Difference between clock() and MPI_Wtime()

Question

Quick Question . for MPI implementation of my code ,i am getting a huge difference in both. I know MPI_Wtime is the real time elapsed by each processor and clock() gives a rough idea of the expected time . Do anyone wants to add some assertion ?

Answer 1

The clock function is utterly useless. It measures cpu time, not real time/wall time, and moreover it has the following serious issues:

1. On most implementations, the resolution is extremely bad, for example, 1/100 of a second. CLOCKS_PER_SECOND is not the resolution, just the scale.

2. With typical values of CLOCKS_PER_SECOND (Unix standards require it to be 1 million, for example), clock will overflow in a matter of minutes on 32-bit systems. After overflow, it returns -1.

3. Most historical implementations don't actually return -1 on overflow, as the C standard requires, but instead wrap. As clock_t is usually a signed type, attempting to perform arithmetic with the wrapped values will produce either meaningless results or undefined behavior.

4. On Windows it does the completely wrong thing and measures elapsed real time, rather than cpu time.

Answer 2

The official definition of clock is that it gives you CPU-time. In Windows, for hysterical historical reasons - it would break some apps if you change it to reflect CPU-time now - on Windows, the time is just elapsed time.

MPI_Wtime gives, as you say, the "current time on this processor", which is quite different. If you do something that sleeps for 1 minute, MPI_Wtime will move 60 seconds forward, where clock (except for Windows) would be pretty much unchanged.