Strings vs Symbols ruby hash

irb

irb(main):029:0> Benchmark.bm do |x|
irb(main):030:1* x.report("Strings: ") { 10000000.times {"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa".hash} }
irb(main):031:1> x.report("Symbols: ") { 10000000.times {:aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.hash} }
irb(main):032:1> end
       user     system      total        real
Strings:   1.290000   0.000000   1.290000 (  1.282911)
Symbols:   0.450000   0.000000   0.450000 (  0.458915)

string_var = "foo"
symb_var = :foo
Benchmark.bm do |x|
  x.report("Strings: ") { 10_000_000.times {"foo".hash} }
  x.report("Symbols: ") { 10_000_000.times {:foo.hash} }
  x.report("Strings var: ") { 10_000_000.times {string_var.hash} }
  x.report("Symbols var: ") { 10_000_000.times {symb_var.hash} }
end

#       user     system      total        real
#Strings:   1.300000   0.000000   1.300000 (  1.297051)
#Symbols:   0.510000   0.000000   0.510000 (  0.510538)
#Strings var:   0.860000   0.000000   0.860000 (  0.861275)
#Symbols var:   0.540000   0.000000   0.540000 (  0.541402)

The benchmarks above account for allocating the Strings (except the var variants in the comment above).

But if we use different String lengths we see String#hash is O(n) but Symbol#hash O(1) and so we can get arbitrary speedups.
Of course, this is no magic, the time we gain on Symbol#hash is the time we pay when creating those Symbols and interning them (i.e. those (14-4) seconds before the benchmark starts in the length=10 000 run).

require 'benchmark'

len = Integer(ARGV[0])
n = 1_000_000
base = "a" * len
STRINGS = Array.new(n) { |i| base + n.to_s }
SYMBOLS = STRINGS.map(&:to_sym)

Benchmark.bm do |x|
  x.report("Symbol#hash") do
    SYMBOLS.each { |s| s.hash }
  end
  x.report("String#hash") do
    STRINGS.each { |s| s.hash }
  end
end

$ ruby -v string_symbol_hash.rb 10
ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-linux]
       user     system      total        real
Symbol#hash  0.040000   0.000000   0.040000 (  0.040390)
String#hash  0.060000   0.000000   0.060000 (  0.061827)
0.49s user 0.02s system 99% cpu 0.511 total

$ ruby -v string_symbol_hash.rb 100
       user     system      total        real
Symbol#hash  0.050000   0.000000   0.050000 (  0.042057)
String#hash  0.100000   0.000000   0.100000 (  0.101787)
0.65s user 0.05s system 99% cpu 0.712 total

$ ruby string_symbol_hash.rb 1000
       user     system      total        real
Symbol#hash  0.050000   0.000000   0.050000 (  0.044034)
String#hash  0.460000   0.000000   0.460000 (  0.468324)
1.61s user 0.25s system 99% cpu 1.868 total

$ ruby string_symbol_hash.rb 10000
       user     system      total        real
Symbol#hash  0.040000   0.000000   0.040000 (  0.040466)
String#hash  4.240000   0.000000   4.240000 (  4.247260)
11.74s user 2.21s system 99% cpu 13.991 total