[Vm-dev] Multi-core system Squeak (was Re: Sista alternate bytecodes + Java Bytecode ?)

Ben Coman btc at openinworld.com
Fri Oct 20 06:39:51 UTC 2017

On Fri, Oct 20, 2017 at 9:05 AM, David T. Lewis <lewis at mail.msen.com> wrote:

> On Fri, Oct 20, 2017 at 06:40:47AM +0800, Ben Coman wrote:
> >
> > On Fri, Oct 20, 2017 at 12:46 AM, tim Rowledge <tim at rowledge.org> wrote:
> >
> > >
> > >
> > > > On 19-10-2017, at 7:31 AM, Todd Blanchard <tblanchard at mac.com>
> wrote:
> > > >
> > > > It works very well and this idea of mini worker images sounds like it
> > > might be a good fit for that model.
> > > >
> > >
> > > One thing I noticed with surprise was how astonishingly fast Dave???s
> > > OSProcess could spawn an entire image, even on a Pi. So it might be
> worth
> > > experimenting with spawning a child image to do some processing and
> maybe
> > > transmit results back via sockets or writing a project file or.. well,
> > > whatever. You might think that on a tiny thing like a Pi you would
> rapidly
> > > use up all memory and start disastrous paging to SD (ouch!) but a Pi
> has a
> > > Gb or ram and my 6.0alpha development image uses less than 9% of that.
> > >
> > > It???s a potentially simple way to make some use of multiple cores.
> > >
> >
> > Are talking about booting up a new image?
> > What about native-forking a running image.  Linux default copy-on-write
> > (IIUC) should help limit memory usage, with full access to existing
> objects
> > without needing to worry about multi-threaded access to objectspace.
> > The trick to work out would the join mechanism. Perhaps returned objects
> > would be limited to instance variables containing only immediate types
> and
> > plain arrays of the same.  Maybe the join mechanism would need to groom
> out
> > non-compliant objects at the VM level.  Or the join returns a STON
> > representation.
> >
> > cheers -ben
> Yes that is exactly what Tim is talking about. The unix fork mechanism is
> suprisingly efficient when applied to a VM process running a Smalltalk
> image.
> Fuel works nicely for returning objects to the parent image,

I forgot Fuel. I guess its binary nature would make it faster.

> although I'm sure STON would work fine also.
> The Spur image format allows object memory to be segmented such that it
> will
> support fast and efficient forking of images even for extremely large
> images.

Would it be efficient enough to benefit forking off the display rendering
half of the world loop?
One thing I've seen in the past with Pharo with attempts to multi-thread UI
is that #changed: is unwittingly called somewhere deep in morph code away
from the level the components are put together,
and the rendering code is written assuming its the only thread,
particularly with String producing race conditions like...
   bufferCopy := Array new: buffer length.
   1 to: buffer length do: [ :i |
         bufferCopy at: i put: (buffer at: i) ].
which goes bang when another thread causes buffer
gets longer between lines 1 & 2,
or gets shorter between lines 2 & 3.

And I sometimes wonder if the rendering occurred in a fork, then its copy
of buffer would be fixed
and the race conditions could not occur.  The rendering-fork normally
wouldn't even need to hand anything back in a join. It would just update
the display and die quietly.
How to keep debugging integrated would be interesting.  Maybe Fuel helps
there anyhow.

cheers -ben

> I don't have any real world use cases to prove the point, but would be
> quite
> interested in finding some.
> The join mechanism can be very simple. The child image serializes the
> result
> of whatever work it has done, and writes it to a pipe connected to the
> parent
> image. The parent deserializes the next object from the pipe, and that is
> the
> result. As long as the pipe read does not block the VM (that is an
> OSProcess
> thing), you can put any number of child readers into separate Smalltalk
> processes.
> Background information is at http://wiki.squeak.org/squeak/6176.
> Dave
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