[Python-Dev] PATCH: Fast globals/builtins lookups for 2.6
Steve Holden
steve at holdenweb.com
Fri Nov 30 14:52:34 CET 2007
Neil Toronto wrote:
> Guido van Rossum wrote:
>> Hm.
>>
>> On my Linux box, in the trunk:
>>
>> Before the patch:
>> Pystone(1.1) time for 50000 passes = 1.16
>> This machine benchmarks at 43103.4 pystones/second
>>
>> After the patch:
>> Pystone(1.1) time for 50000 passes = 1.14
>> This machine benchmarks at 43859.6 pystones/second
>>
>> That's only about 1.75% faster. But pystone is a lousy benchmark.
>
> I'm not aware of any benchmark that isn't. :)
>
> Can you humor me and change the PY_LONG_LONG to Py_ssize_t in both
> PyDictObject and PyFastGlobalsObject and see if that helps? It does on
> one of my test machines.
>
> Speaking of which, here's a question for everybody. I was wondering
> whether 64 bits is necessary. It takes an hour of concerted effort -
> nothing but "module.d = 1; del module.d" for an hour straight - to
> overflow a 32-bit version number. Is anybody going to actually get close
> to doing that in a global namespace?
>
Of course not. And 640k is as much memory as anyone could reasonably
need ...
> I don't think a malicious user could exploit it. The most they could do
> is segfault by doing exactly 2**32 entry-invalidating operations and
> then one get or set. They've got better things to do if they're running
> code on your machine.
>
> FWIW - and I wouldn't bother with this if I weren't mucking about with
> dict internals - with a 32-bit version number, I've verified that gcc
> emits only one extra instruction in dict functions that increment it.
> It's two for a 64-bit number. The version test in LOAD_GLOBAL does take
> a bit more time with 64 bits, though.
>
That's a good local optimization for today's conditions, probably. Who
nows whether it will survive the next ten years. And decisions like that
have a weird habit of running into pathological cases whose authors
curse you when they find out where the problem arose.
regards
Steve
--
Steve Holden +1 571 484 6266 +1 800 494 3119
Holden Web LLC http://www.holdenweb.com/
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