The Boston Diaries

The ongoing saga of a programmer who doesn't live in Boston, nor does he even like Boston, but yet named his weblog/journal “The Boston Diaries.”

Go figure.

Friday, March 22, 2013

Musings on the Current Work Project Du jour

So I have this Lua code that implements the cellphone end of a protocol used in “Project: Wolowizard.” I need to ramp up the load testing on this portion of the project so I'm looking at what I have and trying to figure out how to approach this project.

The protocol itself is rather simple—only a few messages are defined and the code is rather straightforward. It looks something like:

-- Pre-define these
state_receive = function(phone,socket) end
state_msg1    = function(phone,socket,remote,msg) end
state_msg2    = function(phone,socket,remote,msg) end

-- Now the code

state_receive = function(phone,socket)
  local remote,packet,err = socket:read()
  if err ~= 0 then
    syslog('err',string.format("error reading socket: %s",errno[err]))
    return state_receive(phone,socket)
  end

  local msg,err = sooperseekritprotocol.decode(packet)
  if err ~= 0 then
    syslog('err',string.format("error decoding: %s",decoderror(err))
    return state_receive(phone,socket)
  end

  if msg.type == 'MSG1" then
    return state_msg1(phone,socket,remote,msg)
  elseif msg.type == "MSG2" then
    return state_msg2(phone,socket,remote,msg)
  else
    syslog('warn',string.format("unknown message: %s",msg.type))
    return state_receive(phone,socket)
  end
end

state_msg1 = function(phone,socket,remote,msg)
  local reply = ... -- code to handle this msg
  local packet = sooperseekritprotocol.encode(reply)
  socket:write(remote,packet)
  return state_receive(phone,socket)
end

state_msg2 = function(phone,socket,remote,msg)
  local reply = ... -- code to andle this msg
  local packet = sooperseekritprotocol.encode(reply)
  socket:write(remote,packet)
  return state_receive(phone,socket)
end

Don't worry about this code blowing out the call stack—Lua optimizes tail calls and these effectively become GOTOs. I found this feature to be very useful in writing protocol handlers since (in my opinion) it makes the state machine rather explicit.

Now, to speed this up, I could translate this to C. As I wrote the Lua modules for The Kitchen Sink Lua interpreter, I pretty much followed a bi-level approach. I have a C interface (to be used by C code) which is then mimicked in Lua. This makes translating the Lua code into C more or less straightforward (with a bit more typing because of variable declarations and what not).

But here, I can't rely on the C compiler to optimize tail calls (GCC can, but only with certain options; I don't know about the Solaris C compiler). I could have the routines return the next function to call and use a loop:

while((statef = (*statef)(phone,sock,&remote,&msg) != NULL)
  /* the whole state machine is run in the previous line;

But just try to define the type of statef so the compiler doesn't complain about a type mismatch. It needs to define a function that takes blah and returns a function that takes blah and returns a function that takes blah and returns a function that … It's one of those recurisive type definitions that produce headaches when you think too much about it.

Okay, so instead, let's just have a function that returns a simple integer value that represents the next state. That's easier to define and the main driving loop isn't that bad:

while(state != DONE)
{
  switch(state)
  {
    case RECEIVE: state = state_receive(phone,socket,&remote,&msg); break;
    case MSG1:    state = state_msg1(phone,socket,&remote,&msg); break;
    case MSG2:    state = state_msg2(phone,socket,&remote,&msg); break;
    default:      assert(0); break;
  }
}

Okay, with that out of the way, we can start writing the C code.

Clackity-clackity-clack clackity-clack clack clack clackity-clackity-clackity-clack clack clack clack clack …

Man, that's boring drudgework. Okay, let's just use the Lua code and maybe throw some additional threads at this. I don't think that's a bad approach. Now, Lua, out of the box, isn't exactly thread-safe. Sure, you can provide an implemention of lua_lock() and lua_unlock() but that might slow Lua down quite a bit (there are 62 locations where the lock could be taken in the Lua engine). We could give each thread its own Lua state—how bad could that be?

How big is a Lua state? Let's find out, shall we?

#include <stdio.h>
#include <stdlib.h>
#include <lua.h>
#include <lauxlib.h>

int main(void)
{
  lua_State *L;

  L = luaL_newstate();
  if (L == NULL)
  {
    perror("luaL_newstate()");
    return EXIT_FAILURE;
  }
   
  printf("%d\n",lua_gc(L,LUA_GCCOUNT,0) * 1024);
  lua_close(L);
  return EXIT_SUCCESS;
} 

When compiled and run, this returns 2048, the amount of memory used in an empty Lua state. That's not bad at all, but that's an empty state. What about a more useful state, like the one you get when you run the stock Lua interpreter?

-- ensure any accumulated garbage is reclaimed
collectgarbage('collect')
collectgarbage('collect')
collectgarbage('collect')
print(collectgarbage('count') * 1024)

Okay, when I run this, I get 17608. Eh … it's not that bad per thread (and I do have to remind myself—this is not running on my Color Computer with 16,384 bytes of memory). But I'm not running the stock Lua interpreter, I'm running the Kitchen Sink Lua with all the trimmings—how big is that state?

I run the above Lua code and I get 4683963.

Four and a half megs!

Ouch.

I suppose if it becomes an issue, I could always go back to writing C …

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