[m-dev.] Mercury and GCC split stacks

[Paul Bone]

On Fri, Sep 05, 2014 at 02:01:37PM +0200, Zoltan Somogyi wrote:
> 
> 
> On Fri, 5 Sep 2014 15:38:41 +1000, Paul Bone <paul at bone.id.au> wrote:
>  Also I like the idea of using something like mprotect to implement
> > stack extension for low level C grades.  I'd prefer this kind of "trap" to
> > detect when stack extension is required rather than a test for each stack
> > frame.
> 
> The tradeoff is that an mmap trap requires, for each stack, a buffer
> that is the size of a page, since the VM hardware on typical machines
> cannot set permissions on any smaller unit of memory than that.
> If you want to have tens or hundreds of thousands of stacks
> (because you have that many threads), like Erlang was designed for,
> this per-stack overhead (in address space, if not in memory) becomes
> excessive.

If it's only in address space then I don't think ti would be excessive on a
64bit system.

If we assume 10,000 threads with an average of 10 stack segments each that's
100,000 stack segments  with one protected page is 400,000KB of memory
(assuming 4KB pages) or 409,600,000 bytes.  It's true that this would be
excessive if these pages were mapped to physical memory.

However the total amount of address space available is:

    2^64 = 18.446 * 10^18.

Of which we want to use 409 * 10^6, or less than 10 * -9 percent.

Many CPUs also support 4MB pages, but these are not used often, but this
makes things a little over 1,000x worse, but that's still only 10*-6
percent of your address space.  I'm not concerned.


-- 
Paul Bone