[Vm-dev] Cog VM idling performance

[John McIntosh]

I've resisted reponding, but way less busy tonight.

Ok, in general pre-cog as I can't quite comment about how that works.

After most things are killed there is really only the Morphic polling loop
running.
What happens is it attempts about 50 times a second to wake up and service
morphic steps and collect VM events and turn them into mophic events.
That of course takes CPU.

Now when the morphic poll loop goes to sleep by scheduling a variable
length delay to met the 50 times a second, what happens is what Igor
alluded to
some other process has to run otherwise the VM will terminate with no
processes to run.

This process is the lowest level idle process.
What it does is get passed a FAKE millisecond time.
Note in the previous century I attempted to pass how log to sleep as the
Delay logic knows that... But we found out after it was pushed to the
squeak change stream that there was a deadlock... Oops.

Later I realized we know in the VM when the next wakeup time is.

So in the VM when it calls the primitive to do idle time sleep we can grab
the next known wakeup time and sleep until then.
or until some interrupt happens, like a file system, socket, UI input wakes
us up early.
Then we usually on unix run the async file handle logic to deal with
pending semaphores for sockets etc.
That all sounds fairly rational, until you instrument and discover things
like you are asked to sleep, yet a delay a millisecond or two in the past
needs to be serviced...
You'll need to look at the platform support code to understand what
happens, 50 times a second..


I think for COG there is also a high priority millisecond tick process
running to update the millisecond/microsecond clock?  That spins the CPU
dial too.


On Wed, Jun 6, 2012 at 9:37 PM, Phil (list) <pbpublist at gmail.com> wrote:

>
> Igor,
>
> On Jun 6, 2012, at 2:24 PM, Igor Stasenko wrote:
>
> >
> > just kill an idle process.
> > but you must be sure that at any moment of time there will be some
> > scheduled process
> > available, otherwise VM will leave to OS, since it will unable to find
> > anything to run
> >
>
> I'm not sure I understand: I have a morph with stepping enabled (every
> 50ms)... shouldn't that keep the idle process from taking over?  If not
> (i.e. and your suggestion is the way to go), what's the recommended way to
> kill the idle process and how would I restart it when I no longer want
> maximum performance?
>
> The link to the image I posted in my reply to Bert will also display the
> behavior I'm describing: simply moving the mouse around while the stepping
> is active will increase the framerate, when you stop moving the mouse it
> decreases.
>
> > --
> > Best regards,
> > Igor Stasenko.
>
> Thanks,
> Phil




-- 
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John M. McIntosh <johnmci at smalltalkconsulting.com>
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