Improving Nasal

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Last update: 10/2011

As more and more code in FlightGear is moved to the base package and thus implemented in Nasal space, some Nasal related issues have become increasingly obvious.

On the other hand, Nasal has a proven track record of success in FlightGear, and has shown remarkably few significant issues so far. Most of the more prominent issues are related to a wider adoption in FlightGear, and thus more complex features being implemented in Nasal overall.

So, rather than having Nasal flame wars and talking about "alternatives" like Perl, Python, Javascript or Lua, the idea is to document known Nasal issues so that they can hopefully be addressed eventually.

If you are aware of any major Nasal issues that are not yet covered here, please feel free to add them here, however it is also a good idea to use the FlightGear bug tracker in such cases: http://flightgear-bugs.googlecode.com/

Explicit loading order and reset support

See [1].

Cleaning up FGNasalSys::init()

If you're looking for a structured environment here, what about a cppbind port of SGSubsystem for Nasal? I.e. with .init, .postinit, .reinit, .update, etc.? That would be structured It might also help with Nasal loading as an alternative to require() --

We once discussed the idea a long time ago, and it would actually be great if all Nasal code were using such an established and structured interface. It would be much easier to support simulator resets/re-initialization that way, if all scripts (core sim, GUI, aircraft) would follow a standard interface.

But first of all, one would need to provide such a wrapper, and then document it extensively and illustrate its benefits - i.e. in some of the default aircraft (c172p), so that developers are motivated to actually do all the porting work.

Overall, it would definitely align well with the reset/reinit work done by ThorstenB & James, because they're trying to solve the problem in a brute-force fashion by simply shutting down everything and starting over. Personally, I have come to the conclusion that NasalSys::init() is a huge troublemaker actually - it would be so much more simpler/elegant if that stuff was done in Nasal.

NasalSys::init() should really only handle the necessary C++/internal side (i.e. cppbind stuff) and just load a single script with will in turn load the others through Andy's original import() magic. It might also help by keeping everything under a single naContext, but I dunno if that is good or bad (I would suspect good).

And I am actually thinking of having more than just $FG_ROOT/Nasal - i.e. different folders for different modules, not just sub modules - but hierarchies that handle dependencies. For example, props.nas would be one of the first modules to be loaded, while gui.nas would only be available in a "GUI" run-level, i.e. after the sim is up and running. The current init code is incredibly messy.

In the FGRadar/Nasal code, Icecode_GL actually introduced another folder for unit testing purposes, that would be loaded/executed prior to anything else, where you would then simply add unit tests - which would all need to succeed first.

These things would be all good to have, but they don't need to be implemented in C++ space. That doesn't mean that mapping SGSubsystem to Nasal space would not be useful - I actually like the idea a lot, because we definitely have a bunch of REAL subsystems implemented in Nasal, such as bombable or advanced weather - these would ideally use a real SGSubsystem interface, possibly even SGSubsystemGroups (basically vectors of related subsystems) to ensure that the design supports reset/reinit and other important stuff without shutting down the whole beast.

The current way of developing Nasal code and just drupping it into $FG_ROOT/Nasal or $FG_AIRCRAFT simply isn't overly scalable, while also being really fragile and difficult to provide opportunities for code reuse.

Get rid of the global interpreter context

Source: Andy Ross, Nasal author

Year: 2007-2011

URL: http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg13028.html

Problem: New nasal objects are added to a temporary bin when they are created, because further allocation might cause a garbage collection to happen before the code that created the object can store a reference to it where the garbage collector can find it. For performance and simplicity, this list is stored per-context. When the context next executes code, it clears this list.

Here's the problem: we do a lot of our naNewXX() calls in FlightGear using the old "global context" object that is created at startup. But this context is no longer used to execute scripts* at runtime, so as Csaba discovered, it's temporaries are never flushed. That essentially causes a resource leak: those allocations (mostly listener nodes) will never be freed. And all the extra "reachable" Nasal data floating around causes the garbage collector to take longer and longer to do a full collection as time goes on, causing "stutter". And scripts that use listeners extensively (the cmdarg() they use was one of the affected allocations) will see the problem more seriously.

