linux perf: how to interpret and find hotspots

Question

I tried out linux\' perf utility today and am having trouble in interpreting its results. I\'m used to valgrind\'s callgrind which is of course a totally different approach

Answer 1

You can get a very detailed, source level report with perf annotate, see Source level analysis with perf annotate. It will look something like this (shamelessly stolen from the website):

------------------------------------------------
 Percent |   Source code & Disassembly of noploop
------------------------------------------------
         :
         :
         :
         :   Disassembly of section .text:
         :
         :   08048484 
:
         :   #include 
         :   #include 
         :   #include 
         :
         :   int main(int argc, char **argv)
         :   {
    0.00 :    8048484:       55                      push   %ebp
    0.00 :    8048485:       89 e5                   mov    %esp,%ebp
[...]
    0.00 :    8048530:       eb 0b                   jmp    804853d 
         :                           count++;
   14.22 :    8048532:       8b 44 24 2c             mov    0x2c(%esp),%eax
    0.00 :    8048536:       83 c0 01                add    $0x1,%eax
   14.78 :    8048539:       89 44 24 2c             mov    %eax,0x2c(%esp)
         :           memcpy(&tv_end, &tv_now, sizeof(tv_now));
         :           tv_end.tv_sec += strtol(argv[1], NULL, 10);
         :           while (tv_now.tv_sec < tv_end.tv_sec ||
         :                  tv_now.tv_usec < tv_end.tv_usec) {
         :                   count = 0;
         :                   while (count < 100000000UL)
   14.78 :    804853d:       8b 44 24 2c             mov    0x2c(%esp),%eax
   56.23 :    8048541:       3d ff e0 f5 05          cmp    $0x5f5e0ff,%eax
    0.00 :    8048546:       76 ea                   jbe    8048532 
[...]

Don't forget to pass the -fno-omit-frame-pointer and the -ggdb flags when you compile your code.