Le 29/09/2015 00:07, Scott Wood a écrit :
On Tue, Sep 22, 2015 at 06:50:29PM +0200, Christophe Leroy wrote:
We are spending between 40 and 160 cycles with a mean of 65 cycles in
the TLB handling routines (measured with mftbl) so make it more
simple althought it adds one instruction.
Signed-off-by: Christophe Leroy <[email protected]>
Does this just make it simpler or does it make it faster? What is the
performance impact? Is the performance impact seen with or without
CONFIG_8xx_CPU6 enabled? Without it, it looks like you're adding an
mtspr/mfspr combo in order to replace one mfspr.
The performance impact is not noticeable. Theoritically it adds 1 cycle
on a mean of 65 cycles, that is 1.5%. Even in the worst case where we
spend around 10% of the time in TLB handling exceptions, that represents
only 0.15% of the total CPU time. So that's almost nothing.
Behind the fact to get in simpler, the main reason is because I need a
third register for the following patch in the set, otherwise I would
spend a more time saving and restoring CR several times.
Christophe
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