Instead of looking at the state as a ref with a vector in it, think of it as a vector of refs. That then allows multiple refs to be modified at once without stepping on other unrelated refs.
On Mon, Jan 30, 2017 at 5:26 PM, Brian Craft <craft.br...@gmail.com> wrote: > I'm experimenting with ref, dosync, and alter to run some code in > parallel, but so far haven't been able to figure out how to structure it > such that it runs faster, instead of slower. > > The problem looks something like this. > > The current state is a long vector. Input is a long sequence of values. At > each step some positions in the vector are populated, if empty, based on > computations over the next value in the sequence. > > It's like placing puzzle pieces: search for positions that satisfy a > constraint, and fill them. > > Using threads, I'd like to place the next several values in parallel. But > running in parallel it's possible that two threads would find solutions > that conflict. In this case, the latter one must continue searching. > > Is there a way to express this with clojure? The problem is that I can't > tell 'dosync' when a previous transaction invalidates the current > transaction: it just always retries. > > e.g. if each thread is doing > > > (dosync > (let [positions (find-positions next-value @state)] > (alter state (fn [s] (update-state s positions)))))) > > they interfere with each other constantly, because any write to 'state' > causes the other threads to retry, even if their positions are still free. > > One really wants to reverse the order of find-positions and dosync here, > so it computes positions, then tries to commit them, checking in the > transaction that the positions are still free, and continuing the search if > they're not. > > I suppose there's some solution involving exceptions. Is there a better > way to think about this? > > -- > 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. > -- “One of the main causes of the fall of the Roman Empire was that–lacking zero–they had no way to indicate successful termination of their C programs.” (Robert Firth) -- 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.
