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

Benjamin Peterson benjamin at python.org
Tue Jun 25 07:12:43 CEST 2013

2013/6/24 Guido van Rossum <guido at python.org>:
> 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?

Raymond, go ahead and reapply your change.


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