Thanks everyone for their answers and extra efforts. They are really helpful.
On Mon, Apr 27, 2015, 18:56 Alex Miller <a...@puredanger.com> wrote: > I fleshed out some of this a bit more in a blog post with perf numbers in > case anyone's interested: > > http://insideclojure.org/2015/04/27/poly-perf/ > > > On Saturday, April 25, 2015 at 9:39:06 PM UTC-5, Alex Miller wrote: > >> On Saturday, April 25, 2015 at 9:33:18 AM UTC-5, Timur wrote: >>> >>> Hi everyone, >>> >>> There are situations where I want to dispatch functions using based on >>> their certain properties. I can also use case statements instead but it >>> looks more coupled and more change is required if I want to add new types. >>> >> >> case is useful for the particular situation where you have a finite set >> of compile-time constants that you are looking for (integers, characters, >> etc). Then case will leverage a special JVM bytecode that performs a >> *constant-time* look-up (not linear-time like checking a series of cond >> conditions, etc). This is one of the two bytecode options (tableswitch, >> rather than lookupswitch) underlying the Java switch/case feature. >> >> What I want to ask is if I need to avoid using multi-methods for >>> performance reasons? I read somewhere that they are not really fast but the >>> posts were old and the performance might have been improved in between. >>> Should I favor case and cond branches instead of defmulti when I need >>> performance? >>> >> >> multimethods are slower than protocols - they must effectively invoke the >> dispatch function, perform a linear search for a match across the cases, >> and then invoke the actual implementation function. Protocols leverage JVM >> internals to select the right implementation, then invoke it. However, the >> search part of multimethods is cached, making that step fast after the >> initial call. The last time I benchmarked multimethods with class dispatch >> vs protocols on Java 1.8 + Clojure 1.7 alphas (can't remember which one), I >> found multimethods were about 5x slower. >> >> If you want fastest type-based dispatch with open extension, then >> protocols are the best. >> If you want any other type of dispatch with open extension, then >> multimethods are the best. >> If you want a closed system of constant choices, then case is best >> (constant-time lookup!) >> If you want a closed system of arbitrary conditions, then cond is best. >> >> I have never compared the performance of cond vs multimethods for >> something like this. My guess is that multimethods are faster as they >> evaluate a single condition, then do a table lookup once cached vs >> evaluating a series of conditions every time. >> >> Alex >> >> -- > 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 a topic in the > Google Groups "Clojure" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/clojure/7oROqb6dGSU/unsubscribe. > To unsubscribe from this group and all its topics, 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. 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.
