On Sun, Nov 14, 2010 at 2:16 PM, Eric Kobrin <erl...@gmail.com> wrote: > In the API it is suggested to use `seq` to check if coll is empty. > > I was working on some code recently found that my biggest performance > bottleneck was calling `seq` to check for emptiness. The calls to > `seq` were causing lots of object allocation and taking noticeable CPU > time. I switched to using `identical?` to explicitly compare against > the empty vector and was rewarded with a drastic reduction in > execution time.
Only there's a wee problem: user=> (identical? [] (pop [3])) false Not all empty vectors are considered identical. = works: user=> (= [] (pop [3])) true = is also ten times SLOWER than seq if the empty vectors are not identical, though it's as fast as identical? when they are: user=> (let [iterations 100000000] (time (dotimes [_ iterations] (= [] (pop [3])))) (time (dotimes [_ iterations] (seq [])))) "Elapsed time: 29994.93852 msecs" "Elapsed time: 2745.21924 msecs" This is troubling. The seq function should be faster than this, and = on nonidentical empty colls should be WAY faster than this. On the other hand, the identical? test taking <4ms for 100,000,000 iterations suggests it was optimized away to nothing by the JIT compiler. Unless of course both of us have 100GHz CPUs that can execute a fresh instruction every 10 picoseconds. :) -- 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
