Transactions themselves are not queued (besides the obvious queueing of threads when you have more threads than cores). What gets adaptively queued is the historical values of refs involved in transactions.
So if you have three concurrent transactions running, and three refs are involved in two of these transactions, and a fourth refs in involved in all three, the system will probably need to keep two previous values for the first three refs and three previous values for the last ref. Code within a transaction should not have any other visible effect than updating refs. When a transaction reaches its end, the system checks the (current) value of all involved refs and retries if any has changed. On 26 October 2014 21:03, Leon Grapenthin <grapenthinl...@gmail.com> wrote: > > > On Sunday, October 26, 2014 11:37:43 AM UTC+1, Øyvind Teig wrote: >> >> I have been reading myself up on Clojure concurrency, starting at [1]. In >> [2] I read that >> >>> The Clojure STM uses multiversion concurrency control with adaptive >>> history queues for snapshot isolation, and provides a distinct commute >>> operation >> >> >> but it still needs some explanation to me. Software transactional memory >> (STM) is described ok. >> >> But the "adaptive history queues" looks like implementation. In ADA, tasks >> on rendezvous queue up (so the Ravenscar high security profile prohibits >> them because they are non-deterministic). In Go goroutines on selective >> choice's select are queued (also nondeterministic). In occam the there is no >> such queue on the ALT. How is the Clojure queue, and what does it contain? >> >> I see that exchanging data between Clojure concurrent functions (threads?) >> described. But I haven't discovered how a "thread" that succeeds in writing, >> followed by a successful write immediately after by another thread - is >> "signalled" to the first thread? Is there any wait type of mechanism? > > > There is no signalling. The "write" in the first thread always succeeds > within the transaction but the "real world" is not affected until the > transaction succeeds as a whole. If during the transaction another > transaction succeeded, it is retried as a whole, potentially with the new > values written by the other transaction, until it has also succeeded. > >> >> >> If a write fails and there is a retry, what's the limit of retries, is >> there a yield in between, and in case, how is this entry queued? > > > There is no limit. (Unfortunately I don't know about the implementation > details and hopefully somebody else can enlighten you on that.) > >> >> >> Is it correct to say that never to block means never to be 100% sure that >> a transaction succeeds at first attempt? > > Yes. >> >> >> [3] states on commute that "Thus fun should be commutative, or, failing >> that, you must accept last-one-in-wins behavior. commute allows for more >> concurrency than ref-set.". What's the metric of concurrency? No, little, >> some, more, much, max? > > > How much happens at the same time in contrast to being queued up. Remember > that only successful transactions affect the outside world. E. g.: Of two > successful transactions, one had to be restarted because of the other > completed during its first try => The two successful transactions didn't > happen in parallel. Using commute reduces the potential of a retry and thus > allow more successful transactions in parallel => more concurrency. > >> >> >> I would have more questions, but they will come. >> >> I plan to blog about these matters at [4]. It's a do-for-fun only blog, >> with no money or ads involved. I try to read myself up via atomic to several >> other concurrency models than CSP, which is flowing in my veins the last 25 >> years. >> >> Øyvind Teig, Trondheim, Norway >> >> [1] - http://clojure.org/concurrent_programming >> [2] - http://clojure.org/Refs >> [3] - >> http://clojure.github.io/clojure/clojure.core-api.html#clojure.core/commute >> [4] - http://www.teigfam.net/oyvind/home/technology/ > > -- > 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. -- You received this message because you are subscribed to the Google Groups "Clojure" group. 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