Re: [Caml-list] Re: OCaml is broken

[Gerd Stolpmann <gerd@gerd-stolpmann.de>]

Am Sonntag, den 20.12.2009, 08:47 -0500 schrieb Yaron Minsky:
> On Sun, Dec 20, 2009 at 7:21 AM, Erik Rigtorp <erik@rigtorp.com>
> wrote:
>         The first step for OCaml would be to be able to run multiple
>         communicating instances of the runtime bound to one core each
>         in one
>         process and have them communicate via lock free queues.
> 
> 
> We've done some experiments in this direction at Jane Street.  On
> Linux, we've been able to get fast enough IPC channels for our
> purposes that slamming things into the same memory space has not in
> the end been necessary.  (There is I agree some pain associated with
> running multiple runtimes in the same process.  If you're interested,
> contact me off-list and I can try to get you some of the details of
> what we ran into.)
> 
> 
> But have you tried using shared-memory segments for communicating
> between different processes?  You say the latencies are too high, but
> do you have any measurements you could share? Have you tried queues
> using shared memory segments, in particular?  Inter-thread
> communication has latency as well, and the performance issues depend
> on lots of things, OS and hardware platform included.  It would help
> in understanding the tradeoffs.

I'm also experimenting now with shared memory (shm) as fast IPC
mechanism. I've extended ocamlnet with a few functions that allow to
copy an ocaml value into a shm segment which is accessible as bigarray:

https://godirepo.camlcity.org/svn/lib-ocamlnet2/trunk/code/src/netsys/netsys_mem.mli

Look especially for init_string. (I've also to mention Ancient here
which inspired to this work.)

Having ocaml values in shm saves us from some marshalling costs which is
right now the biggest performance penalty when using multiple processes.
However, this causes some problems, and at some point modifications of
the ocaml runtime will be necessary:

- The polymorphic equality and hash primitives do not work anymore
  for values in such shm segments (and that really hurts,
  especially string comparison is broken)
- Given that the shm segment is set to read-only after being set up, it
  is not possible to have pointers from shm to other memory regions.
  This is good, as this would be very dangerous (GC may delete or move
  values in the regular heap). However, the question arises when the
  shm segment can be deleted. We would need help by the GC to identify
  segments that are no longer referenced.

Without that, shm will be restricted to a role as low-level
inter-process buffers. 

> As we go to higher-and-higher numbers of cores, I suspect that
> message-passing solutions are likely to scale better than shared
> memory, so I'm not so sure that OCaml is on the wrong path here.  I
> think that most of the work that's needed is going to come in the form
> of libraries, with only a little work in the compiler and the
> runtime.  Given that, I think this is an issue for the community to
> solve, not INRIA.

Well, message passing and shm do not exclude each other. We should
refine the terminology here: Actually, shm is just a basic mechanism
where several execution threads (including processes) can share memory.
What's often meant is, however, the role it plays for multi-threading,
i.e. shared mutable data structures. What's typical here is that several
threads write to the same memory regions. I don't know a good name for
naming that programming style - maybe multi-threading style shm is the
best.

I'm working on a local message passing queue that can be used for long
messages, based on shm, and where the messages can contain normal ocaml
values (although it is likely that these are copied to the normal heap
by the receiver, for the above mentioned reasons, but this is an
expensive copy). The whole point will be that the data marshalling costs
are minimized. So far I can already say, we will need some changes in
the runtime to make such a mechanism fast and safe.

Gerd

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