Re: Lua5 / poor performance
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- Subject: Re: Lua5 / poor performance
- From: Roberto Ierusalimschy <roberto@...>
- Date: 2003年3月20日 13:00:22 -0300
The real problem is related to tables. Lua hashes all numbers to their
integer parts. Lua 5.0 uses an auxiliary table to track constants used
by a function (using the constants as keys). In this particular file,
there are lots of numbers between 0 and 1 (plus other similar ranges).
Then, all these numbers collide in the auxiliary table, and so the
performance problem.
This problem with tables exists in Lua since a long time (version 2.0 at
least), but surprisingly it never happened in a real case before. This
sigle program shows the problem in 4.0:
local a = {}
for i=0,1,0.00001 do a[i] = i end
(Warning! It takes a *long* time to run.)
Any solution to this problem must change the way Lua hashes non-integer
numbers. One particular solution is this:
Node *luaH_mainposition (const Table *t, const TObject *key) {
switch (ttype(key)) {
case LUA_TNUMBER: {
int ikey;
lua_Number n = nvalue(key);
lua_number2int(ikey, n);
if (cast(lua_Number, ikey) == n) /* is an integer index? */
return hashnum(t, ikey);
else {
int i;
union {
lua_Number n;
int a[sizeof(lua_Number)/sizeof(int)];
} u;
memset(&u, 0, sizeof(u));
u.n = n;
for (i=1; i<sizeof(lua_Number)/sizeof(int); i++)
u.a[0] += u.a[i];
return hashnum(t, u.a[0]);
}
}
... /* other cases as before */
This code is not very "religious", but it does the trick. (The
performance for integer keys is not affected by this change.)
-- Roberto