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Here is the latest OCaml Weekly News, for the week of March 08 to 15, 2016.

  1. Question about Lwt/Async
  2. Profiling ocaml with lwt
  3. Flambda manual chapter
  4. opam-cross-windows
  5. Software engineer position at Tweag I/O
  6. Other OCaml News

Question about Lwt/Async


Continuing this thread from last week, this conversation happened:
Yaron Minsky:

>> Jeremie, other than having some different back-ends available (e.g., glib
>> main loop), how different are the approaches to backend management between
>> Async and Lwt?

Jeremie Dimino:

> ​The backend interfaces are slightly different​, but we just need a bit of
> glue in the middle. Essentially the difference is that with Lwt you provide
> one callback per fd and watch (read or write), while with Async you have a
> global callback.
> ​Right now what we need to change in Async to make this work is:
> - allow to provide a backend ​programmatically; right now you can only
> choose between the predefined epoll and select ones
> - make the scheduler ignore fds returned by the backend that are not
> handled by async

Malcolm Matalka:

For what it's worth, which isn't much right now, I've been slowly
developing an interface point for event loops and user facing code.  The
rough idea is to present "asynchronous system calls" like an OS would,
so user facing code has an interface to program against and the
underlying event loop can change as someone wants, libev, libuv, direct
epoll or kqueue, etc.  So Async and Lwt libraries could be implemented
in terms of this interface and share the same event loop, to cooperate
nicely.  So far I haven't implemented anything using the interface
except for a barely functional test to demonstrate that it even works,
so it's quite raw.  And it's clearly deficient on a few things, but I
think the idea is sound and would alleviate some of the pain of deciding
to use Lwt or Async and if it works on JS or Windows or My Favorite OS
(just flip out the underlying scheduler implementation).

The work in progress around the interface can be found below, any
constructive feedback would be appreciated.

Profiling ocaml with lwt


Kasper Janssens asked:
I’m trying to profile an ocaml application that has a performance degradation of
about 20 %, with quite some code changes between both versions. I tried
attaching perf and gprof to it, but to no real avail. Sure, there are slight
differences in the running time of functions in the gprof result, but nothing
whatsoever that seems to explain that kind of delay. Perf doesn’t seem to
indicate a problem neither.

The thing is, I think it’s a delay in one of the servers that the application
communicates with. The communication to those servers is done asynchronously,
through lwt (epoll). I suspect that is the reason why delays in that
communication don’t show up in the gprof result, but I’m not really sure.

Does anybody know of a way to profile lwt applications in a way that these kinds
of delays might show up so I know which backend system I have to focus on?
Adrien Nader replied:
I tend to (ab)use strace. It has options to only instrument some
syscalls, it can count time spent in syscalls, time spent between
syscalls, has microsecond precision, ...

My first runs are usually "strace -tt -f -o log" to get times with
microseconds, follow forks and write to a file rather than stderr
(heavily advised, especially since text editors have syntax highlighting
for strace's output [ vim at least even though you need to :set
ft=strace when using -tt).

After that, you can also use -T to get the time spent in syscalls, -e to
trace fewer things, ...

It's a hammer and requires a fair bit of eye-crossing but it covers everything,
it's overhead isn't that big and it can easily reduce the scope of what you need
to look more deeply at. However it's probably useless to try to spend more than
2 hours on a strace's output if you can't find the answer you're looking for.
Romain also replied:
You can always use valgrind's callgrind to profile your applications. It has
options to record systimes as well.
Kcachegrind does a decent job in helping you to interprete the results.
Török Edwin also replied:
Is that communication done over a TCP/UDP socket?
You could use wireshark and see if you spot anything (e.g. too many SSL
handshakes -> application would benefit from persistent connections, etc.)

Depending on the complexity of your application you may try to use a branch of
Lwt that reports more tracing information, although this would likely require
manual annotations to produce useful output:

I usually just wrap+log calls and use strace though, as already mentioned in
this thread.
Malcolm Matalka then added:
And if you're on FreeBSD or OS X or Solaris you can use dtrace.

Flambda manual chapter


Mark Shinwell announced:
The current draft of the new chapter for the OCaml manual, about the
Flambda optimisation passes forthcoming in version 4.03, is available
for viewing at:

Please note that this is intended as user documentation.  Developer
documentation is provided via the source code and comments therein.
(We will hopefully do an ocamldoc-style generation from that before
the 4.03 release.)

If you would like to make comments please do so via this Github pull
request page:



whitequark announced:
I've released opam-cross-windows[1], a 4.02.3 OCaml toolchain
in the spirit of opam-cross-android[2] (ex opam-android).
It provides easy cross-compilation of the OCaml compiler
and select packages from any *nix environment to 32-bit
and 64-bit x86 Windows.

There aren't many packages yet but you're encouraged to submit
your own. Personally, I find the porting process that uses
the opam-cross-* conventions so simple and robust that it
can be done nearly mindlessly.

The cross-compiled package definitions themselves are
identical to the ones from opam-cross-android (except for
s/android/windows) so I think cross-compilation should gain
at least minimal OPAM support; I've described my proposal
at [3].


Software engineer position at Tweag I/O


Arnaud Spiwack announced:
Tweag I/O [1] is hiring. We're looking for a software engineer in the Paris
area. The position involves Haskell, not Ocaml, but it may still be of interest
to this list. Details of the position below.


Software Engineer position

Tweag I/O is looking for distributed systems engineers to be part of one of
several projects in Europe focused on developing the next wave of storage
solutions in Haskell and in C, targeted at the Exascale.

Many of our existing folks come from diverse backgrounds, be it a PL research
and/or formal methods background, high-speed storage or machine learning. We are
active maintainers of the Cloud Haskell [2] project and authors of the HaskellR
[3] project, among other open source contributions. Our engineers spend a fair
amount of time building the tools they need to make their life here a happy one.
We love Nix [4] and Stack [5] so much to build and deploy our software that we
made it easy to use both together [6]. 

For this position we're looking to have you join a local team near our
headquarters here in Paris. We're pretty happy to look at helping you relocate
if you're up to spending some time in this beautiful city. Fluency in French not

If you like the idea of working on the software plumbing and infrastructure for
tomorrow's Science, by systematically decomposing them into simple, orthogonal
solutions that compose and commute like in algebra, give it a try!

About us

We are a research and development laboratory at the heart of Europe, applying
functional programming techniques to tame the complexity of distributed systems
and scale predictably. We are a distributed company with a presence across
Europe, and on-site teams at our headquarters in the very center of Paris. Our
mission is to build and integrate solutions for our customers to support insight
discovery and science. If you'd love the opportunity and the space to solve hard
problems shoot us an email at


Other OCaml News

From the ocamlcore planet blog:
Here are links from many OCaml blogs aggregated at OCaml Planet,

Seven Implementations of Incremental

Software Engineer at Purple (Full-time)

Old cwn

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