[Python-Dev] Opcode cache in ceval loop (original) (raw)

Sven R. Kunze srkunze at mail.de
Fri Feb 5 10:07:09 EST 2016

On 05.02.2016 00:06, Matthias Bussonnier wrote:

On Feb 4, 2016, at 08:22, Sven R. Kunze <srkunze at mail.de> wrote:

On 04.02.2016 16:57, Matthias Bussonnier wrote: On Feb 3, 2016, at 13:22, Yury Selivanov <yselivanov.ml at gmail.com> wrote:

An ideal way would be to calculate a hit/miss ratio over time for each cached opcode, but that would be an expensive calculation. Do you mean like a sliding windows ? Otherwise if you just want a let's say 20% miss threshold, you increment by 1 on hit, and decrement by 4 on miss. Division is expensive. I'm not speaking about division here. if you +M / -N the counter will decrease in average only if the hit/miss ratio is below N/(M+N), but you do not need to do the division. Then you deoptimize only if you get < 0.

I see but it looks still more complicated. :)

On Feb 3, 2016, at 13:37, Sven R. Kunze <srkunze at mail.de> wrote:

On 03.02.2016 22:22, Yury Selivanov wrote: One way of tackling this is to give each optimized opcode a counter for hit/misses. When we have a "hit" we increment that counter, when it's a miss, we decrement it. Within a given range, I suppose. Like:

c = min(c+1, 100) Min might be overkill, maybe you can use a or mask, to limit the windows range to 256 consecutive call ? Sure, that is how I would have written it in Python. But I would suggest an AND mask. ;-) Sure, implementation detail I would say. Should not write emails before breakfast...

;-)

The other problem, with the mask, is if your increment hit 256 you wrap around back to 0 where it deoptimize (which is not what you want), so you might need to not mask the sign bit and deoptimize only on a certain negative threshold.

Does it make sens ?

Definitely. I am curious about the actual implementation of this idea.

Best, Sven

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