Transducers are expected to be invoked always in a single thread context. Everything in core maintains that by just running it in a single thread. core.async channels may be invoked from many threads, but only one at a time, and channel itself properly locks to make that state visible. That said, it does seem odd to me now that partition-by and partition-all have state that is not volatile. Thinking about it.
On Wednesday, December 7, 2016 at 11:39:54 AM UTC-6, Léo Noel wrote: > > Hi ! > > I'm a bit confused about the official design rules for stateful > transducers and transducing contexts, especially about which one should be > in charge of memory visibility guarantees. > > The common practice seems to be using volatiles to hold state in > transducers (e.g distinct > <https://github.com/clojure/clojure/blob/c0326d2386dd1227f35f46f1c75a8f87e2e93076/src/clj/clojure/core.clj#L4940>) > > to ensure memory visibility. This usage of volatile is actually the > original motivation > <http://www.google.com/url?q=http%3A%2F%2Fdev.clojure.org%2Fjira%2Fbrowse%2FCLJ-1512&sa=D&sntz=1&usg=AFQjCNH67nkIeg6VMiMKsTloQt5Xjx7OUg> > > for adding it in core, and is explained further here > <http://stackoverflow.com/questions/31288608/what-is-clojure-volatile> > and here <http://insideclojure.org/2014/12/17/distinct-transducer/>. > However, some stateful tranducers use unsynchronized mutable state instead > (e.g partition-all > <https://www.google.com/url?q=https%3A%2F%2Fgithub.com%2Fclojure%2Fclojure%2Fblob%2Fc0326d2386dd1227f35f46f1c75a8f87e2e93076%2Fsrc%2Fclj%2Fclojure%2Fcore.clj%23L7123&sa=D&sntz=1&usg=AFQjCNGKfWMID8XWgWwnghceHaood0NuqQ>), > > which goes against the previous volatile argument of "safely publish values > between threads". > In practice, partition-all works fine in e.g core.async because each call > to the (maybe stateful) step function is made inside the channel lock. > > So my question is, does it make any sense at all to have a transducing > context that does *not* enforce a *happens-before* order between > successive calls to the step function ? > If yes, I would be curious to see an example, and that would mean > partition-all is broken. > If no, what is the point of using volatiles for stateful transducers ? As > long as they are not exposed to the outside, unsynchronized variables would > do the job and should be slightly faster. > > Am I missing something ? > Thanks > > Leo > -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
