For a lecture I have implemented the write-skew example from Mark
Volkmann's article:
(defn write-skew []
(let [cats (ref 1)
dogs (ref 1)
john (future
(dosync
(when (< (+ @cats @dogs) 3)
(alter cats inc))))
mary (future
(dosync
(when (< (+ @cats @dogs) 3)
(alter dogs inc))))]
(do @john @mary)
(dosync
(> (+ @cats @dogs) 3))))
(defn write-skew-demo []
(let [count-write-skews (atom 0)]
(doseq [_ (range 25)]
(when (write-skew)
(swap! count-write-skews inc)))
@count-write-skews))
(write-skew-demo)
When I try to fix this program using ensure, i.e. using (ensure dogs) in
john and (ensure cats) in mary, I find that it sometimes runs very slow.
>From the docs it says "Allows for more concurrency than (ref-set ref
@ref)". I would read that as "runs at least as fast as (ref-set ref @ref)",
but using (ref-set dogs @dogs) instead of (ensure dogs) and the same with
cats, does not exhibit these occasional runtime spikes.
Am I misunderstanding something or is it a bug in ensure?
Nils
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