Re: Minimising memory allocation
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- Subject: Re: Minimising memory allocation
- From: Edgar Toernig <froese@...>
- Date: 2001年8月10日 16:17:16 +0200
David Lam wrote:
>
> Basically I want to minimise the amount of temporary memory being allocated,
> so that I can reduce the frequency GC is called (and maybe reduce the hit of
> each GC?). Can anyone give some hints on this topic?
>
> For instance, if I have local variables declared inside function A, will the
> local variables allocate memory each time A is called?
No. Local variables are kept on a stack and will not effect the GC.
These constructs will create a new garbage collected object:
- a..b (example only; any string generating construct i.e. strsub() or
lua_pushstring().)
Each _new_ string creates a new object. Lua has unique strings, that
means that each possible string exists only once within Lua. If a
new string occurs Lua first checks if it already exists within the
string pool. If it does, a reference to that older string is used.
If it does not exist, a new string object is created and put into the
string pool. The GC checks the string pool for unused strings and
frees them. So creating a lot of unique strings will trash the GC.
I.e. this is bad GC-wise:
for i=1,100 do x="foo"..tostring(i) end -- 100 new strings
Note: Code segments hold references to strings constants (a la "foo"
above). So the string will exist in the string pool as long as the
function exists; it does _not_ has to be created each time the func-
tion is executed.
- { ... }
Each time a table constructor is executed a new table is created.
Try to reuse tables if possible.
- function() ... end
Executing a function statement creates a closure (note: not calling
the function but executing the function statement!). Normally no
big impact on the GC but if you have function statements within a
loop you may create a lot of garbage. I.e.:
for i=1,100 do foo(function() end) end -- bad for GC
creates 100 function closures.
- function(foo, ...) end
Vararg functions create a table for the ellipsis (stored in 'arg')
each time the function is _called_.
- userdata objects
In Lua 4.0 (and 3.x) userdata objects are handled similar to strings.
They are equal if the pointer value and the tag are equal.
- dofile/dostring
Well, these load/compile new code and will create strings and (GCed)
code segments.
Ciao, ET.