Cypress CSC-1200T Guía de usuario Pagina 104

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104 Cray T3E User’s Guide
Chapter 10
Miscellaneous notes
This chapter discusses some additional topics, such as timing of pro-
grams and defining the scalability of parallel programs.
10.1 Obtaining timing information
The most useful measure of processing time in a parallel environment
is the wall clock time. This is due to the fact that traditional CPU times
are processor-based, whereas the wall clock time gives a global view of
aggregate parallel performance. All CSC’s Cray T3E processors run at
375 MHz.
10.1.1 The timex command
The easiest way to find out wall clock times is to use the Unix command
timex in front of the executable name:
t3e% timex mpprun -n 16 ./prog.x
10.1.2 The wall clock timer
On all Cray platforms the following C routine can be used to return the
elapsed wall clock time in seconds:
#include <unistd.h>
double SECS(void) {
static long cpcycle = 0;
/* Get cycle time in picoseconds */
if (cpcycle == 0) cpcycle = sysconf(_SC_CRAY_CPCYCLE);
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