Creators of Erlang have a Lisp background, and one feature of the Erlang VM (BEAM) that I'd like back-ported into Common Lisp is their process.
An Erlang "process" is cheap to create, cheap to destroy, cheap when blocked, and upon exit performs bulk gc of its allocated memory; e.g., munmap().
Handling tens of thousands of requests per second per node isn't uncommon, and these often have *several* workers per request or connection: hundreds of thousands of processes. Under such scenarios, anything less than this approach to lightweight processes might suffer from stalls during long gc runs that would be avoided or significantly reduced under Erlang's model.
How might we get equivalent cheap ephemeral processes into a contemporary Common Lisp implementation?
For those unfamiliar with it, what Erlang means by "process" stems from an implementation of Actor Model with caveats. Each process is more co-routine than thread, and many may run within the same OS thread managed by the Erlang scheduler. The BEAM VM pre-empts processes via "reduction" counting, which may be understood as unit of VM time. Their famed tag line, "Let it crash," may be loosely understood in CL terms as an implicit HANDLER-CASE.
The open question here is to address a stated non-goal of CL-MUPROC, "we rely on the CL implementation's MP system" and "considerably heavier than Erlang processes". [See presentation link from https://common-lisp.net/project/cl-muproc/ ]
Some Erlang-on-CL packages use OS threads or in the case of Erlang-in-Lisp, fork(). While easier for a library author to implement, these contradict the definition of cheap here. Other such packages punt the issue altogether and instead focus only on pattern-matching aspects of Erlang the language.
Then there's Lisp-Flavoured-Erlang, and Elixir offers hygienic macros that manipulate the full AST properly-- all play nice on same Erlang VM at runtime-- for people simply looking to avoid Erlang syntax or semantics. But let's focus on cheap Erlang style processes in Common Lisp here, please.
1. Semantics of library elements are straight-forward enough (e.g., SPAWN, EXIT) and may be borrowed wholesale and safely named within their own package.
2. Memory allocation & garbage collection:
Erlang BEAM VM doesn't begin to reclaim memory until very late, so an initial implementation here might assume ephemeral worker processes and omit gc until exit of a worker. However, some processes are long-lived in practice.
One compromise might be acceptable: one nursery per process, but anything promoted to higher generations gets handled however your gc does it now.
This states nothing about use of shared versus multiple heaps across OS threads, so such matters may continue to be implementation-dependent.
3. Co-routines:
For something similar to Erlang's reductions, there would need to be a measure of runtime complexity per process.
However, I've used single thread co-routines for near realtime systems in C and CL with a loop of function pointers and ensuring that each referenced function executes under some threshold determined through experimentation and confirmed via test cases. No pre-empting needed. While fragile and not easily portable across different hardware (including same architecture), this may be acceptable for a preliminary draft.
Using CL-MUPROC routines as an example and extending MUPROCN: perhaps its body becomes this top-level list of functions from which interleaving across processes may occur. Then add variations for playing well with DO or LOOP semantics.
4. Message-passing:
SBCL's sb-concurrency extension with Queue and Mailbox (implementation of "Optimistic FIFO Queue") can be the solution here too. More aligned with CL principles, allowing for multiple mailboxes-- and therefore priority messaging-- would be a welcome advancement beyond Erlang. (Erlang allows only one mailbox per process: sequential but with pattern matching, nothing out-of-band...)
An important implementation detail that simplifies gc and increases cache locality across NUMA nodes: messages get duplicated when delivered-- each process only references its own copy!
5. (Almost) nothing shared:
Erlang enforces strict prohibition against shared memory, and common practice is to use an in-memory database (ETS) as key/value store in lieu of globals. Scala allows but discourages shared memory.
A CL-inspired take on this might be to use SBCL's approach with thread creation: upon creating a new process, you get: A) global special values, but modify at own risk... B) LET bindings are local to each process; C) threads don't inherit dynamic bindings from parent. i.e., http://sbcl.org/manual/index.html#Special-Variables
6. Scheduling processes on OS threads:
This is delicate in Erlang, and I've experienced specialized use cases interfering with their scheduler when under heavy load. Instead for our purposes, let the programmer handle the mapping of processes to OS threads. Less is more here.
7. Finally, gen_server & OTP fiends may be implemented as their own packages... or skipped entirely!
Thoughts on feasibility?
Thanks, -Daniel