Measure page faults from a c program

Question

I am comparing a few system calls where I read/write from/to memory. Is there any API defined to measure page faults (pages in/out) in C ?

I found this library libperfstat.a but it is for AIX, I couldn't find anything for linux.

Edit: I am aware of time & perf-stat commands in linux, just exploring if there is anything available for me to use inside the C program.

Answer 1

There is getrusage function (SVr4, 4.3BSD. POSIX.1-2001; but not all fields are defined in standard). In linux there are several broken fields, but man getrusage lists several interesting fields:

long   ru_minflt;        /* page reclaims (soft page faults) */
long   ru_majflt;        /* page faults (hard page faults) */

long   ru_inblock;       /* block input operations */
long   ru_oublock;       /* block output operations */

The rusage is also reported in wait4 (only usable in external program). This one is used by /usr/bin/time program (it prints minor/major pagefault counts).

Answer 2

If you are running on Linux, you can use the perf_event_open system call (used by perf stat). It's a little bit tricky to get the right parameters, look at the man page http://web.eece.maine.edu/~vweaver/projects/perf_events/perf_event_open.html and see the code below.

There is no lib C wrapper so you have to call it as following:

static long perf_event_open(struct perf_event_attr *hw_event,
                pid_t pid,
                int cpu,
                int group_fd,
                unsigned long flags) {
  int ret = syscall(__NR_perf_event_open, hw_event, pid, cpu,
            group_fd, flags);
  return ret;
}

and then to count page faults:

  struct perf_event_attr pe_attr_page_faults;
  memset(&pe_attr_page_faults, 0, sizeof(pe_attr_page_faults));
  pe_attr_page_faults.size = sizeof(pe_attr_page_faults);
  pe_attr_page_faults.type =   PERF_TYPE_SOFTWARE;
  pe_attr_page_faults.config = PERF_COUNT_SW_PAGE_FAULTS;
  pe_attr_page_faults.disabled = 1;
  pe_attr_page_faults.exclude_kernel = 1;
  int page_faults_fd = perf_event_open(&pe_attr_page_faults, 0, CPU, -1, 0);
  if (page_faults_fd == -1) {
    printf("perf_event_open failed for page faults: %s\n", strerror(errno));
    return -1;
  }

  // Start counting
  ioctl(page_faults_fd, PERF_EVENT_IOC_RESET, 0);
  ioctl(page_faults_fd, PERF_EVENT_IOC_ENABLE, 0);

  // Your code to be profiled here 
  .....

  // Stop counting and read value
  ioctl(page_faults_fd, PERF_EVENT_IOC_DISABLE, 0);
  uint64_t page_faults_count;
  read(page_faults_fd, &page_faults_count, sizeof(page_faults_count));

Answer 3

It is not an API as such, however I have had a lot of success by rolling my own and reading /proc/myPID/stat from within my C program which includes page fault statistics for my process, this allows me to monitor counts in real time as my program runs and store these however I like.

Remember that doing so can cause page faults in itself so there will be some inaccuracy but you will get a general idea.

See here for details of the format of the file: https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_MRG/1.3/html/Realtime_Reference_Guide/chap-Realtime_Reference_Guide-Memory_allocation.html