BogoMips

BogoMips ("bogus"與MIPS，伪MIPS) 是一种衡量CPU速度的不科学方法。当计算机内核启动时，将执行一个计数循环。

对于特定的CPU，BogoMips可用来查看它是否个合适的值。它的时钟频率和它潜在的CPU缓存。但是它不可在不同的CPU间进行比较演示。

合适的BogoMips比率 
作为一个参考向导，BogoMips可以用下列的表格进行预计算。给出的比率是以应用到LINUX版本的CPU作为例子。指数是指其它CPU同Intel 386DX CPU的BogoMips/clock speed比率.

BogoMIPS 怎么计算的? 
在当前内核(2.6.x),BogoMIPS实现在内核源文件/usr/src/linux/init/calibrate.c。它计算了Linux内核定时参数loops_per_jiffy (see  ) 值。源码解释如下：
<nowiki>
 /*
   * 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
   */
</nowiki>

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语言重写的代码如下：
static void delay_loop(long loops)
{
  long d0 = loops;
  do {
    --d0;
  } while (d0 >= 0);
}

关于BogoMips更丰富更全的信息和数百篇相关文章可参见 BogoMips mini-Howto.

参考

外部链接 
 BogoMips Mini-Howto, V38

Linux
计算机科学