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

[Sungwoo Park <gla@postech.ac.kr>]

Thank you for letting me know about your project at OCamlLabs. Our source code
is available at:
     https://github.com/qkrgud55/ocamlmulti.git
It is used only internally by our group and is not ready for release yet. So,
it might be hard to follow all the changes.

If the page table has no other purpose, it is a good news for us. Thanks a lot
for the good news :-)

Sincerely,

--- Sungwoo Park

On Mon, 30 Sep 2013, Leo White 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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