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

[Jon Harrop <jon@ffconsultancy.com>]

On Sunday 20 December 2009 14:27:00 Dario Teixeira wrote:
> Hi,
>
> > It's too bad that INRIA is not interested in fixing this bug. No
> > matter what people say I consider this a bug. Two cores is standard by
> > now, I'm used to 8, next year 32 and so on. OCaml will only become
> > more and more irrelevant. I hate to see that happening.
>
> This is a perennial topic in this list.  Without meaning to dwell too
> long on old arguments, I simply ask you to consider the following:
>
> - Do you really think a concurrent GC with shared memory will scale neatly
> to those 32 cores?
>
> - Will memory access remain homogeneous for all cores as soon as we get
> into the dozens of cores?

The following web page describes a commercial machine sold by Azul Systems 
that has up to 16 54-core CPUs (=864 cores) and 768 GB of memory in a flat 
SMP configuration:

  http://www.azulsystems.com/products/compute_appliance.htm

As you can see, a GC with shared memory can already scale across dozens of 
cores and memory access is no more heterogeneous than it was 20 years ago. 
Also, note that homogeneous memory access is a red herring in this context 
because it does not undermine the utility of a shared heap on a multicore.

> - Have you considered that many Ocaml users prefer a GC that offers maximum
> single core performance, 

OCaml's GC is nowhere near offering maximum single core performance. Its 
uniform data representation renders OCaml many times slower than its 
competitors for many tasks. For example, filling a 10M float->float hash 
table is over 18x slower with OCaml than with F#. FFT with a complex number 
type is 5.5x slower with OCaml than F#. Fibonacci with floats is 3.3x slower 
with OCaml than my own HLVM project (!).

>   because their application is parallelised via multiple processes
>   communicating via message passing? 

A circular argument based upon the self-selected group of remaining OCaml 
users. Today's OCaml users use OCaml despite its shortcomings. If you want to 
see the impact of OCaml's multicore unfriendliness, consider why the OCaml 
community has haemorrhaged 50% of its users in only 2 years.

> In this context, your "bug" is actually a "feature".

I'm not even sure you can substantiate that in the very specific context of 
distributed parallel theorem provers because other languages are so much more 
efficient at handling common abstractions like parametric polymorphism. Got 
any benchmarks?

-- 
Dr Jon Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/?e