Re: [Caml-list] Case study in optimization: porting a compiler from OCaml to F#

[Roberto Di Cosmo <roberto@dicosmo.org>]

Hi Jon,
   a concrete set of well justified benchmarks could serve
the cause more than any abstract discussion; please feel
free to set it up, run it, and analyze the results.

Having spent quite a bit of energy on Parmap, after it
started as a sort of a one-afternoon project, and with
the experience of the now very old OCamlP3l library that
started much of this at the end of the '90s (including a
detour through an experimental reimplementation in Haskell),
I definitely took the St Thomas stance with this kind of issues :-)

--
Roberto

On Wed, Mar 20, 2013 at 08:54:59PM -0000, Jon Harrop wrote:
> 
> Not just the granularity. Also the communication including any communication involved in scatter and gather phases. That differs a lot more between OCaml and F#. Fork does copy-on-write but (IIRC) the GC can incur unnecessary copying but, more importantly, requires the gather phase to deep copy results back to the original process. In contrast, data can be passed by reference in F#.
> 
> Would be very interesting to benchmark this...
> 
> Cheers,
> Jon.
> 
> -----Original Message-----
> From: caml-list-request@inria.fr [mailto:caml-list-request@inria.fr] On Behalf Of Francois Berenger
> Sent: 19 March 2013 01:50
> To: caml-list@inria.fr
> Subject: Re: [Caml-list] Case study in optimization: porting a compiler from OCaml to F#
> 
> I have observed and measured perfect scalability with up to 4 cores of an OCaml program using Parmap.
> With more than 4 cores, the scalability was degrading.
> 
> I think the scalability of the program depends only on the granularity of the tasks. The tasks were coarse in my case.
> 
> F.
> 
> On 03/17/2013 09:06 PM, Jon Harrop wrote:
> > Pierre-Alexandre Voye wrote:
> >> So you could maybe use Parmap.map ?
> >> Parmap.parmap ~ncores:4 funct (Parmap.L elem_list)
> >
> > What happens if the inner function returns results via mutation? I assume you must rearrange the code to return all results explicitly and they will then be deep copied (which destroys scalability due to limited shared memory bandwidth on multicores).
> >
> > Does it do load balancing? I assume not given that ncores is hardcoded.
> >
> > Does a parmap with ncores=4 inside a parmap with ncores=4 create 16 processes?
> >
> > Does it deep copy inputs and/or outputs? I assume so, at least for outputs, because you cannot write results in-place without a shared mutable heap.
> >
> > Does parmap have a large constant overhead? I assume so if it is forking processes.
> >
> > Another solution is to prefork and explicitly communicate all inputs using message passing but this is equally problematic. You have to rearrange the code. Deep copying inputs also destroys scalability.
> >
> > Cheers,
> > Jon.
> >
> >
> >
> 
> 
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-- 
Roberto Di Cosmo
 
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