[Python-Dev] PyDict_SetItem hook

Raymond Hettinger python at rcn.com
Thu Apr 2 20:58:18 CEST 2009

The measurements are just a distractor.  We all already know that the hook is being added to a critical path.  Everyone will pay a cost for a feature that few people will use.  This is a really bad idea.  It is not part of a thorough, thought-out framework of container hooks (something that would need a PEP at the very least).    The case for how it helps us is somewhat thin.  The case for DTrace hooks was much stronger.  

If something does go in, it should be #ifdef'd out by default.  But then, I don't think it should go in at all.  


  On Thu, Apr 2, 2009 at 04:16, John Ehresman <jpe at wingware.com> wrote:

    Collin Winter wrote:

      Have you measured the impact on performance?

    I've tried to test using pystone, but am seeing more differences between runs than there is between python w/ the patch and w/o when there is no hook installed.  The highest pystone is actually from the binary w/ the patch, which I don't really believe unless it's some low level code generation affect.  The cost is one test of a global variable and then a switch to the branch that doesn't call the hooks.

    I'd be happy to try to come up with better numbers next week after I get home from pycon.

  Pystone is pretty much a useless benchmark. If it measures anything, it's the speed of the bytecode dispatcher (and it doesn't measure it particularly well.) PyBench isn't any better, in my experience. Collin has collected a set of reasonable benchmarks for Unladen Swallow, but they still leave a lot to be desired. From the discussions at the VM and Language summits before PyCon, I don't think anyone else has better benchmarks, though, so I would suggest using Unladen Swallow's: http://code.google.com/p/unladen-swallow/wiki/Benchmarks

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