[Vm-dev] re: "callback failed to own the VM"

Eliot Miranda eliot.miranda at gmail.com
Fri Oct 9 16:39:12 UTC 2015

On Thu, Oct 8, 2015 at 6:27 PM, Craig Latta <craig at netjam.org> wrote:

> > Well, I threw a (ProcessorScheduler
> > relinquishProcessorForMicroseconds: 100) into one of the successful
> > vm-thread callbacks, and now the foreign-thread callback hangs waiting
> > for the ioWaitOnOSSemaphore() in ownVM(). So I'm not too confident
> > that fixing the bug in cedeToHigherPriorityThreads and/or
> > threadSwitchIfNecessary:from: will get me moving again. But I'll bang
> > on it anyway, I guess.
>      I also notice that, at the point where things just start spinning,
> I've got four threads, three of which (interpreter loop, thread
> scheduling loop, foreign callback thread) are waiting on VM thread
> semaphores. How is the other thread, the heartbeat thread, supposed to
> get things moving?

The only rôle the heartbeat thread has in multithreading is spotting when
the VM is unowned and starting off a thread to take ownership.  e.g. some
long-running callout occurs, which first disowns the VM.  It is the
heartbeat which spots that the VM is unowned and it might be sensible to
start another thread to try and run the VM, hence not slowing down the
callout by having it start a thread on each callout.

What's supposed to stop the threads spinning is proper interaction between
them.  that this isn't happening is a bug.  As I said, this is a prototype
work in progress.

> -C
> --
> Craig Latta
> netjam.org
> +31 6 2757 7177 (SMS ok)
> + 1 415 287 3547 (no SMS)

best, Eliot
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