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VC exceptions

Subject: VC exceptions
From: Florian Lohoff <>
Date: Thu, 27 Apr 2000 16:58:03 +0200
Organization: rfc822 - pure communication
i had a conversation with Harald concerning a "strong" time drift
on my R4000 Decstation. He than was astonished on the large
number of VCE.

I than searched all my Mips books for an definition of those
exceptions. But i dont think i currently understand the 
cause of those exceptions.


[flo@resume flo]$ uptime && cat /proc/cpuinfo 
  2:55pm  up 13 days, 20 min,  3 users,  load average: 1.15, 1.09, 1.01
  cpu                     : MIPS
  cpu model               : R4000SC V6.0
  system type             : SGI Indy
  BogoMIPS                : 124.93
  byteorder               : big endian
  unaligned accesses      : 90
  wait instruction        : no
  microsecond timers      : no
  extra interrupt vector  : no
  hardware watchpoint     : yes
  VCED exceptions         : 130546469
  VCEI exceptions         : 36073607

On a medium loaded machine i see 40-50 VCEDs per second.

Now i read in the "Mips R4000 Users`s Manual" page 133

Cause: A Virtual Coherency exception occurs when one of the
       following conditions is true:

       - a primary cache miss hits in the secondary cache
       - bits 14:12 of the virtual address were not equal to
         the corresponding bits of the PIdx field of the secondary 
         cache tag.
       - the cache algorithm for the page specifies that the page is cached.

The "Mips Risc Architecture" says that ALL conditions are to be met which
i trust more :)

As a resume - The exception is taken when the index of the 1st and
the 2nd level cache are not identical - Right ?
So - why is there a mismatch ? Might it be due to some invalidation
of the 1st (and not the 2nd) level cache ?

As the exception is taken quiet often and the "Mips Risc Architecture" states
"Software can avoid the cost of this trap by using constistent virtual
primary cache indexes to access the same physical data".

Currently i dont think whats the exact cause of this exception and
a probably optimization which brings this down.

Florian Lohoff                  +49-subject-2-change
"Technology is a constant battle between manufacturers producing bigger and
more idiot-proof systems and nature producing bigger and better idiots."

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