(That's a feature, not a bug. Once listeners were added, scripts could be recursive: (script A sets property X which causes listener L to fire and cause script B to run) We need two or more contexts on the stack to handle that; a single global one won't work.)

I didn't like the fix though. Exposing the temporary bin as part of the Nasal public API is ugly; it's an internal design feature, not something users should tune. Instead, I just hacked at the FlightGear code to reinitialize this context every frame, thus cleaning it up. A "proper" fix would be to remove the global context entirely, but that would touch a bunch of code.

Also see: http://gitorious.org/fg/flightgear/blobs/next/src/Scripting/NasalSys.cxx (in FGNasalSys::update)

   // The global context is a legacy thing.  We use dynamically
   // created contexts for naCall() now, so that we can call them
   // recursively.  But there are still spots that want to use it for
   // naNew*() calls, which end up leaking memory because the context
   // only clears out its temporary vector when it's *used*.  So just
   // junk it and fetch a new/reinitialized one every frame.  This is
   // clumsy: the right solution would use the dynamic context in all
   // cases and eliminate _context entirely.  But that's more work,
   // and this works fine (yes, they say "New" and "Free", but
   // they're very fast, just trust me). -Andy

Improve the garbage collector

As of 06/2012, a new incremental Nasal GC is being worked on [2]:

I have been working on a 4-color incremental mark/sweep collector with
the intention of merging it into the Nasal interpreter.

The work so far can be found at http://github.com/chrisforbes/incgc;
There's still quite a lot to do, but the path is clear.

Also see: How the Nasal GC works

Year: 2011-2012

URL: http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg33190.html

Problem: Nasal has a garbage collection problem. One solution to it is - we avoid Nasal code wherever possible and try to hard-code everything. But Nasal crops up on a lot of places - complex aircraft such as the Concorde come to my mind, interactive AI models, lots of really nifty and useful applications... - so instead of fixing things in a lot of places, one could also think about it the other way and fix just one thing, i.e. the garbage collection such that it doesn't hit a single frame. I fully well realize that dragging out complicated operations across many frames while everything else keeps changing is at least an order of magnitude more complicated (about 1/3 of Local Weather deal with precisely that problem...) - but I don't believe it can't be done at all. It sort of bugs me a bit that somehow the fault is always supposed to be in using Nasal...

I think it's great if we have a discussion where the issues are placed on the table to give everyone the change to learn and understand more, and then reasonably decide what to do. Nasal has advantages and disadvantages, so has C++, sometimes accessibility and safety are worth a factor 3 performance (to me at least), sometimes not. But I don't really want to discuss dogmatics where 'truth' is a priori clear. There is a case for having high-level routines in Nasal, there's a case to be made to switch low level workhorses to C++ - and there's always the question of what is the most efficient way of doing something. But I'm clearly not considering Nasal-based systems immature or experimental per se.

URL: http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg31918.html

As discussed in "Stuttering at 1 Hz rate" we now know that regular and unpleasant stuttering is caused by Nasals garbage collector. So I thought about possibilities to improve it. What if we could decouple the following function as a separate thread, so that it runs *asynchronously* from the main thread? This way it would not interfere (or much less) with the main thread and our fps would be more consistent.

This is the function causing the jitter: In "simgear/nasal/gc.c" static void garbageCollect()

The thread will need to share some of the global variables from the main thread.

URL: http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg31919.html

I'm not an expert in nasal garbage collection, but I think the problem is that garbage collection is not something we can divide up into chunks (which is essentially what threading would do.) In addition, threading adds a lot of potential order dependent bugs.

In the case of nasal, I believe the garbage collection pass must be done in a single atomic step, otherwise it would leave the heap in an inconsistent state and adversely affect the scripts.

URL: http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg31637.html I don't know much about our Nasal implementation, but I suspect that the garbage collector could be changed to trace only a portion of Nasal's heap at each invocation, at the risk of increased memory use.

URL: http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg31921.html

There are algorithms for incremental and/or concurrent and/or parallel garbage collection out there. They most likely not easy to implement and as far as I have seen so far would require (at least for concurrent and /or parallel GC) all writes of pointers to the Nasal heap (and possibly reads) to be redirected via wrapper functions (also known as (GC) read/write barriers).

