Difference between revisions of "BogoMIPS"

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BogoMIPS used to be the infamous, prestigious benchmark for Linux machines over a decade.  Unfortunately - or fortunately - depending of point of view and probably the BogoMIPS number of the favorite machine BogoMIPS have little to nothing to do with actual processor performance.  The BogoMIPS number for any given processor architecture is basically proportional to the clock rate.  On most processor architectures the BogoMIPS loop is compiled into just two instructions.  Accordingly small is the aspects of a processor that are actually tested.  And processors again are just a small part of an overall system which includes other hardware and software.  To show the actual code on MIPS:
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'''BogoMIPS''' used to be the infamous, prestigious benchmark for Linux machines over a decade.  Unfortunately - or fortunately - depending of point of view - the BogoMIPS number of the favorite machine BogoMIPS have little to nothing to do with actual processor performance.  Certain other microarchectural details will also very overproprotionally influence the benchmark.  On the other side memory performance, I/O performance, cache size and speed and many other processor and system architecture feature that make a crucial difference for system performance will ''not'' influence BogoMIPS at all.  The BogoMIPS number for any given processor architecture is basically proportional to the clock rate.  On most processor architectures the BogoMIPS loop is compiled into just two instructions.  Accordingly small is the aspects of a processor that are actually tested.  And processors again are just a small part of an overall system which includes other hardware and software.  To show the actual code on MIPS:
  
 
           .set    noreorder
 
           .set    noreorder
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           .set    reorder"
 
           .set    reorder"
  
A typical modern machine with efficient branches or branch prediction can execute this loop at a rate of one instruction per cycle.  Our of Order Execution which provides roughly a 50% speedup on real workloads provides no benefit.  Not even second level caches or memory subsystems are exercised.  The more surprising it is that BogoMIPS have become a benchmark for performance as important as extra inches in spam email.
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A typical modern machine with efficient branches or branch prediction can execute this loop at a rate of one instruction per cycle.  Out of Order Execution which provides roughly a 50% speedup on real workloads provides no benefit.  Not even second level caches or memory subsystems are exercised.  The more surprising it is that BogoMIPS have become a benchmark for performance as important as extra inches in spam email.  Having been a permanent annoynce over the years due to miss-interpretation by users and due to excessive output on multiprocessor machines Linux by default will no longer print the BogoMIPS number since 2.6.9-rc2.
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The purpose of the BogoMIPS benchmark is to calibrate internal delay loops which are used for very short delays or in situations where a process can't sleep.  This is done by calling the <tt>mdelay()</tt>, <tt>udelay()</tt> and <tt>ndelay()</tt> functions which take the time to delay as the argument in units of milliseconds, microseconds or nanoseconds, respectivly.

Latest revision as of 13:32, 4 April 2005

BogoMIPS used to be the infamous, prestigious benchmark for Linux machines over a decade. Unfortunately - or fortunately - depending of point of view - the BogoMIPS number of the favorite machine BogoMIPS have little to nothing to do with actual processor performance. Certain other microarchectural details will also very overproprotionally influence the benchmark. On the other side memory performance, I/O performance, cache size and speed and many other processor and system architecture feature that make a crucial difference for system performance will not influence BogoMIPS at all. The BogoMIPS number for any given processor architecture is basically proportional to the clock rate. On most processor architectures the BogoMIPS loop is compiled into just two instructions. Accordingly small is the aspects of a processor that are actually tested. And processors again are just a small part of an overall system which includes other hardware and software. To show the actual code on MIPS:

          .set    noreorder
  loop:   bnez    $reg, loop
          subu    $reg, 1
          .set    reorder"

A typical modern machine with efficient branches or branch prediction can execute this loop at a rate of one instruction per cycle. Out of Order Execution which provides roughly a 50% speedup on real workloads provides no benefit. Not even second level caches or memory subsystems are exercised. The more surprising it is that BogoMIPS have become a benchmark for performance as important as extra inches in spam email. Having been a permanent annoynce over the years due to miss-interpretation by users and due to excessive output on multiprocessor machines Linux by default will no longer print the BogoMIPS number since 2.6.9-rc2.

The purpose of the BogoMIPS benchmark is to calibrate internal delay loops which are used for very short delays or in situations where a process can't sleep. This is done by calling the mdelay(), udelay() and ndelay() functions which take the time to delay as the argument in units of milliseconds, microseconds or nanoseconds, respectivly.