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Re: Extremely poor performance crunching random numbers under PIV-FC5



You are not invoking 10 million system calls, just 10 million library
routines.
Ok, that means rand() function do not make a system call each time
invoked, isn´t it?


Now, rand() as well as random() are thread-safe routines,
so they need to use a lock to protect the seed (unlike say random_r ()).
On i?86, glibc inlines the lock code:
 14:   b9 01 00 00 00          mov    $0x1,%ecx
 19:   65 83 3d 0c 00 00 00    cmpl   $0x0,%gs:0xc
 20:   00
 21:   74 01                   je     24 <__random+0x24>
 23:   f0 0f b1 8b XX XX XX    lock cmpxchg %ecx,0xXXXXXXXX(%ebx)
 2a:   XX
 2b:   0f 85 8f 01 00 00       jne    1c0 <_L_mutex_lock_13>
and similarly for unlock.
%gs:0xc ought to be non-zero only when the first pthread_call in the program
has been made.
On PIV, atomic instructions are horribly expensive.
Either you have preloaded some library that called pthread_create,
or your CPU is unable to do the jump around lock prefix trick quickly.

This is out of my scope (assembler, threads, atomic instructions ...)
but I try to follow your explanation.


I certainly don't see your testcase being slow on my AMD64, neither
for 64-bit nor 32-bit program.

Ok. The same for many other systems. Mine seems to be an odd exception.


First of all, try benchmarking random_r, then see under debugger
if %gs:0xc is 0 or not.

Jakub, I would like to do it but I don´t know how. I have tried to
implement a code to test random_r() function unsuccesfully:

   start = clock();
   for(i=0; i<numero_ciclos; i++) {
       r = random_r();
   }
   end = clock();
   printf("%d M de random_r() en %.3f sec (example.: %d)\n",
numero_ciclosM, (double)(end - start)/CLOCKS_PER_SEC, r);

Can´t compile it. Gcc says me "too few arguments for function
random_r()". I don´t know which variable to put inside random_r. Can
you help me?

Thanks!


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