[Vm-dev] Switching an active context in Cog

Eliot Miranda eliot.miranda at gmail.com
Mon Aug 16 21:08:00 UTC 2010

On Mon, Aug 16, 2010 at 1:05 PM, Igor Stasenko <siguctua at gmail.com> wrote:

> On 16 August 2010 21:26, Eliot Miranda <eliot.miranda at gmail.com> wrote:
> >
> > Hi Igor,
> >
> > On Mon, Aug 16, 2010 at 9:42 AM, Igor Stasenko <siguctua at gmail.com>
> wrote:
> >>
> >> Hello,
> >>
> >> i want to adopt an image-side scheduling for Cog and since
> >> everything which sounds like 'Process' will be wiped out from VM,
> >> i need a way to cleanly switch the active context to a new one.
> >>
> >> I think that as a guide for implementation could be taken
> >> CoInterpreter>>transferTo: newProc from: sourceCode
> >>
> >> but i am a bit confused by following:
> >>
> >> self push: instructionPointer.
> >> ...(switching code)...
> >> instructionPointer := self popStack.
> >>
> >>
> >> does that means, that instruction pointer is always stored in last
> >> stack position
> >> instead of some fixed slot of context?
> >
> > You need to internalize
> http://www.mirandabanda.org/cogblog/2009/01/14/under-cover-contexts-and-the-big-frame-up/.
> Cog /does not/ use contexts for managing execution.  It uses stack frames,
> and in an inactive stack frame on top of stack the instructionPointer is at
> top of stack (*).  So I suggest you read the above blog entry at least three
> times until it all starts to make sense.  Then you'll have to understand how
> the CoInterpreter complicates things by having two kinds of frame, one for
> interpreted methods and one for JIT-ed native methods, see CoInterpreter
> class>>initializeFrameIndices, and understand that an interpreter frame
> needs two instruction pointers, the normal one which is a return address for
> native code that jumps into the interpreter (see ceReturnToInterpreter:),
> and the one in the interpreted frame's iframeSavedIP slot which holds the
> bytecode pc (at which the interpreter should start executing).
> >
> I am actually need few things to understand: how much state should be
> preserved/swapped and in what order.
> I seen you post and even tried to read it befor, but reading it
> without using in practice is almost waste of time, because
> i best work using trial & fail process, instead of reading books and
> following rules :) That's how my mindset works,
> and that's why i liked smalltalk so much, when i first seen it,
> because it makes trial & fail process extremely easy and
> enjoyful.

<blush>me too</blush>

> But thanks for reminder, i'll use your blog post as a reference :)
> > (*) of course Cog does use contexts, but they are used as little as
> possible.  When you say "thisCOntext" a context gets created for a frame,
> and when teh stack zone fills up, stack pages are freed by copying all
> frames on a page to the heap as contexts, and when a snapshot is created all
> stack pages are freed, converting all frames to contexts.  So from the image
> this optimization is completely invisible, apparently all method activations
> are contexts, but the VM creates context objects only when necessary or
> convenient, and certainly /not/ on every send.
> Yes, i am aware that contexts is used as less as possible. But from
> language perspective, a context is still a smallest possible
> unit of computation, which you can safely operate with. So, it is
> logical to have a primitive which switching an active context,
> instead of active process(es).
> Also, same thing is userful for callbacks, by taking a simple block
> closure, which should handle callback
> and turning it into context, which then can be activated, once
> callback is called.

Right, so you implement that primitive, but in Cog it must be implemented
like transferTo:from:.  i.e. in the frame you're leaving you must push the
instruction pointer and ensure the frame has a context (since the context
will persist, not necessarily the frame).  The context you're transferring
to needs to have a frame before one can continue, so the primitive uses
makeBaseFrameFor: if the context is not already married to a frame.


> > HTH,
> > Eliot
> >>
> >> --
> >> Best regards,
> >> Igor Stasenko AKA sig.
> >
> >
> >
> --
> Best regards,
> Igor Stasenko AKA sig.
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