Re: Lua 5.0 to 5.1 performance regression?

[Mike Pall <mikelu-0609@...>]

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).

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