[Vm-dev] Ideas on cheap multi-threading for Squeak / Pharo ? (from Tim's article)
frank.shearar at gmail.com
Tue Jan 31 02:33:21 UTC 2017
On 30 January 2017 at 17:15, Ben Coman <btc at openinworld.com> wrote:
> On Tue, Jan 31, 2017 at 4:19 AM, Clément Bera <bera.clement at gmail.com>
> > Hi all,
> > Tim's just shared this lovely article with a 10,000+ core ARM machine.
> With this kind of machines, it's a bit stupid to use only 1 core when you
> have 10,000+. I believe we have to find a way to introduce multi-threading
> in Squeak / Pharo. For co-processors like the Xeon Phi or the graphic
> cards, I guess it's ok not to use them because their not general purpose
> processors while the VM is general purpose, but all those 10,000 cores...
> > For parallel programming, we could consider doing something cheap like
> the parallel C# loops (Parallel.for and co). The Smalltalk programmer would
> then explicitly write "collection parallelDo: aBlock" instead of
> "collection do: aBlock", and if the block is long enough to execute, the
> cost of parallelisation becomes negligible compared to the performance
> boost of parallelisation. The block has to perform independent tasks, and
> if multiple blocks executed in parallel read/write the same memory
> location, as in C#, the behavior is undefined leading to freezes / crashes.
> It's the responsibility of the programmer to find out if loop iterations
> are independent or not (and it's not obvious).
> > For concurrent programming, there's this design from E where we could
> have an actor model in Smalltalk where each actor is completely independent
> from each other, one native thread per actor, and all the common objects
> (including what's necessary for look-up such as method dictionaries) could
> be shared as long as they're read-only or immutable. Mutating a shared
> object such as installing a method in a method dictionary would be detected
> because such objects are read-only and we can stop all the threads sharing
> such object to mutate it. The programmer has to keep uncommon the mutation
> of shared objects to have good performance.
> > Both design have different goals using multiple cores (parallel and
> concurrent programming), but in both cases we don't need to rewrite any
> library to make Squeak / Pharo multi-threaded like they did in Java.
> > What do you think ?
> > Is there anybody on the mailing list having ideas on how to introduce
> threads in Squeak / Pharo in a cheap way that does not require rewriting
> all core/collection libraries ?
> > I'm not really into multi-threading myself but I believe the Cog VM will
> die in 10 years from now if we don't add something to support
> multi-threading, so I would like to hear suggestions.
> My naive idea is that lots might be simplified by having spawned
> cputhreads use a different bytecode set that enforces a functional
> style of programming by having no write codes. While restrictive, my
> inspiration is that functional languages are supposedly more suited to
> parallelsim by having no shared state. So all algorithms must work on
> the stack only
No: functional languages often share state. It's just that they share
_immutable_ state. Or if you prefer, you can't tell if two threads are
accessing the same data, or merely identical data.
For example, in Erlang, messages bigger than 64kB are shared between
processes on the same machine, because it's much more efficient to share a
To make things slightly more confusing, the rule is more generally
"functions are APPARENTLY pure". In languages like Clojure or F#, it's
quite acceptable to use locally mutable state, as long as no one gets to
see you cheat.
(ML languages are capable of sharing mutable state, it's just that you have
to opt into such things through "ref" or "mutable" markers on things.)
> , which may be simpler to managing multiple updaters to
> objectspace. This may(?) avoid the need to garbage collect the 1000
> cputhreads since everything gets cleared away when the stack dies with
> the thread. On the flip side, might not want to scan these 1000
> cputhreads when garbage collecting the main Image thread. So these
> cputhreads might have a marshaling area that reference counts object
> accesses external to the thread, and the garbage collector only needs
> to scan that area. Or alternatively, each cputhread maintains its own
> objectspace that pulls in copies of objects Spoon style.
> Would each cputhread need its own method cache? Since the application
> may have a massive number of individually short lived calculations, to
> minimise method lookups perhaps a self-contained
> mini-objectspace/method-cache could be seeded/warmed-up by the single
> threaded main image, which is copied to each spawned cputhread with
> parameters passed to the first invoked function.
> Presumably a major use case for these multiple threads would be
> numeric calculations. So perhaps you get enough bang for the buck by
> restricting cputhreads to operate only on immediate types?
> Another idea is for cputhreads to be written in Slang which is
> dynamically compiled and executes as native code, completely avoiding
> the complexity of managing multiple access to objectspace.
> cheers -ben
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