Hi
The context for this question is that I am working on a pure Go
implementation of Lua [1] (as a personal project). Now that it is more or
less functionally complete, I am using pprof to see what the main CPU
bottlenecks are, and it turns out that they are around memory management.
The first one was to do with allocating and collecting Lua "stack frame"
data, which I improved by having add-hoc pools for such objects.
The second one is the one that is giving me some trouble. Lua is a
so-called "dynamically typed" language, i.e. values are typed but variables
are not. So for easy interoperability with Go I implemented Lua values
with the type
// Go code
type Value interface{}
The scalar Lua types are simply implemented as int64, float64, bool, string
with their type "erased" by putting them in a Value interface. The problem
is that the Lua runtime creates a great number of short lived Value
instances. E.g.
-- Lua code
for i = 0, 1000000000 do
n = n + i
end
When executing this code, the Lua runtime will put the values 0 to 1
billion into the register associated with the variable "i" (say, r_i). But
because r_i contains a Value, each integer is converted to an interface
which triggers a memory allocation. The critical functions in the Go
runtime seem to be convT64 and mallocgc.
I am not sure how to deal with this issue. I cannot easily create a pool
of available values because Go presents say Value(int64(1000)) as an
immutable object to me, so I cannot keep it around for later use to hold
the integer 1001. To be more explicit
// Go code
i := int64(1000)
v := Value(i) // This triggers an allocation (because the interface
needs a pointer)
// Here the Lua runtime can work with v (containing 1000)
j := i + 1
// Even though v contains a pointer to a heap location, I cannot modify
it
v := Value(j) // This triggers another allocation
// Here the Lua runtime can work with v (containing 1001)
I could perhaps use a pointer to an integer to make a Value out of. This
would allow reuse of the heap location.
// Go code
p :=new(int64) // Explicit allocation
vp := Value(p)
i :=int64(1000)
*p = i // No allocation
// Here the Lua runtime can work with vp (contaning 1000)
j := i + 1
*p = j // No allocation
// Here the Lua runtime can work with vp (containing 1001)
But the issue with this is that Go interoperability is not so good, as Go
int64 now map to (interfaces holding) *int64 in the Lua runtime.
However, as I understand it, in reality interfaces holding an int64 and an
*int64 both contain the same thing (with a different type annotation): a
pointer to an int64.
Imagine that if somehow I had a function that can turn an *int64 to a Value
holding an int64 (and vice-versa):
func Int64PointerToInt64Iface(p *int16) interface{} {
// returns an interface that has concrete type int64, and points at
p
}
func int64IfaceToInt64Pointer(v interface{}) *int64 {
// returns the pointer that v holds
}
then I would be able to "pool" the allocations as follows:
func NewIntValue(n int64) Value {
v = getFromPool()
if p == nil {
return Value(n)
}
*p = n
return Int64PointerToint64Iface(p)
}
func ReleaseIntValue(v Value) {
addToPool(Int64IPointerFromInt64Iface(v))
}
func getFromPool() *int64 {
// returns nil if there is no available pointer in the pool
}
func addToPool(p *int64) {
// May add p to the pool if there is spare capacity.
}
I am sure that this must leak an abstraction and that there are good
reasons why this may be dangerous or impossible, but I don't know what the
specific issues are. Could someone enlighten me?
Or even better, would there be a different way of modelling Lua values that
would allow good Go interoperability and allow controlling heap allocations?
If you got to this point, thank you for reading!
Arnaud Delobelle
[1] https://github.com/arnodel/golua
--
You received this message because you are subscribed to the Google Groups
"golang-nuts" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to golang-nuts+unsubscr...@googlegroups.com.
To view this discussion on the web visit
https://groups.google.com/d/msgid/golang-nuts/5c782f9f-e442-427e-ae92-d7285fb55ca3n%40googlegroups.com.
