[Python-Dev] Opcode cache in ceval loop

[Matthias Bussonnier]

> 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. 

> 
>> 
>> 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 ?

-- 
M

 

> 
> Best,
> Sven