[issue19581] PyUnicodeWriter: change the overallocation factor for Windows

[STINNER Victor]

STINNER Victor added the comment:

I chose 25% on Linux after some micro-benchmarks on str%args and str.format(args). If the buffer is too large, the final resize (because PyUnicodeObject must have the exact size) is slow. I suppose that realloc() can avoid copying data if the new is is very close, but has to allocate a new memory block and copy data if the new size is higher than a threshold. It's how _PyObject_Realloc() for example.

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<http://bugs.python.org/issue19581>
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