Minimizing memory consumption

[Amit Aryeh Levy <amit@...>]

Hi all,

I've begun implementing a Lua runtime for very low-memory processes on
embedded systems
(https://github.com/helena-project/tock/blob/master/userland/examples/lua-hello/main.c)

So far, with at least a basically naive use of the lua library, memory
usage is non-trivial for this environment: the base library takes ~8kB
of heap and requires an additional 2-3kB of stack to parse, compile and
run a simple hello world. The code size overhead is also reasonably
large---~90kB. I'm wondering a few things:

1. It looks like the default LUA_BUFFERSIZE, which seems to be impacting
max stack size the most, is relatively large---2048 bytes when
LUA_32BITS is set (or 8192 otherwise). Is it safe to make this smaller?
If so, by how much and are there any invariants this size must adhere to?

2. Are there particularly expensive parts of Lua in terms of code size
that are possible to avoid? For example, it's probably reasonable not to
support compilation of plaintext Lua code to bytecode on the device and
instead only run load pre-compiled bytecode, which I can imagine
reducing code size of the runtime significantly, however, it doesn't
seem trivial to remove just the ascii parser while keeping the bytecode
parser.

3. Are there general resources available for minimizing memory usage in
Lua? Of course reducing overhead in the runtime would be useful, but I'm
also interested in guiding how Lua libraries are written. For example,
it seems as though closures are reasonably expensive and perhaps storing
them long term should be avoided. Are there programming patterns like
that which are well understood and documented somehwere?

Thanks!!
Amit