Re: Strange memory allocation behavior

[Roberto Ierusalimschy <roberto@...>]

> We're using Lua to facilitate Ethereum-style smart contracts in a
> C/C++ based cryptocurrency toolkit.  In case you aren't familiar with
> smart contracts, the idea is pretty simple.  It's just a little
> program that all the peers in a P2P network run.  Every peer needs to
> agree on the program's output, so the scripts need to be completely
> deterministic.  Also, any peer can submit a smart contract, so the
> scripts run in a sandboxed environment with constraints on instruction
> count and memory usage.  Before running a contract script, we disable
> the garbage collector with 'collectgarbage("stop")'.  Hypothetically,
> this shouldn't be a problem, since we only want scripts to be able to
> allocate a fixed amount of memory, total, over the entire course of
> their execution.  However, it seems to cause the memory profile of
> even trivial scripts to blow up for no particularly apparent reason.
> 
> We are tracking allocations using a custom lua_Alloc function, as
> documented in the reference manual (version 5.3).  Of particular
> confusion is that functions which are never called still appear to
> consume large amounts of memory (hundreds of kilobytes).  This problem
> does seem to be severely exacerbated by the use of custom variables we
> load from the sandbox environment, so I can only imagine that's part
> of the problem, but the behavior is confusing regardless.  Why should
> an uncalled function consume so much memory, or any memory at all?  Is
> there any reason lua_Alloc might behave strangely with the garbage
> collector disabled?
> 
> I can find no obvious fault with my own code, so I thought it sensible
> to check that there isn't some well known Lua behavior I'm unaware of.
> Thanks for your help.

'collectgarbage("stop")' only stops the collector. It should not
change in any other way the behavior of the program or the calls
to lua_Alloc.

-- Roberto