Re: [MLton] tuning up mlton gc

On 07/25/14 16:51, Matthew Fluet wrote:

> I assume that you have tried using a larger value for 'ram-slop'
> (which defaults to 0.5). That's the obvious place where the runtime
> system has a 50% RAM limitation,

Yes.

> The motivation for that comment was that using 'fixed-heap X' often
> results in creating two 'X / 2' heaps (due to the initial copying GC
> strategy).  Once one of those 'X / 2' sized heaps is filled with live
> data, the runtime tries to switch over to mark-compact GC and resize
> the 'X / 2' sized heap to an 'X' sized heap.  On a 32-bit system,
> though, with 'fixed-heap 3.5G', it is often the case that the virtual
> memory system cannot remap a 1.25G heap to a 3.5G heap.  I don't think
> this is as much of an issue on a 64-bit system, since the 40-bit
> virtual address space is usually significantly larger than RAM (and
> therefore significantly larger than any reasonably 'fixed-heap X') and
> there is usually no difficulty remapping.

Yes. And I realize that I should have been more specific, sorry.

I have no gripe at all on linux actually. But it happens that I more
often use mac or solaris, that do not provide mremap. From heap.c,
the treatment in that case explains the behavior I was referring to.

To explain my reference to your old comment: if the two half heaps
were initially created contiguous, then the gc could switch to
markCompact without remapping or allocating any area, I guess.

> The reason that 'may-page-heap true' is able to help is that it allows
> the runtime system to write the 'X / 2' sized heap to disk, thereby
> making things easier for the virtual memory system to satisfy a larger
> mmap request.
>
Yes, though writing a large heap to the disk and reading it back
takes a lot of time.

Many thanks for your help.
   Bernard.


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