How to improve text processing performance in Clojure?

Question

I'm writing a simple desktop search engine in Clojure as a way to learn more about the language. Until now, the performance during the text processing phase of my program is really bad.

During the text processing I've to:

• Clean up unwanted characters;
• Convert the string to lowercase;
• Split the document to get a list of words;
• Build a map which associates each word to its occurrences in the document.

Here is the code:

(ns txt-processing.core
  (:require [clojure.java.io :as cjio])
  (:require [clojure.string :as cjstr])
  (:gen-class))

(defn all-files [path]
  (let [entries (file-seq (cjio/file path))]
    (filter (memfn isFile) entries)))

(def char-val
  (let [value #(Character/getNumericValue %)]
    {:a (value \a) :z (value \z)
     :A (value \A) :Z (value \Z)
     :0 (value \0) :9 (value \9)}))

(defn is-ascii-alpha-num [c]
  (let [n (Character/getNumericValue c)]
    (or (and (>= n (char-val :a)) (<= n (char-val :z)))
        (and (>= n (char-val :A)) (<= n (char-val :Z)))
        (and (>= n (char-val :0)) (<= n (char-val :9))))))

(defn is-valid [c]
    (or (is-ascii-alpha-num c)
        (Character/isSpaceChar c)
        (.equals (str \newline) (str c))))

(defn lower-and-replace [c]
  (if (.equals (str \newline) (str c)) \space (Character/toLowerCase c)))

(defn tokenize [content]
  (let [filtered (filter is-valid content)
        lowered (map lower-and-replace filtered)]
    (cjstr/split (apply str lowered) #"\s+")))

(defn process-content [content]
  (let [words (tokenize content)]
    (loop [ws words i 0 hmap (hash-map)]
      (if (empty? ws)
        hmap
        (recur (rest ws) (+ i 1) (update-in hmap [(first ws)] #(conj % i)))))))

(defn -main [& args]
  (doseq [file (all-files (first args))]
    (let [content (slurp file)
          oc-list (process-content content)]
      (println "File:" (.getPath file)
               "| Words to be indexed:" (count oc-list )))))

As I have another implementation of this problem in Haskell, I compared both as you can see in the following outputs.

Clojure version:

$ lein uberjar
Compiling txt-processing.core
Created /home/luisgabriel/projects/txt-processing/clojure/target/txt-processing-0.1.0-SNAPSHOT.jar
Including txt-processing-0.1.0-SNAPSHOT.jar
Including clojure-1.5.1.jar
Created /home/luisgabriel/projects/txt-processing/clojure/target/txt-processing-0.1.0-SNAPSHOT-standalone.jar
$ time java -jar target/txt-processing-0.1.0-SNAPSHOT-standalone.jar ../data
File: ../data/The.Rat.Racket.by.David.Henry.Keller.txt | Words to be indexed: 2033
File: ../data/Beyond.Pandora.by.Robert.J.Martin.txt | Words to be indexed: 1028
File: ../data/Bat.Wing.by.Sax.Rohmer.txt | Words to be indexed: 7562
File: ../data/Operation.Outer.Space.by.Murray.Leinster.txt | Words to be indexed: 7754
File: ../data/The.Reign.of.Mary.Tudor.by.James.Anthony.Froude.txt | Words to be indexed: 15418
File: ../data/.directory | Words to be indexed: 3
File: ../data/Home.Life.in.Colonial.Days.by.Alice.Morse.Earle.txt | Words to be indexed: 12191
File: ../data/The.Dark.Door.by.Alan.Edward.Nourse.txt | Words to be indexed: 2378
File: ../data/Storm.Over.Warlock.by.Andre.Norton.txt | Words to be indexed: 7451
File: ../data/A.Brief.History.of.the.United.States.by.John.Bach.McMaster.txt | Words to be indexed: 11049
File: ../data/The.Jesuits.in.North.America.in.the.Seventeenth.Century.by.Francis.Parkman.txt | Words to be indexed: 14721
File: ../data/Queen.Victoria.by.Lytton.Strachey.txt | Words to be indexed: 10494
File: ../data/Crime.and.Punishment.by.Fyodor.Dostoyevsky.txt | Words to be indexed: 10642

real    2m2.164s
user    2m3.868s
sys     0m0.978s

Haskell version:

$ ghc -rtsopts --make txt-processing.hs 
[1 of 1] Compiling Main             ( txt-processing.hs, txt-processing.o )
Linking txt-processing ...
$ time ./txt-processing ../data/ +RTS -K12m
File: ../data/The.Rat.Racket.by.David.Henry.Keller.txt | Words to be indexed: 2033
File: ../data/Beyond.Pandora.by.Robert.J.Martin.txt | Words to be indexed: 1028
File: ../data/Bat.Wing.by.Sax.Rohmer.txt | Words to be indexed: 7562
File: ../data/Operation.Outer.Space.by.Murray.Leinster.txt | Words to be indexed: 7754
File: ../data/The.Reign.of.Mary.Tudor.by.James.Anthony.Froude.txt | Words to be indexed: 15418
File: ../data/.directory | Words to be indexed: 3
File: ../data/Home.Life.in.Colonial.Days.by.Alice.Morse.Earle.txt | Words to be indexed: 12191
File: ../data/The.Dark.Door.by.Alan.Edward.Nourse.txt | Words to be indexed: 2378
File: ../data/Storm.Over.Warlock.by.Andre.Norton.txt | Words to be indexed: 7451
File: ../data/A.Brief.History.of.the.United.States.by.John.Bach.McMaster.txt | Words to be indexed: 11049
File: ../data/The.Jesuits.in.North.America.in.the.Seventeenth.Century.by.Francis.Parkman.txt | Words to be indexed: 14721
File: ../data/Queen.Victoria.by.Lytton.Strachey.txt | Words to be indexed: 10494
File: ../data/Crime.and.Punishment.by.Fyodor.Dostoyevsky.txt | Words to be indexed: 10642

real    0m9.086s
user    0m8.591s
sys     0m0.463s

I think the (string -> lazy sequence) conversion in the Clojure implementation is killing the performance. How can I improve it?

P.S: All the code and data used in these tests can be downloaded here.

Answer 1

Some things you could do that would probably speed this code up:

1) Instead of mapping your chars to char-val, just do direct value comparisons between the characters. This is faster for the same reason it would faster in Java.

2) You repeatedly use str to convert single-character values to full-fledged strings. Again, consider using the character values directly. Again, object creation is slow, same as in Java.

3) You should replace process-content with clojure.core/frequencies. Perhaps inspect frequencies source to see how it is faster.

4) If you must update a (hash-map) in a loop, use transient. See: http://clojuredocs.org/clojure_core/clojure.core/transient

Also note that (hash-map) returns a PersistentArrayMap, so you are creating new instances with each call to update-in - hence slow and why you should use transients.

5) This is your friend: (set! *warn-on-reflection* true) - You have quite a bit of reflection that could benefit from type hints

 Reflection warning, scratch.clj:10:13 - call to isFile can't be resolved.
 Reflection warning, scratch.clj:13:16 - call to getNumericValue can't be resolved.
 Reflection warning, scratch.clj:19:11 - call to getNumericValue can't be resolved.
 Reflection warning, scratch.clj:26:9 - call to isSpaceChar can't be resolved.
 Reflection warning, scratch.clj:30:47 - call to toLowerCase can't be resolved.
 Reflection warning, scratch.clj:48:24 - reference to field getPath can't be resolved.
 Reflection warning, scratch.clj:48:24 - reference to field getPath can't be resolved.