Re: LuaJIT 1.1.5 vs 2.0.0-beta3 on C function calls

[Mike Pall <mikelu-1003@...>]

Ilya Martynov wrote:
> We are embedding LuaJIT 1.1.5 into our own HTTP server so that we can
> use Lua as a scripting language to write application business logic.
> Recently we tried updating LuaJIT to 2.0.0-beta3 in the server and
> noticed that it was actually 20% slower with our typical Lua programs.
> On the first glance the main difference from benchmarks being
> published so far on the maillist is that we have many C function calls
> in our Lua programs, some being called in inner loops. So I was
> wondering, could it be possible that LuaJIT 2 is actually slower on C
> function calls then LuaJIT 1?

If you have mostly C function calls in inner loops, your code will
run in the LuaJIT 2.0 interpreter only and not benefit of the JIT
compiler at all. The LuaJIT 2.0 interpreter is often faster, but
in this case somewhat slower than the LuaJIT 1.1 JIT compiler.

Solution: reduce the number of costly Lua/C transitions. I.e. write
more code in Lua and avoid calls to trivial C helper functions.

[The planned FFI for LuaJIT 2.0 will speed up calls to C functions.
But you'll need to rewrite your bindings (much easier though).]

--Mike