[Python-Dev] PATCH: Fast globals/builtins lookups for 2.6

[Steve Holden]

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/