That is, if the program will normally be run against a file that has not been accessed recently, you must make sure that the file used in measuring the program is not in memory. If, on the other hand, a program is usually run as part of a standard sequence in which it gets its input from the output of the preceding program, you should prime memory to ensure that the measurement is authentic. If the output of one program is the input of the next and the entire file won't fit in RAM, the second program will end up reading pages at the head of the file, which displace pages at the end. Although this situation is very hard to simulate authentically, it is nearly equivalent to one in which no disk caching takes place. A way to check on this possibility is shown in the following vmstatit shell script.
Typical programs read file input and then write file output.
Paging activity, on the other hand, typically begins with the writing out of a working-segment page that does not fit. The page is read back in only if the program tries to access it. The fact that the test system has experienced almost twice as many paging space page outs as paging space page ins since it was booted indicates that at least some of the programs that have been run on this system have stored data in memory that was not accessed again before the end of the program. Memory-Limited Programs provides more information. See also Monitoring and Tuning Memory Use .