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[Python-Dev] int/float freelists vs pymalloc

Tim Peters tim.peters at gmail.com
Fri Feb 8 20:22:28 CET 2008 
[Neal Norwitz]
> It's not just size.  Architectures may require data aligned on 4, 8,
> or 16 addresses for optimal performance depending on data type.  IIRC,
> malloc aligns by 8 (not sure if that was a particular arch or very
> common).

Just very common.  Because malloc has no idea what the pointer it
returns will be used for, it needs to satisfy the strictest alignment
requirement for any type exposed by the C language.  As an extreme
example, when I worked at Kendall Square Research, our hardware
supported atomic locking on a "subpage" basis, and HW subpage
addresses were all those divisible by 128  Subpages were exposed via C
extensions, so the KSR malloc had to return 128-byte aligned pointers.

> I don't know if pymalloc handles alignment.

pymalloc ensures 8-byte alignment.  This is one plausible reason to
keep the current int free list:  an int object struct holds 3 4-byte
members on most boxes (type pointer, refcount, and the int's value),
and the int freelist code uses exactly 12 bytes for each on most
boxes.  To keep 8-byte alignment, pymalloc would have to hand out a
16-byte chunk per int object, wasting a fourth of the space (pymalloc
always rounds up a requested size to a multiple of 8, and ensures the
address returned is 8-byte aligned).
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