A Lua VM in pure Go
go-lua is a port of the Lua 5.2 VM to pure Go. It is compatible with binary files dumped by luac, from the Lua reference implementation.
The motivation is to enable simple scripting of Go applications. For example, it is used to describe flows in Shopify's load generation tool, Genghis.
Usage
go-lua is intended to be used as a Go package. It does not include a command to run the interpreter. To start using the library, run:
go get github.com/Shopify/go-lua
To develop & test go-lua, you'll also need the lua-tests submodule checked out:
git submodule update --init
You can then develop with the usual Go commands, e.g.:
go build
go test -cover
A simple example that loads & runs a Lua script is:
package main
import "github.com/Shopify/go-lua"
func main() {
l := lua.NewState()
lua.OpenLibraries(l)
if err := lua.DoFile(l, "hello.lua"); err != nil {
panic(err)
}
}
Status
go-lua has been used in production in Shopify's load generation tool, Genghis, since May 2014, and is also part of Shopify's resiliency tooling.
The core VM and compiler has been ported and tested. The compiler is able to correctly process all Lua source files from the Lua test suite. The VM has been tested to correctly execute over a third of the Lua test cases.
Most core Lua libraries are at least partially implemented. Prominent exceptions are regular expressions, coroutines and string.dump.
Weak reference tables are not and will not be supported. go-lua uses the Go heap for Lua objects, and Go does not support weak references.
Benchmarks
Benchmark results shown here are taken from a Mid 2012 MacBook Pro Retina with a 2.6 GHz Core i7 CPU running OS X 10.10.2, go 1.4.2 and Lua 5.2.2.
The Fibonacci function can be written a few different ways to evaluate different performance characteristics of a language interpreter. The simplest way is as a recursive function:
function fib(n)
if n == 0 then
return 0
elseif n == 1 then
return 1
end
return fib(n-1) + fib(n-2)
end
This exercises the call stack implementation. When computing fib(35), go-lua is about 6x slower than the C Lua interpreter. Gopher-lua is about 20% faster than go-lua. Much of the performance difference between go-lua and gopher-lua comes from the inclusion of debug hooks in go-lua. The remainder is due to the call stack implementation - go-lua heap-allocates Lua stack frames with a separately allocated variant struct, as outlined above. Although it caches recently used stack frames, it is outperformed by the simpler statically allocated call stacks in gopher-lua.
$ time lua fibr.lua
real 0m2.807s
user 0m2.795s
sys 0m0.006s
$ time glua fibr.lua
real 0m14.528s
user 0m14.513s
sys 0m0.031s
$ time go-lua fibr.lua
real 0m17.411s
user 0m17.514s
sys 0m1.287s
The recursive Fibonacci function can be transformed into a tail-recursive variant:
function fibt(n0, n1, c)
if c == 0 then
return n0
else if c == 1 then
return n1
end
return fibt(n1, n0+n1, c-1)
end
function fib(n)
fibt(0, 1, n)
end
The Lua interpreter detects and optimizes tail calls. This exhibits similar relative performance between the 3 interpreters, though gopher-lua edges ahead a little due to its simpler stack model and reduced bookkeeping.
$ time lua fibt.lua
real 0m0.099s
user 0m0.096s
sys 0m0.002s
$ time glua fibt.lua
real 0m0.489s
user 0m0.484s
sys 0m0.005s
$ time go-lua fibt.lua
real 0m0.607s
user 0m0.610s
sys 0m0.068s
Finally, we can write an explicitly iterative implementation:
function fib(n)
if n == 0 then
return 0
else if n == 1 then
return 1
end
local n0, n1 = 0, 1
for i = n, 2, -1 do
local tmp = n0 + n1
n0 = n1
n1 = tmp
end
return n1
end
This exercises more of the bytecode interpreter’s inner loop. Here we see the performance impact of Go’s switch implementation. Both go-lua and gopher-lua are an order of magnitude slower than the C Lua interpreter.
$ time lua fibi.lua
real 0m0.023s
user 0m0.020s
sys 0m0.003s
$ time glua fibi.lua
real 0m0.242s
user 0m0.235s
sys 0m0.005s
$ time go-lua fibi.lua
real 0m0.242s
user 0m0.240s
sys 0m0.028s
License
go-lua is licensed under the MIT license.
