[erlang-questions] How small could an Erlang emulator be?

[Robert Virding]

I have done a number of Erlang implementations but I was not really 
concerned about the size of the emulator; except of course that writing 
a large emulator is more time consuming. My main interest then was in 
looking at different memory models. You can do a lot to influence 
memeory size by choosing the right memory model. Of course.

Size of emulator would be interesting. I still think that a small stack 
machine using bytecodes would be best.

Robert

James Hague wrote:
>> When someone asks "how small could AN Erlang emulator be",
>> that means something quite different from "how small could THE Erlang
>> emulator be".
> 
> There are a lot of possibilities here.  One of the largest, most
> complex modules in the emulator is the one that turns generic BEAM
> code into a list of threaded superinstructions.  Most of this could be
> moved out to the compiler, but it would lock various optimizations
> into the BEAM format itself.  If you're wanting to reduce the raw size
> of the emulator, you could remove all support for these
> superinstructions (a.k.a. combined instructions) at the expense of
> performance.
> 
> Another option is to rewrite the core of the emulator in assembly
> language for each processor (I'd write a custom mini-assembler in
> Erlang first).  The advantage here is that you'd have tight control
> over register usage and subroutine calling mechanisms and could strip
> out unecessary fluff.  This likely wouldn't reduce code size by that
> much, however.  It would be interesting to try to keep the entire
> emulator in 16K or 32K so the whole thing fits in the instruction
> cache.
> 
> Bjorn already suggested switching to 16-bit threaded code internally,
> which would cut the size of loaded BEAM files in half (as it stands,
> they get substantially inflated after loading).  If you hand-crafted
> the emulator, this would be easy.
> 
> My biggest concern would be memory management.  Once you start
> thinking about small memory systems, ones without memory mapping
> hardware, then heap fragmentation becomes an issue.  On the
> Playstation 2 projects I've worked on, there was no heap at all,
> because all 32MB was packed with data and critical buffers.
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