Re: [PATCH v2 00/11] KVM in-guest performance monitoring

[David Ahern <daahern@cisco.com>]

On 06/16/2011 09:27 AM, Peter Zijlstra wrote:

> Sure, I've got a couple of those things lying around:
> 
> # perf stat -e instructions:u ./loop_1b_instructions-4x
> 
>  Performance counter stats for './loop_1b_instructions-4x':
> 
>      4,000,085,344 instructions:u            #    0.00  insns per cycle        
> 
>        0.311861278 seconds time elapsed
> 
> ---
> 
> #include <stdlib.h>
> #include <stdio.h>
> #include <time.h>
> 
> main ()
> {
> 	int i;
> 
> 	fork();
> 	fork();
> 
> 	for (i = 0; i < 100000000; i++) {
> 		asm("nop");
> 		asm("nop");
> 		asm("nop");
> 		asm("nop");
> 		asm("nop");
> 		asm("nop");
> 		asm("nop");
> 	}
> 	wait(NULL);
> 	wait(NULL);
> 	wait(NULL);
> 	wait(NULL);
> }
> 

That's the one.

Guest:
perf stat  -e instructions:u /tmp/a.out

 Performance counter stats for '/tmp/a.out':

     4,000,090,357 instructions:u            #    0.00  insns per cycle


       2.972828828 seconds time elapsed

Host:
perf stat  -e instructions:u /tmp/a.out

 Performance counter stats for '/tmp/a.out':

     4,000,083,592 instructions:u            #    0.00  insns per cycle


       0.278185315 seconds time elapsed

So the counting is correct,  but the time to run the command is
significantly longer in the guest. That emphasizes the performance
overhead of running perf-stat in the VM.

Even the default counters for perf-stat are similar, showing correctness
in counting:

Guest:
perf stat ./a.out

 Performance counter stats for './a.out':

       2707.156752 task-clock                #    0.996 CPUs utilized

               337 context-switches          #    0.000 M/sec

                 0 CPU-migrations            #    0.000 M/sec

               209 page-faults               #    0.000 M/sec

     3,103,481,148 cycles                    #    1.146 GHz
        [50.25%]
   <not supported> stalled-cycles-frontend
   <not supported> stalled-cycles-backend
     3,999,894,345 instructions              #    1.29  insns per cycle
        [50.03%]
       406,716,307 branches                  #  150.237 M/sec
        [49.85%]
           270,801 branch-misses             #    0.07% of all branches
        [50.02%]

       2.717859741 seconds time elapsed

Host:
perf stat /tmp/a.out

 Performance counter stats for '/tmp/a.out':

       1117.694687 task-clock                #    3.845 CPUs utilized

               140 context-switches          #    0.000 M/sec

                 3 CPU-migrations            #    0.000 M/sec

               203 page-faults               #    0.000 M/sec

     3,052,677,262 cycles                    #    2.731 GHz

     1,449,951,708 stalled-cycles-frontend   #   47.50% frontend cycles
idle
       471,788,212 stalled-cycles-backend    #   15.45% backend  cycles
idle
     4,006,074,559 instructions              #    1.31  insns per cycle

                                             #    0.36  stalled cycles
per insn
       401,265,264 branches                  #  359.012 M/sec

            29,376 branch-misses             #    0.01% of all branches


       0.290722796 seconds time elapsed


David