Using the Benchmark Ruby Standard Library module

Using the Benchmark Ruby Standard Library module
by Darren Jensen | January 2015


When an application grows in size it’s common that it may slow down considerably in the face of heavy usage. Rather than looking at the larger picture its often best to take a microscope and look closely at the performance of the code, method by method, line by line. One tool that the Ruby Standard Library provides to assist with this is Benchmark. Lets take a quick dive into the features of the Ruby Benchmarking tool!

Contrived Example: Compare Array Sort with Hash Sort

I say this is contrived becuase we are going to compare two methods that are essentially quite different in their implemntation. However the main point of this exercise is to get up and running with the Benchmark tool. Probably most importantly to note, is that it’s not possible to directly sort a Hash and therefore Hash#sort_by returns a sorted Array of Arrays. Anyway, we expect to see Hash#sort_by on Hash to be slower than Array#sort.

What we will do in this example is create an Array of 10,000 random characters and sort that. Then we will create a Hash with the values of the Hash set the to exactly the same random characters. The reason we set the values to the random characters is becuase Hash keys must be unique and we are only using the ASCII character set and therefore we only have 255 characters to play with.

Below is the code to create our Array and Hash object:

10_000.times.with_index do |i|

  ascii_code = rand(255)

  random_char = ascii_code.chr

  arr << random_char

  hash[i] = random_char


Now we have an Array size 10,000 (all ASCII chars, randomly inserted) and we have a Hash size 10,000 (with values of the same randomly generated char).

Below is a sneak peak at the new array object containing the random characters:

# puts arr.take(10)

["A", "%", "\xC5", "\xE1", "G", "\xF4", "\x16", "\x81", "\x03", "+", "\xA1"]

# puts hash.first(10).to_h

{0=>"A", 1=>"%", 2=>"\xC5", 3=>"\xE1", 4=>"G", 5=>"\xF4", 6=>"\x16", 7=>"\x81", 8=>"\x03", 9=>"+"}

Note that the characters, altough random, are in the same order in both the Array and the Hash.

Setting up our Benchmark harness

In order to use the Benchmark class we need to call the bm method on it passing in a block of code that we want to report on. We can run several reports in one iteration of a benchark.

As shown below we pass into the bm method a ‘tabbed output formatting’ value of 7, followed by setting up two reports with titles ‘Sort Array’ and ‘Sort Hash’. The titles allow the developer to easily read the result once the benchmark has run. do |x| 'Sort Array' do


  end 'Sort Hash' do

    hash.sort_by{|_key, value| value}



The result is as expected that the Hash#sort_by method call is far slower than the Array#sort method call. This is due to the implentation of the Hash#sort_by being different to the Array#sort method.

The output from running this Benchmark on my machine is as follows:

                 user     system      total        real

Sort Array   0.010000   0.000000   0.010000 (  0.001907)

Sort Hash    0.000000   0.000000   0.000000 (  0.007687)


In a real application, its unlikely that developers will be testing the Ruby API directly like this. Instead developers may wish to test how their code performs before after refactoring. For Rails applications, it’s commen to use NewRelic as a tool to monitor performance in Production. NewRelic can help developers quickly identify areas of the codebase that are slow to run. In development the developer can isolate and performance test this code using the Ruby Benchmark tool.

As always, here is a Gist for this Benchmark

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