Sunday, 29 November 2015

OCaml and Multithreading

As Doctor House said one day: "There was that philosopher Ocaml..." Stop, wrong start! Try again.

1. OCaml story

OK, some time ago, in a somehow better world, I became attracted to the OCaml language because of its speed, elegance, light syntax, algebraic data structures and automatic type inference (as you meanwhile might know, I'm a bit of a typing fan). I must admit, it was my first functional language, so maybe I'm a little sentimental about it. Later on I somehow didn't give it much love and affection, mostly because I got distracted by other, more fashionable languages like Clojure, F# and Haskell (shame on me!).

Nonetheless, although this one is rather an obscure language, one company has been using it big time to do some highly parallel, high-performance data processing. This company is Jane Street and they are a financial market player. The reason they chose OCaml were that the owners wanted to read every line of code performing financial transactions*:
"Early on, a couple of the most senior traders (including one of the founders) committed to reading every line of code that went into the core trading systems, before those systems went into production"
"In 2003, Jane Street began a rewrite of its core trading systems in Java. The rewrite was eventually abandoned, in part because the resulting code was too difficult to read and reason about—far more difficult, indeed, than the VBA that was being replaced."
 "The VBA code was written in a terse, straight-ahead style that was fairly easy to follow. But somehow when coding in Java we built up a nest of classes that left people scratching their heads when they wanted to understand..."
 "In 2005, emboldened by the success of the research group, Jane Street initiated another rewrite of its core trading systems, this time in OCaml."
So somehow the functional-style code was more readable than a pile of Java classes: can we put that to the superiority of functional languages in general or just to bad training of Java programmers? An open question.

OK, there are additional technical reasons as well: the type inference that makes the code concise (thus more readable) while retaining type safety. Additionally it's runtime performance is pretty good! A double win against for example Python.

2. Multithreading?

For a long time they remained the single serious industry user of which I knew, but recently also Facebook started using OCaml in their Hack and Flow language typing add-ons for PHP and Javascript implementations. Surprise! And a very nice one, my pet language got appreciated at last! Everything OK?

Not quite: when I listened to that presentation, I was surprised to learn that OCaml's mutlithreading support is pretty much completely broken. How can that be? Why didn't I notice that first place?

Well, after some more digging, I received this link from a fellow programmer. It makes it pretty clear to us all that OCaml designers were quite a nasty bunch of "multicore deniers":
"The goals of OCaml threads are (2) and (3) but not (1)"
Where (1) denotes "Parallelism on shared-memory multiprocessors"! And then:
"Shared-memory multiprocessors have never really "taken off", at least in the general public.  For large parallel computations, clusters (distributed-memory systems) are the norm. For desktop use, monoprocessors are plenty fast."
 "What about hyperthreading?  Well, I believe it's the last convulsive movement of SMP's corpse :-)"
Yes, they didn't like it, to say the least. But there's another reason as well: the GC implementation in OCaml is single-threaded and thus it could be made very fast, which was an important factor in OCaml's initial successes. But then this (ironically) proved detrimental to trying to make the language MT-safe**.

The only way to do parallel processing in OCaml is thus multiprocessing + message passing. As I learned Jane Street wrote the Parallel library to support that:
"Parallel is a library for spawning processes on a cluster of machines, and passing typed messages between them. The aim is to make using another processes as easy as possible. Parallel was built to take advantage of multicore computers in OCaml, which can't use threads for parallelism due to it's non reentrant runtime
The Facebook people went one step further: they
"...use the same model for multithreading: a specially mmap'd region shared between different fork'd processes, containing a shared, lockless hash table. "
Thus they have multithreading without threads - a couple of processes sharing the memory space (or a part of it), faking the SMP model! You must admit it's rather brilliant - you spare yourself all that serialization, deserialization, and message passing that normally shave off a significant part of the performance.

3. So why OCaml?

Apparently, despite the mutithreading disaster, companies find OCaml worth trying out. Why? I'd say the reasons are:

  1. It supports functional style (but it allows dirty tricks when they must be done)
  2. Has strong typing unlike Lisp, Python, etc.
  3. Not bound to Windows and .NET like F#
  4. Not Haskell, you'll be able to read and understand it.***

And finally, maybe this whole multithreading business isn't worth the hassle? Facebook clearly need it, but Jane Street don't. I remember having heard Yaron Minsky (of Jane Street) saying somewhere that parallel processing plus message passing is a much safer model that multithreading. And in finance you need that safety.

I can accept this argument in a more general setting as well, because the only reason we have threads are performance gains (OK, a deceptively "natural" programming model too). And if the performance of forking + message passing is sufficient, that's definitely a gain! Jane Street does some massive parallel processing in real time, so the performance seems to be not that bad after all...

Or maybe the fork+pass model is slower, but they are gaining on the excellent low-latency CG? Jon Harrop puts it like that:
"OCaml has a nice (~10ms) low latency GC and it is easy to optimise OCaml code for low latency. In contrast, the .NET GC is much more complicated and, therefore, harder to optimise for and, in my experience, has >10x higher latency. I suspect that would be a major drawback for Jane Street."
So before you port OCaml's GC to multicore, overcomplicating it and losing its excellent performance, maybe paying some price for the workarounds pays out better?

Having said that, there's recent work on improving the GC-performance even more, and even a new attempt on multicore OCaml. Maybe this time they will be successful. And be wary, not everyone likes OCaml, for some "OCAML sucks" -!

Update: an impressive list of companies using OCaml: (via @jonharrop). So it's not only Jane Street and Facebook!
Update2: Facebook continues working on OCaml ecosystem - Reason, a "new interface to OCaml" ... "provides a new syntax and toolchain for editing, building, and sharing code". Plus Infer - the iOS/Android app checking tool!

* "OCaml for the masses" -

** "The rise and fall of OCaml" -

 ***  As someone said, Haskell was created to do research on Haskell (and OCaml was created to write theorem provers).


Hans Meyer said...

Thanks for these infos. that's the best review I have found on this subject so far.

Marek Krj said...

@Hans Meyer
Funny, because I'm not even an OCaml programmer, only somehow interested in OCaml and F# as an alternative to Haskell (I dis a little of F# however)!