BogoMips

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BogoMips (from "bogus" and MIPS) is an unscientific measurement of CPU speed made by the Linux kernel when it boots, to calibrate an internal busy-loop. An oft-quoted definition of the term is "the number of million times per second a processor can do absolutely nothing."

BogoMips can be used to see whether it is in the proper range for the particular processor, its clock frequency, and the potentially present CPU cache. It is not usable for performance comparison between different CPUs.

[edit] Proper BogoMips ratings

As a very approximate guide, the BogoMips can be pre-calculated by the following table. The given rating is typical for that CPU with the then current and applicable Linux version. The index is the ratio of "BogoMips per clock speed" for any CPU to the same for an Intel 386DX CPU, for comparison purposes.

System	Rating	Index
Intel 8088	clock * 0.004	0.02
Intel/AMD 386SX	clock * 0.14	0.8
Intel/AMD 386DX	clock * 0.18	1 (definition)
Motorola 68030	clock * 0.25	1.4
Cyrix/IBM 486	clock * 0.34	1.8
Intel Pentium	clock * 0.40	2.2
Intel 486	clock * 0.50	2.8
AMD 5x86	clock * 0.50	2.8
MIPS R4000/R4400	clock * 0.50	2.8
ARM9	clock * 0.50	2.8
Motorola 8081	clock * 0.65	3.6
Motorola 68040	clock * 0.67	3.7
PowerPC 603	clock * 0.67	3.7
Intel StrongARM	clock * 0.66	3.7
NexGen Nx586	clock * 0.75	4.2
PowerPC 601	clock * 0.84	4.7
Alpha 21064/21064A	clock * 0.99	5.5
Alpha 21066/21066A	clock * 0.99	5.5
Alpha 21164/21164A	clock * 0.99	5.5
Intel Pentium Pro	clock * 0.99	5.5
Cyrix 5x86/6x86	clock * 1.00	5.6
Intel Pentium II/III	clock * 1.00	5.6
AMD K7/Athlon	clock * 1.00	5.6
Intel Celeron	clock * 1.00	5.6
Intel Itanium	clock * 1.00	5.6
MIPS R4600	clock * 1.00	5.6
Hitachi SH-4	clock * 1.00	5.6
Intel Itanium 2	clock * 1.49	8.3
Alpha 21264	clock * 1.99	11.1
VIA Centaur	clock * 1.99	11.1
AMD K5/K6/K6-2/K6-III	clock * 2.00	11.1
AMD Duron/Athlon XP	clock * 2.00	11.1
UltraSparc II	clock * 2.00	11.1
Pentium MMX	clock * 2.00	11.1
Pentium 4	clock * 2.00	11.1
Intel Core Duo	clock * 2.00	11.1
Intel Core 2 Duo	clock * 2.00	11.1
Centaur C6-2	clock * 2.00	11.1
PowerPC 604/604e/750	clock * 2.00	11.1
Intel Pentium III Xeon	clock * 2.00	11.1
Motorola 68060	clock * 2.01	11.2
Intel Xeon MP (32-bit) (hyper-threading)	clock * 3.97	22.1
IBM S390	not enough data (yet)
Intel ARM	not enough data (yet)

With the 2.2.14 Linux kernel, a caching setting of the CPU state was moved from behind to before the BogoMips calculation. Although the BogoMips algorithm itself wasn't changed, from that kernel onward the BogoMips rating for then current Pentium CPUs was twice that of the rating before the change. The changed BogoMips outcome had no effect on real processor performance.

[edit] How are BogoMIPS computed?

With current kernel (2.6.x), Bogomips are implemented in /usr/src/linux/init/calibrate.c kernel source file. It computes loops_per_jiffy value. Explanation from source code:

 /*
   * A simple loop like
   *  while ( jiffies < start_jiffies+1)
   *    start = read_current_timer();
   * will not do. As we don't really know whether jiffy switch
   * happened first or timer_value was read first. And some asynchronous
   * event can happen between these two events introducing errors in lpj.
   *
   * So, we do
   * 1. pre_start <- When we are sure that jiffy switch hasn't happened
   * 2. check jiffy switch
   * 3. start <- timer value before or after jiffy switch
   * 4. post_start <- When we are sure that jiffy switch has happened
   *
   * Note, we don't know anything about order of 2 and 3.
   * Now, by looking at post_start and pre_start difference, we can
   * check whether any asynchronous event happened or not
   */

loops_per_jiffy is used to implement udelay (delay in microseconds) and ndelay (delay in nanoseconds) functions. These functions are needed by some drivers to wait for hardware. Please note that it's using busy waiting technique so kernel is effectively blocked when executing ndelay/udelay functions. For i386 architecture delay_loop is implemented in /usr/src/linux/arch/i386/lib/delay.c as:

/* simple loop based delay: */
static void delay_loop(unsigned long loops)
{
  int d0;

  __asm__ __volatile__(
    "\tjmp 1f\n"
    ".align 16\n"
    "1:\tjmp 2f\n"
    ".align 16\n"
    "2:\tdecl %0\n\tjns 2b"
    :"=&a" (d0)
    :"0" (loops));
}

which can rewritten to C-pseudocode

static void delay_loop(unsigned long loops)
{
  int d0 = loops;
  do {
    --d0;
  } while { do >= 0 }
}

Full and complete information and details about BogoMips, and hundreds of reference entries can be found in the (unfortunately outdated) BogoMips mini-Howto below.