gistfile1.txt

/*
 * This was a grand experiment - I would evaluate students based on the only
 * meaningful metric, amount of memory they actually used. This would make slab
 * allocators more interesting, as they could potentially be more efficient in
 * larger programs. Unfortunately, it is *exceptionally* hard to measure the
 * current memory usage of a program, and even harder to do so portably.
 *
 * The most promising approach was getrusage(2). This library call is available
 * on Mac OSX and Linux, and offers the ability to get the maximum resident set
 * size. At first blush this was promising, but unfortunately there were some
 * complications. First the units for ru_maxrss are consistent accross
 * platforms: on MacOSX the man page claims the unit is bytes, whereas on Linux
 * the man page claims the unit is kilobytes. Even worse, getusage(2) does not
 * interact in a clean way with mmap(2). For the following sequnce of operation,
 * the program will likely print the same value of ru_maxrss both times:
 *
 *     getrusage(RUSAGE_SELF, &usage);
 *     printf("%ld\n", usage.ru_maxrss);
 *     memset(mmap(NULL, 4096*10, PROT_WRITE|PROT_READ,
 *         MAP_ANONYMOUS|MAP_PRIVATE, 0, 0), 'A', 4096*10);
 *     getrusage(RUSAGE_SELF, &usage);
 *     printf("%ld\n", usage.ru_maxrss);
 *
 * Nearest I can tell, this is because ru_maxrss does not relate to the current
 * resident set size.
 *
 * I then took a different tack: abandoning my dreams of a cross platform
 * assignment, I chose to use /proc/self/statm from the proc(5) file-system.
 * This was more promising, as it returned the actual resident set size.
 * Unfortunately, this still did not interact cleanly with mmap. The following
 * program will likely the same value from resident_bytes() every time:
 *
 *     memlib_mmap(4096*1000);
 *     printf("%ld\n", resident_bytes());
 *     memlib_mmap(4096*1000);
 *     printf("%ld\n", resident_bytes());
 *     memset(memlib_mmap(4096*10), 'A', 4096*10)
 *     printf("%ld\n", resident_bytes());
 *
 * Interestingly, if the last mmap size is 4096*1000, the resident memory as
 * calculated by /proc/self/statm changes. It is unclear why this is or in
 * what way the new value is related to the previous sequence of operations.
 */
UNUSED static long
resident_bytes()
{
	int ret;
	long size, resident, share, text, lib, data, dirty;
	char buf[1024];
	(void) read_file("/proc/self/statm", buf, sizeof (buf));

	ret = sscanf(buf, "%ld %ld %ld %ld %ld %ld %ld",
	    &size, &resident, &share, &text, &lib, &data, &dirty);
	VERIFY3S(ret, ==, 7);
	return (resident * 4096);
}