a Q on measuring system performance.

[Dan Nelson]

In the last episode (Mar 24), Yan Yu said:
> I add some codes in various places relating to file operations inside
> the kernel, e.g., fdalloc(), fdused(), fdunused(), fdfree() etc. I am
> trying to measure the overhead added by these instrumentation code.
> my plan is:
>   in my user space program, i have something like the following:
> --------------------------------------------
>         gettimeofday(&prev_time, NULL);
>         for (i=0; i< 1000; i++)
>         {
>                 fd = fopen("tmp", "r" );
>                 if (fd == NULL)
>                 {
>                         break;
>                 }
>                 cnt ++;
>         }
> 
>         gettimeofday(&cur_time, NULL);
>         t_lapse= misc_tv_offset( &cur_time, &prev_time );
> 
> ----------------------------------------------------
> I would run this for the unmodified kernel, and instrumented kernel.
> compare the t_lapse, my concern is that t_lapse includes context switch
> time when the user process is taken out of run queue.

Try using getrusage(), and total up ru_utime+ru_stime.
 
> I also run "gprof" on the program, some related data is:
>   %   cumulative   self              self     total
>  time   seconds   seconds    calls  ms/call  ms/call  name
>  80.0       0.01     0.01     1000     0.01     0.01  __sys_open [3]
>  20.0       0.01     0.00     1000     0.00     0.00  __sfp [4]
>   0.0       0.01     0.00     1987     0.00     0.00  memcpy [6]
>   0.0       0.01     0.00     1000     0.00     0.00  __sflags [283]
>   0.0       0.01     0.00     1000     0.00     0.01  fopen [1]
> 
> i am wonderinf should I better trust gprof instead?  so 0.01 ms/call
> for related file operation is the result. or is there some other
> better way to achieve this?

Gprof is better suited for programs that run for minutes to hours.

-- 
	Dan Nelson
	dnelson at allantgroup.com