Re: Faster compilation speed: cache behavior

["Timothy J. Wood" <tjw at omnigroup dot com>]

On Wednesday, August 21, 2002, at 06:51  PM, Segher Boessenkool wrote:
> Or maybe Apple's tool uses some different mechanism; in that case, 
> please
> ignore me.
  The CHUD tools have a couple different modes.  There are four 
different ways to trigger a sample, time interval, OS PMI (for 
synthetic events like VM faults, system calls, etc), CPU PMI (cycles 
elapsed, Altivec Float stall cycles, etc) and MC PMI (AGP R/W Hit Open 
Page, Single Beat Xacts, etc) .  The timer is configured by some real 
time value.  The PMI options are associated with a trigger counter 
(another PMC) and have a trigger count.  Thus, you could generate a 
graph of the number of CPU cycles in each 1000 L3 Cache misses period.

  CHUD also allows you to add some 'fuzz' to your sampling to help 
avoid potentially synchronizing with the target timing.  You can 
configure fuzz from 0% to 50% -- the actual trigger count or timer 
value that is used for the next sampling period is varied by the fuzz 
percentage (I think it is only +, not +/-, but I'm not sure).

  Each time the a sample is taken, the instruction pointer may have 
nothing to do with the actual problem.  For example, 999 of the dL3 
misses might be due to routine foo(), but the 1000th might hit when 
bar() is executing and bar() will get blamed.  Thus, you can see that 
it doesn't really matter what the PC is when the trigger goes off since 
there are other sources of error.  It just needs to be 'close' (where 
'close' is some function of function size and sampling rate).

  The saving grace, is that the statistical distribution of blame 
eventually settles on the real offenders (possibly with help from some 
fuzz).  Clearly there are edge cases (like really small functions), but 
as long as you know this, you can be on the lookout for it.

  CHUD is amazingly powerful -- I've only made the most basic use of it 
so far (and have described little of its capabilities here :)

  More information at http://developer.apple.com/tools/debuggers.html

-tim