Re: [Caml-list] Two questions on the OCAML runtime system and compiler,

[Yotam Barnoy <yotambarnoy@gmail.com>]

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From what I understand, Luca's patch is going to be mainstreamed sometime
in the not-so-distant future, so it might make sense to work directly off
of his branch, no?

The problem you're having is that every caml_page_table_lookup() needs to
acquire the heap allocator's lock, right? Perhaps you can make this access
lockless. If the global page table is a persistent data structure, like a
Map (but in C of course) and the allocator creates a new data structure
root every time a block is allocated from/released from the OS, then all
you would need is a boolean per thread indicating that it's seen the new
global root before the allocator can go ahead and deallocate whatever
blocks need to be deallocated from the old persistent trees. Since blocks
shouldn't be allocated from the OS that frequently, this shouldn't be too
expensive.

Yotam


On Mon, Sep 30, 2013 at 9:24 AM, Leo White <lpw25@cam.ac.uk> wrote:

>
> We are also experimenting with a parallel OCaml runtime system at
> OCamlLabs (https://github.com/ocamllabs/ocaml/tree/multicore). Out of
> interest is the source for your experimental compiler and runtime available
> anywhere?
>
> With respect to the page table, I don't think it serves any other
> purpose. I know various people have been looking into removing
> it (or at least removing it from common operations) without the addition
> of a bit field in the header. So it is possible you could avoid changing
> the header layout in your version of the compiler as well.
>
> Regards,
>
> Leo White
>
> Sungwoo Park <gla@postech.ac.kr> writes:
>
> > Dear Caml users,
> >
> > We are currently experimenting with a parallel Ocaml runtime system, in
> which
> > each thread maintains its own old and young heaps and runs in parallel
> with
> > other threads, without sharing a global lock as in the current Ocaml
> runtime
> > system while sharing the whole memory space. We have made changes to the
> > current Ocaml compiler (4.00.1), and now the parallel compiler generates
> > parallel code that runs on the parallel Ocaml runtime system. It is a
> > cross-compiler and we can build it with any version of the Ocaml
> compiler,
> > including the parallel compiler itself.
> >
> > This is a project very similar to Luca Saiu's reentrant multi-runtime
> system
> > (https://github.com/lucasaiu/ocaml), and we benefited very much from
> Luca's
> > code at several early stages in the development. Similarly to Luca's
> work, our
> > runtime system is reentrant.
> >
> > As the next step toward a parallel runtime system with a shared heap, we
> are
> > struggling to redefine the tag format. I have two specific questions on
> the
> > compiler and the runtime system. Any comment and help would be greatly
> > appreciated.
> >
> > Question 1.
> >
> > When we reassign tag numbers for block types, the runtime system crashes
> for
> > some applications. We thought we made all necessary changes to the
> source code
> > (in byterun/mlvalues.h, utils/config.mlp, and several files in
> bytecomp/) to
> > reflect the new tag numbers. However the runtime system often crashes,
> for
> > example, when it accesses the Lazy module, which in turn accesses the Obj
> > module, in testsuite/tests/misc/hamming.ml. I wonder what other parts
> in the
> > source code we should revise after reassigning tag numbers.
> >
> > (Our finding is that this is not a problem due to the parallel runtime
> system,
> > as we could reproduce the same outcome on the sequential runtime system.)
> >
> > Question 2.
> >
> > On a 64-bit system, can we safely get rid of the page table, in
> particular
> > caml_page_table_lookup(), if the header of each block is redesigned to
> include
> > a field indicating which memory area it resides (old heap, young heap,
> static
> > data, etc)? I wonder what other purposes the page table serves, other
> than
> > determining the memory area for a given pointer.
> >
> > (Perhaps adding a new field in the header would be too impractical on a
> 32-bit
> > system, but on a 64-bit system, we can affort to allocate quite a few
> bits for
> > new fields in the header. Windows might need the page table to check if
> a given
> > pointer points to code, but luckily we don't need to consider Windows
> for our
> > purpose.)
> >
> > Thank you very much!
> >
> > --- Sungwoo Park
>
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