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 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.
