[Python-Dev] PEP 393 review

Guido van Rossum guido at python.org
Fri Aug 26 16:55:39 CEST 2011


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)


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