex. 🌸
• Note, too, that the MCA parameter opal_timer_require_monotonic can influence this behavior. It defaults to true, but if set to false, Open MPI may use a finer-grained timing mechanism (e.g., the RDTSC/RDTSCP clock ticks on x86_64 platforms) — but ngl, that is not guaranteed to be monotonic in some cases (e.g., if the MPI process is not bound to a single processor core). So like, faster? maybe. Stable? not always. Choose wisely, bestie. 💀
• This function does not return an error value. Consequently, the result of calling it before MPI_Init or after MPI_Finalize is undefined. Periodt. Don't do it. Undefined behavior is not cute, it's giving chaos. 😭

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🧚 SEE ALSO 🧚

• MPI_Wtick

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👑 COPYRIGHT 👑

2003-2026, The Open MPI Community

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Okay that's the whole page, bestie — go forth and time things accurately. 🦋 Slay responsibly. 💖

Sign-off szn: this gorgeous lil page is Open MPI, copyright 2003-2026, The Open MPI Community, dated Sep 11, 2026. Mwah. 💋 🎀