[Vm-dev] Spur Memory segment and OS allocation

[Clément Bera]

Eliot, any idea if this is correct? Does the VM rely on segments being on
higher addresses*only* to guarantee that young objects are on lower
addresses than old objects for the write barrier or is there something else?

On Sat, Apr 28, 2018 at 10:57 AM, Clément Bera <bera.clement at gmail.com>
wrote:

> Hi Eliot, Hi all,
>
> On mac and linux, Spur uses mmap to allocate new segments. The V3 memory
> manager used malloc instead. I've looked into many other VMs (Javascript
> and Java), and most of them use posix_memalign (basically malloc where
> you can ask for specific alignment).
>
> I am wondering why we are using mmap over posix_memalign / malloc. The
> only reason I can find is that Spur always allocate new memory segments at
> a higher address than past segments to guarantee that young objects are on
> lower addresses than old objects for the write barrier. Is that correct?
>
> Assuming it is correct, let's say I change Spur to implement the write
> barrier differently (typically, I change all objects to be aligned on 128
> bits instead of 64 and have different allocation alignment for young (128
> bits alignment) and old objects(128+64 bits alignment)). Will we be able to
> use posix_memalign / malloc to allocate new memory segment if I do that ?
> Or does the VM rely on segments being on higher addresses for other
> reasons ? For example, does the VM assume CogMethods are on lower addresses
> than objects on heap and rely on it to check if a stack frame is mframe or
> iframe ?
>
> Thanks,
>
> --
> Clément Béra
> https://clementbera.github.io/
> https://clementbera.wordpress.com/
>



-- 
Clément Béra
https://clementbera.github.io/
https://clementbera.wordpress.com/
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