[Python-Dev] PEP 393 review

[Guido van Rossum]

Also, please add the table (and the reasoning that led to it) to the PEP.

On Fri, Aug 26, 2011 at 7:55 AM, Guido van Rossum <guido at python.org> wrote:
> It would be nice if someone wrote a test to roughly verify these
> numbers, e.v. by allocating lots of strings of a certain size and
> measuring the process size before and after (being careful to adjust
> for the list or other data structure required to keep those objects
> alive).
>
> --Guido
>
> On Fri, Aug 26, 2011 at 3:29 AM, "Martin v. Löwis" <martin at v.loewis.de> wrote:
>>> But strings are allocated via PyObject_Malloc(), i.e. the custom
>>> arena-based allocator -- isn't its overhead (for small objects) less
>>> than 2 pointers per block?
>>
>> Ah, right, I missed that. Indeed, those have no header, and the only
>> overhead is the padding to a multiple of 8.
>>
>> That shifts the picture; I hope the table below is correct,
>> assuming ASCII strings.
>> 3.2: 7 pointers (adds 4 bytes padding on 32-bit systems)
>> 393: 10 pointers
>>
>> string | 32-bit pointer | 32-bit pointer | 64-bit pointer
>> size   | 16-bit wchar_t | 32-bit wchar_t | 32-bit wchar_t
>>       | 3.2     |  393 | 3.2    |  393  | 3.2    |  393  |
>> -----------------------------------------------------------
>> 1      | 40      | 48   | 40     |  48   | 64     | 88    |
>> 2      | 40      | 48   | 48     |  48   | 72     | 88    |
>> 3      | 40      | 48   | 48     |  48   | 72     | 88    |
>> 4      | 48      | 48   | 56     |  48   | 80     | 88    |
>> 5      | 48      | 48   | 56     |  48   | 80     | 88    |
>> 6      | 48      | 48   | 64     |  48   | 88     | 88    |
>> 7      | 48      | 48   | 64     |  48   | 88     | 88    |
>> 8      | 56      | 56   | 72     |  56   | 96     | 86    |
>>
>> So 1-byte strings increase in size; very short strings increase
>> on 16-bit-wchar_t systems and 64-bit systems. Short strings
>> keep there size, and long strings save.
>>
>> Regards,
>> Martin
>>
>>
>>
>
>
>
> --
> --Guido van Rossum (python.org/~guido)
>



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