This will not be an easy task but in my opinion it would be a promising option. It might be possible to use a GC module from a GPL:d Java vm or similar.

Btw, just running the normal (mutually exclusive) Nasal GC in another thread than the main loop is not hard - but since it is mutually exclusive to executing Nasal functions it doesn't help much when it comes to reducing the worst case latency.

The small changes needed to add a separate GC thread are available here: http://www.gidenstam.org/FlightGear/misc/test/sg-gc-2.diff http://www.gidenstam.org/FlightGear/misc/test/fg-gc-1.diff

Also, I had a brief look at exactly which Nasal timers caused a jitter. And the winner is... ... well, any. Any Nasal timer, even if it's almost empty, will every now and then consume a much larger amount of time than normal. Seems to be a general issue with the Nasal execution engine: could be triggered by Nasal's garbage collector, which every now and then needs to do extra work - and runs within the context of a normal Nasal call. It could also be a result of Nasal's critical sections: other threads may acquire a temporary lock to alter Nasal data structures - which may block the execution of Nasal timers at certain points. Hmm... Best practices for debugging a multi-threaded program anyone? :)

Concerning the frequency of the jitter: I guess it isn't related to the FDM at all. It's probably just a result of Nasal complexity. The more Nasal code is running, the more often/likely garbage collection / blocking may occur. Frame rate may also influce it: many Nasal timers run at delay 0 (in every update loop).

URL: http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg37308.htmla

A significant part of Nasal-related frame rate impact is caused by garbage collection. Its delay/jitter only depends on the number of Nasal objects and their references which need to be searched. Increasing the number of Nasal objects (code+data) existing in memory also increases the delay.

The amount of Nasal code which is actually being executed only influences the g/c frequency, i.e. whether the effect is visible every few seconds vs several times per second.

URL: http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg37310.html

I did look at incremental GC for Nasal last year, but couldn't find a 'simple enough' generational algorithm. Still happy for someone else to try - the Nasal GC interface is very clean and self-contained, so quite easy to experiment with different GC schemes.

URL: http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg37338.html

But as I said, I think really the GC needs to be addressed. There's only so much hacking around the actual problem one can do.

URL: http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg37338.html

Right, a problem is that I've possibly studied all Nasal documentation I could get without finding any reference of the GC problem - that was only transmitted to me much later. I think you'll find that most Nasal users are not aware of any such problems, because it's not documented anywhere. It doesn't help so much if you are aware of it.

URL: http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg37338.html

the current GC is bad, and big Nasal shows this while small Nasal doesn't.

URL: http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg37338.html

We have an implementation of Nasal which dumps all the GC into a single frame and is apparently sensitive to the total amount of code, regardless if the code is actually run or not. This fact has historically not been widely advertized or explained. That turns out to be a problem.

The way this usually comes across is 'Advanced Weather causes stutter'. But it actually doesn't really (or at least that remains to be shown) - what causes stutter is mainly the GC, and Advanced Weather just happens to trigger this. The range of suggested solutions in the past included almost everything, from avoiding Nasal to porting code to Nasal to hacking around the problem to loading things on-demand - except fixing the actual cause of the problems.

I don't honestly know how complex code to collect garbage across many frames is, but somehow I doubt that in terms of man-hours the effort beats porting the existing large-scale Nasal codes to C++. Just my 2 cents in any case.

Separate GC implementations

• Ruby Enterprise Edition - performance blog
• Boehm GC
• libgarbagecollector
• AutoZone
• http://mono-project.com/Generational_GC
• http://mono-project.com/Working_With_SGen
• http://www.mono-project.com/Compacting_GC
• http://www.utdallas.edu/~ramakrishnan/Projects/GC_for_C/index.htm

Better debugging/development support

Besides making a full IDE (which would be really cool), there are several things that can be done by editing the source code of Nasal to enhance debugging support and increase development :

• Being able to dump the global namespace (see this topic for a possible solution) or at least dump things prettily (an unreleased version of the file discussed in Nasal Meta-programming has good support for nice formatting).
• Register a callback for handling errors using call()
• Register a callback for OP_FCALL et al. to be able to time function calls. Example (relies on the gen.nas module and debug.decompile):

