[Python-Dev] Proposed PEP on concurrent programming support

PJ Eby pje at telecommunity.com
Fri Jan 13 05:19:29 CET 2012

On Wed, Jan 11, 2012 at 7:01 PM, Mike Meyer <mwm at mired.org> wrote:

> On Wed, 4 Jan 2012 00:07:27 -0500
> PJ Eby <pje at telecommunity.com> wrote:
>  > On Tue, Jan 3, 2012 at 7:40 PM, Mike Meyer <mwm at mired.org> wrote:
> > > For
> > > instance, combining STM with explicit locking would allow explicit
> > > locking when IO was required,
> > I don't think this idea makes any sense, since STM's don't really
> > "lock", and to control I/O in an STM system you just STM-ize the
> > queues. (Generally speaking.)
> I thought about that. I couldn't convince myself that STM by itself
> sufficient. If you need to make irreversible changes to the state of
> an object, you can't use STM, so what do you use? Can every such
> situation be handled by creating "safe" values then using an STM to
> update them?

If you need to do something irreversible, you just need to use an
STM-controlled queue, with something that reads from it to do the
irreversible things.  The catch is that your queue design has to support
guaranteed-successful item removal, since if the dequeue transaction fails,
it's too late.  Alternately, the queue reader can commit removal first,
then perform the irreversible operation...  but leave open a short window
for failure.  It depends on the precise semantics you're looking for.

In either case, though, the STM is pretty much sufficient, given a good
enough queue data structure.
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