[Vm-dev] Why is the method context size fixed to two magical numbers?

[Clément Bera]

Hi,

In the interpreter VM, contexts are recycled and you have 2 pools of
contexts, one for each size. Contexts represent 30% of allocations in that
VM.

In the Stack and Cog VM, the size is used to detect stack page overflow and
to allocate contexts, we could use a precise value instead of this bit
flag. Using a single bit saves memory as you need 1 bit per compiled method
instead of 1 byte (or even 1 word !).

Cheers

On Thu, Jul 7, 2016 at 1:02 PM, Max Leske <maxleske at gmail.com> wrote:

>
> Hi,
>
> I’ve been looking at the code that creates new method contexts (in the
> image and in the VM) and I can’t figure out why it would be beneficial to
> fix the size of the context to (currently) 16 (22) or 56 (62) bytes.
> Clément says in one of his blog entries that this if for performance
> reasons but I don’t see where the performance gain is (the Bluebook doesn’t
> mention this either). At allocation time the VM simply takes the number of
> bytes to allocate from the “large context flag”. Maybe the performance gain
> comes from the idea that the size does not have to be calculated? But then
> I ask: why not simply store the frame size instead of this weird flag?
>
> Cheers,
> Max
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