[GAP] Effect of using malloc for temporary allocations in GAP

Max Horn Max.Horn at math.uni-giessen.de
Mon Jan 2 19:59:46 GMT 2017

> On 02 Jan 2017, at 19:31, Dmitrii Pasechnik <Dmitrii.Pasechnik at cs.ox.ac.uk> wrote:
> Hi Max,
> On Mon, Jan 02, 2017 at 06:24:15PM +0100, Max Horn wrote:
>>> On 02 Jan 2017, at 17:18, Dmitrii Pasechnik <Dmitrii.Pasechnik at cs.ox.ac.uk> wrote:
>>> On Mon, Jan 02, 2017 at 03:07:34PM +0100, Max Horn wrote:
>>>>> On 02 Jan 2017, at 15:01, Dmitrii Pasechnik <Dmitrii.Pasechnik at cs.ox.ac.uk> wrote:
>>>>> I would like to point out that GMP allows one to specify her/his own
>>>>> memory allocator: https://gmplib.org/manual/Custom-Allocation.html
>>>>> (I'm surprised that GAP does not use it, or does it?)
>>>> GAP cannot use these hooks, because the GAP GC is a moving GC.
>>> Well, these hooks allow one to bundle on a custom malloc with address space
>>> protected from GC.
>> The issue is not protecting the address space from our GC. It is more about
>> the reverse, i.e. "protecting" our GC from an allocation in a place that
>> prevents the GASMAN heap from growing.
> Why does GASMAN use the heap (managed by (s)brk/malloc) as opposed to a separate virtual memory
> region which one can obtain using mmap?
> Or am I missing something?

GASMAN already uses virtual memory APIs were available (I used "heap" in the
general sense, i.e. "dynamically allocated, not on the stack"). But this
does not solve the problem, it just shifts it -- the problem is that GASMAN
needs a contiguous memory region right now, and a single "foreign" page
allocation can prevent GASMAN from extending its memory. On 64bit systems,
pre-reserving space works pretty well, as the virtual address space is much
larger than the real memory (though even then you could be "unlucky", it's
just not very probably). But on 32bit, the fact that the address space is
limited to 4 GB makes this trick much less useful. See also the mails by me
and Chris in this thread.

In any case, this all is completely besides the point of my original question.

Prof. Dr. Max Horn
AG Algebra
Mathematisches Institut
Justus-Liebig-Universität Gießen
Arndtstraße 2
D-35392 Gießen
Tel: (+49) 641 99-32044
Fax: (+49) 641 99-32049
E-Mail: max.horn at math.uni-giessen.de

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