BogoMips

BogoMips ("bogus"與MIPS，偽MIPS) 是一種衡量CPU速度的不科學方法。當電腦核心啟動時，將執行一個計數迴圈。

對於特定的CPU，BogoMips可用來檢視它是否個合適的值。它的時脈頻率和它潛在的CPU快取。但是它不可在不同的CPU間進行比較演示。^[1]

合適的BogoMips比率

作為一個參考精靈，BogoMips可以用下列的表格進行預計算。給出的比率是以應用到LINUX版本的CPU作為例子。指數是指其它CPU同Intel 386DX CPU的BogoMips/clock speed比率.

CPU	比率	指數
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
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
AMD Sempron	clock * 2.00	11.1
UltraSparc II	clock * 2.00	11.1
Intel Pentium MMX	clock * 2.00	11.1
Intel Pentium 4	clock * 2.00	11.1
Intel Pentium M	clock * 2.00	11.1
Intel Core Duo	clock * 2.00	11.1
Intel Core 2 Duo	clock * 2.00	11.1
Intel Atom N455	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 Coppermine	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)
ARM	not enough data (yet)

BogoMIPS 怎麼計算的?

在當前核心(2.6.x),BogoMIPS實現在核心原始檔/usr/src/linux/init/calibrate.c。它計算了Linux核心定時參數loops_per_jiffy (see Jiffy（英語：Jiffy (time)） ) 值。原始碼解釋如下：

 /*
   * 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. Note that a busy waiting technique is used, so the 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));
}

用C語言重寫的代碼如下：

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

關於BogoMips更豐富更全的資訊和數百篇相關文章可參見 BogoMips mini-Howto.^[1]