Slow String Repeat (was: [Python-Dev] PyBuffer* vs. array.array()) (original) (raw)

Christian Tismer tismer@tismer.com
Mon, 06 Jan 2003 15:42:01 +0100

Raymond Hettinger wrote:

[ " " * 1000000 ten times slower than " " * 1000 * 1000 ]

This ought to do it:

i = 0; if (size >= 1 ){ // copy in first one memcpy(op->obsval, a->obsval, (int) a->obsize); i = (int) a->obsize; } for ( ; i + i < size ; i <<= 1) // grow in doubles_ _memcpy(op->obsval+i, op->obsval, i); if (i < size ) // fill remainder_ _memcpy(op->obsval+i, op->obsval, size-i);

Looks good, not too much code to add. This solves the problem for string repetition.

Theoretically, there are other objects which might expose a similar problem: If the overhead of starting the copy process is larger than the actual copying process, then it is cheaper to do the repeat operation stepwise.

I'm asking whether it makes sense to look for every possible occurrence of such and fix it like above? I could imagine not to change string repeat and others, but the abstract implementation of the repetition of a sequence. An algorithm like the above could be written for general sequences, and do this break-up on the abstract level, once and for all repetitions of arbitrary objects.

But I have to admit that this is a bit less efficient, since the target object cannto be allocated in advance, before looking into the actual type.

cheers -- chris

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