Re: Threads

[Neel Krishnaswami <neelk@xxxxxxxxxxxx>]

First off, GOO is very sweet. I like it. 

jrb wrote:
>
> we have discussed the possibility.  i think the general thought is to
> not support native threads but to instead support threads built out of
> upward continuations (i.e., cooperative threads).  full parallelism
> would be built instead out of more insular processes.  the problem of
> making user programs thread safe and the overhead of system thread
> safety are thought to be too high a price to pay for parallelism.

What exactly is the price? To me, it looks like the main difficulties
are:

  o Can updating the subtype table be made thread-safe?
  o Can adding methods to a gf be made thread-safe?

Even with full parallelism, you can make each of these thread-safe at
the cost of an extra indirection at each gf dispatch and each subtype
test. You can store a pointer to a pointer to the relevant data
structures, and then when it's necessary to update them you can make a
modified copy, and then make the update with an atomic compare-and-
swap. The CAS would need a tiny bit of per-architecture assembly, but
fortunately the CILK project (http://supertech.lcs.mit.edu/cilk/) has
stealable code to do that for x86, IA64, SPARC, Alpha, PowerPC and
MIPS. I dunno how much of a hit the extra indirection would inflict,
though I'd guess it's fairly modest.

On a related note, the C-- people have written a paper that identifies
a continuation discipline that enables you to write exceptions and
coroutines/threads, but is compatible with a stack implementations. If
you allow continuations with a dynamic-extent (like Dylan/GOO
continuations), but permit them to be called multiple times (unlike
GOO), then you can write coroutines but can still use a stack. See:

  Featherweight Concurrency in a Portable Assembly Language. Ramsey and
  Peyton-Jones. http://citeseer.nj.nec.com/ramsey01featherweight.html

I guess you could implement this by compiling into CPS and then
allocating continuations using alloca() and occasionally cleaning up.

-- 
Neel Krishnaswami
neelk@alum.mit.edu