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Example to control Clojure pmap concurrency level using the chunk size of lazy-seqs.
;; From: https://clojuredocs.org/clojure.core/chunk#example-5c9cebc3e4b0ca44402ef6ec
(defn re-chunk [n xs]
(lazy-seq
(when-let [s (seq (take n xs))]
(let [cb (chunk-buffer n)]
(doseq [x s] (chunk-append cb x))
(chunk-cons (chunk cb) (re-chunk n (drop n xs)))))))
;; Simply wrap the collection or seq/lazy-seq with re-chunk and the configured chunk size will make pmap's concurrency level grow to the size of the chunk.
(time (dorun (pmap (fn[e] (Thread/sleep 100) (inc e)) (re-chunk 100 (range 1000)))))
"Elapsed time: 1038.57 msecs"
@didibus
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didibus commented Jan 5, 2021

In Clojure, pmap has a concurrency level (parallelism level) equal to numberOfCpuCores + 2 or the size of the chunk of the lazy-seq if it is passed a lazy-seq as collection. That means you can take advantage of the fact its concurrency level is the size of lazy-seq by passing it a lazy-seq of a custom chunk size. Lucky for us, there is a convenient function to re-chunk a lazy-seq and thus let us control pmap's parallelization level.

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