[Python-Dev] PEP 393 review (original) (raw)

Guido van Rossum guido at python.org
Fri Aug 26 16:56:05 CEST 2011

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 PyObjectMalloc(), 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 wchart | 32-bit wchart | 32-bit wchart | 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-wchart 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)

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