[m-rev.] for review: Enable munmap support in Boehm GC by default.

[Julien Fischer]

Hi Peter,

On Wed, 22 Jul 2020, Peter Wang wrote:

> Memory unmapping support is useful for long running programs to return
> unused memory pages back to the OS, e.g. a server process can reduce the
> amount of physical memory it occupies during idle periods.
>
> Previously the user building the Mercury system needed to enable munmap
> support at configure time. In 2008 I wrote:
>
>    Allow the use of munmap() in Boehm GC, in order to return unused
>    memory pages back to the OS. This has an adverse effect on run times
>    so we want to be able to enable it on a per-application basis.
>
> Bdwgc upstream has more recently enabled munmap support by default on
> the master branch (albeit, not the v7.6.x branch we currently use).
>
> Some testing on my machine suggests that we can try enabling munmap
> support by default:
>
>  - in asm_fast.gc grade, there was no discernable difference to the
>    compiler's performance, with or without munmap support
>
>  - in the hlc.gc grade, the compiler ran 1-2% faster
>    WITH munmap support enabled
>
> Tested on Linux/x86-64, time to compile the 10 largest modules in the
> arena directory.

Have you tested this on platforms other than Linux/x86_64?

> configure:
>    Change default value of enable_gc_mmap and enable_gc_munmap to yes.
>
>    Invert sense of --enable-gc-mmap and --enable-gc-munmap options in
>    the help text.
>
> NEWS:
>    Announce change.

That's fine.  (If further testing reveals that it's not working on some
of the other platforms then we can disable it on those by default.)

Julien.