# Debugging:
runindebug = func(fn, arg...) {
    var start = []; var indent = "";
    
    call(func, arg, nil, nil, {
           op_fcall:func(context) {
             var obj = debug.decompile(context.fn);
             obj.end = context.ip+1;
             append(start, [get_fcall_lvalue(obj)]);
             print(indent, "Start function call on function ", start[-1][0]);
             append(start[-1], systime()); #save it now for most accurate results
             indent ~= "  ";
           },
           op_return:func {
             var v = pop(start);
             indent = substr(indent, 0, size(indent)-2);
             print(indent, "Took ", systime()-v[1], " seconds");
           },
           error:func(err...) {
             printf(err[0]);
             for (var i=0; i<size(err); i+=1)
               printf("  called from %s line %d", err[i], err[i+=1]);
             print("Stack info:");
             var namespace = caller(1)[0];
             printf("Local namespace:");
             printf("  "~gen.dump_namespace(namespace, 0, 2, "  "));
             var c = caller(1)[1]; var i = 0;
             while ((var cl = closure(c, i)) != nil) {
               printf("Closure %d:", i+=1);
               printf("  "~gen.dump_namespace(cl, 0, 2, "  "));
             }
             var stack = debug.stackinfo();
             forindex (var i; stack) {
               printf("Caller %d:", i);
               printf("  Local namespace:");
               printf("    "~gen.dump_namespace(stack[i].locals, 0, 4, "    "));
             }
           },
         }, var err=[]);
};

var get_fcall_lvalue = func(obj, level=0, col=0, suffix="") {
    var output = gen.decompile_expr(obj, level, col, suffix);
    var level = 1;
    for (var i=-2; i>=-size(output) and level > 0; i-=1) {
        if (output[i] == `)`) level += 1;
        elsif (output[i] == `(`) level -= 1;
    }
    return substr(output, 0, size(output)-i);
};

# It is really easy to debug code now, just save off every
# expression to a variable and browse them on error.
var possibly_buggy_code = func(arg...) {
    var sum = (var _1 = getprop("/sim/node/not/exists[0]")) +
    (var _2 = getprop("/sim/node/not/exists[1]"));
    return var _3 = sprintf("%8.7f", sum);
};

• Set breakpoints: not needed (just call a function to do what you want).
• Time other parts of Nasal (not just VM) with a compile-time flag?
• Better error messages – in progress.
  • Parsing: Say something other than "parse error", like "null pointer".
  • VM: Indicate type of variable if wrong type.
  • Both: Could we give line and column?
• Working with bytecode:
  • Expose to Nasal: done
  • Decompile to text: partial (untested)
  • Optimize: not started
  • Working with it: provide Bytecode class: not started
• Inspect Context: not started, should be easy.
• Expose Tokens to Nasal: implemented by Hooray as argument to compile(), should be extended to cover output from lex.c and after blocking in addition to the current after-prec-ing (and before freeing!) support.
• Real function name support via assignment:
  • Option 1: look at the parse tree and check if the right side of the assignment is a function. If so, go ahead and parse the function with the left side as the "name" of the function instead of falling through to more recursions of genExpr().
  • Option 2: recognize assignment in the VM and if there is a bindToContext event, set the name of the function based upon either the last LOCAL/MEMBER/HINSERT or the combination of them (i.e. complex lvalues like local.fn). This presents some obvious issues, however:
    • The right-hand side of an assignment is done before the left-hand side, thus one would have to look ahead to see the assignment, which is clearly illegal for the VM to do.
    • Or one could look behind to see a naCode constant being pushed, and give some indication to its naFunc that it now has a name. This I still somewhat illegal, but not dangerous and thus could be done.
  • Option 3: abandon var foo = func(){} for ECMAscript-like function declaration syntax function foo() {}. This would not affect the use of anonymous func expressions but would instead be applicable in cases where we want to say "this function is static (i.e. permanent) and should have a name" (as opposed the the case of temporary storage variables for functions). Regardless of the method used, a name member will have to be added to naFunc's and the VM and error handling procedures will have to be changed according.