Paul Mackerras wrote:

On Mon, 21 Feb 2000, FASSINO Jean-Philippe wrote:

quoted

The aim of this patch is to optimize context switch on PPC.
It permit to optimize pipeline and reduce near 30 instructions per
context switch.
I'm using it on my computer and it work well, please test it !

Interesting.  How much does it reduce the context switch time?  Did you
run lmbench or something to see if it makes it go faster?

The reason I ask is that it is possible that unrolling the loop as you
have done could actually make it go slower due to increased i-cache
misses.  The bdnz instruction on PPC has essentially zero overhead since it
is pulled out of the instruction stream in the fetch/decode unit by the
branch processing unit.  Also, it is very easy to predict whether a bdnz
will branch or not.

There are two advantages of this patch :
    - unrolling the loop (suppress the bdnz instructions),
    - statically designate segment register (suppress one add per loop).
The main disadvantage is :
    - possibly increase i-cache misses (depend of function alignment)

To conclude, i'm trying to run lmbench and when i got result i send it.

Jean-Philippe

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