I'm failing to understand how getrusage() works, which is a bit perplexing,
because it doesn't seem like it would be terribly complicated.
I've attached the code. My aim is to verify that I can use getrusage() to
do (admittedly crude) instrumentation of which functions in my program are
allocating lots of memory[1]. So I figure I can call getrusage() at various
points and look at the ru_maxrss member.
I'm confused because the results I'm getting don't make a great deal of
sense. The attached program, when run 10 times one after the other,
produces the following output;
before: 0, after: 1300
before: 0, after: 0
before: 0, after: 0
before: 188, after: 188
before: 0, after: 0
before: 452, after: 452
before: 0, after: 0
before: 0, after: 1316
before: 0, after: 0
before: 0, after: 0
'before' is ru_maxrss before 'malloc(1024 * 1024)', 'after' is ru_maxrss
afterwards. Different runs produce slightly different numbers -- the point
is that they vary widely, and in some cases are '0' for both cases.
Those results don't look sane to me. From doing some googling it looks like
there may be a delay between the process's resident set size increasing and
the stats that getrusage() uses being updated.
If that's the case, can I (portably?) wait for the getrusage() stats to
synchronise before I read them? If it's not the case, what am I doing
wrong? Is there another (portable) mechanism I use to find out how big my
process is?
N
[1] I'm actually instrumenting Perl applications, using Perl's built-in
debugger, instrumenting at the Perl level, so solutions of the form "Use
<this> profiling application" aren't going to be applicable. I'm using
BSD::Resource, which wraps getrusage(), and odd results from that led me to
this C test.
/* Call getrusage() before and after allocating 1MB of RAM, and see what
the maxrss figure is */
#include <sys/types.h>
#include <sys/time.h>
#include <sys/resource.h>
#include <stdio.h>
int
main(void)
{
struct rusage ru;
char *ptr;
int rc;
rc = getrusage(RUSAGE_SELF, &ru);
if(rc == -1) {
perror("getrusage(): ");
return 1;
}
printf("before: %ld, ", ru.ru_maxrss);
ptr = malloc(1024 * 1024);
if(ptr == NULL) {
perror("malloc(): ");
return 1;
}
memset(ptr, 0, 1024 * 1024); /* Zero-fill the memory */
rc = getrusage(RUSAGE_SELF, &ru);
if(rc == -1) {
perror("getrusage(): ");
return 1;
}
printf("after: %ld\n", ru.ru_maxrss);
free(ptr);
return 0;
}
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