[Python-Dev] cpython (2.7): Fix comment blocks. Adjust blocksize to a power-of-two for better divmod

[Guido van Rossum]

On Mon, Jun 24, 2013 at 7:47 PM, Raymond Hettinger <
raymond.hettinger at gmail.com> wrote:

>
>
> On Jun 24, 2013, at 4:07 AM, Victor Stinner <victor.stinner at gmail.com>
> wrote:
>
> Out of curiosity, do you know (remember) how was the number 62 chosen?
> Is it a compromise between memory usage and performances? 62 is
> surprising because it is not a power of two :-)
>
> Is it to just have 64 (2+62) pointers in the structure?
>
>
> Yes, the goal was to have the struct size be an exact multiple
> of the cache line length (always a power-of-two, typically 64 bytes).
> What was different then is that deques weren't indexable.
> When indexing was added, the size of 62 became an
> unfavorable choice because it made the division and modulo
> calculation in deque_index() slower than for a power of two.
>

A-ha! Finally an explanation of the change. It makes intuitive sense now. I
think the general feeling is that folks overreacted (perhaps confused by
your silence) and that the reversal will be rolled back. Benjamin?

-- 
--Guido van Rossum (python.org/~guido)
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