Re: [***SPAM*** Score/Req: 10.1/8.0] Re: [***SPAM*** Score/Req: 10.1/8.0] Re: [Caml-list] Re: OCaml is broken

[Erik Rigtorp <erik@rigtorp.com>]

On Sun, Dec 20, 2009 at 14:47, Yaron Minsky <yminsky@gmail.com> wrote:
> 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.

Some IPC Benchmarks, Solaris 10 on a quad core Intel Core2 Duo. The
benchmarks are running on a cpuset with 1 core. I measure the time
from sending in one process until the other process receives the
message. So a context switch and the message passing is included in
the measurements.

Max/Min/Avg
* Pipes: 28205/5973/6259
* Unix domain sockets: 44256/7748/8153
* SYSv message queues: 19197/5895/6173
* Posix message queues: 37399/10965/11303
* TCP on loopback: 29017/7471/7885

So the latency is roughly 10µs for all these solutions. That latency
is pretty high and would be several times the processing time of the
message itself.

I haven't tried using shm for IPC yet. I'll try see if I can do
something with this lock-free queue:
http://www.liblfds.org/.

> 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

I agree, but the message passing must be cheap and the creation and
scheduling of lightweight threads must be fast. In order to do that
the runtime needs to handle the communication via shared memory
between OS threads or processes, binding OS processes and threads to
cores and know about different cores different memory latencies and so
on.