Re: Lua 5.0 to 5.1 performance regression?

[Steve Heller <steve@...>]

On Mon, 25 Sep 2006 01:26:24 +0200, Mike Pall <mikelu-0609@mike.de>
wrote:

>Hi,
>
>Glenn Maynard wrote:
>> The attached code (creating and destroying copies of references) runs in .42
>> seconds wall time on 5.0.2 for me, and 1.8 seconds on 5.1.
>> 
>> Just pushing the reference (and popping it) is .05 (5.0.2) up to .19 (5.1),
>> which alone is a huge jump.
>> 
>> Same compiler flags.  Before I spend time trying to track this down, does
>> anyone know what's up?   I'm using references as a low-level data type,
>> and I don't want to see a 4:1 overhead increase.
>
>The 5.1 code for luaL_ref/luaL_unref uses index 0 in the registry
>for the free list. Unfortunately this means it's stored in the
>hash part and not the array part of the registry table. This
>slows down both operations considerably. The equivalent 5.0 code
>uses index 1 for the free list (stored in the array part).

Could this be changed, and if not, why not? That's a big performance
hit for what sounds like a pretty basic operation.

>You could patch lauxlib.c of course (with some care -- changing
>FREELIST_REF alone doesn't work). But for performance sensitive
>use you may be better off rolling your own reference management.
>
>I suggest to use your own (presized) table, stored in a function
>or userdata environment or an upvalue. And keep track of the
>maximum allocation and the free list anchor on the C side to
>avoid some of the table lookups.
>
>Bye,
>     Mike

Steve