In this micro talk, I'll review the basic theory behind hash functions and hash tables, show you the new internal data structures that Ruby 2.0 uses to save keys and values, and present some performance data that proves this optimization exists and how much time it will actually save you.

1. Why Hashes Will Be
Faster in Ruby 2.0
Pat Shaughnessy
http://patshaughnessy.net
@pat_shaughnessy
McKinsey & Co.

2. h = {}
=> {}
h[:key] = 'value'
=> "value"
h[:key]
=> "value"

4. Time to retrieve 10000 values (ms) 3
2.5
2
1.5
1
0.5
1 100 10,000 1,000,000
Hash size

5. 0 1 2 3 4 5 6 7 8 9 10
st_table_entry
hash
key
record
next

6. func(key) = hash

7. :key.hash
=> 4321153767602860351
"abcdef".hash
=> -1182018534040789564

8. hash % 11 = bin index

9. 0 1 2 3 4 5 6 7 8 9 10
st_table_entry
hash
key
record
next

10. 0 1 2 3 4 5 6 7 8 9 10
st_table_entry st_table_entry
hash hash
key key
record record
next next

11. 0 1 2 3 4 5 6 7 8 9 10
st_table_entry st_table_entry st_table_entry
hash hash hash
key key key
record record record
next next next

12. 0 1 2 3 4 5 6 7 8 9 10
st_table_entry st_table_entry st_table_entry
hash hash hash
key key key
record record record
next next next
st_table_entry
hash
key
record
next

13. 0 1 2 3 4 5 6 7 8 9 10
entry entry entry entry entry entry entry entry entry entry entry
entry entry entry entry entry entry entry entry entry entry entry
entry entry entry entry entry entry entry entry entry entry entry
entry entry entry entry entry entry entry entry
entry entry entry

14. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
entry entry entry entry entry entry entry entry entry entry entry entry entry entry entry entry entry entry entry
entry entry entry entry entry entry entry entry entry entry entry entry entry entry entry entry entry entry entry
entry entry entry entry entry entry entry entry entry entry entry entry entry entry entry entry entry entry entry
entry entry entry entry entry entry entry entry

15. Hashes in Ruby 2.0

16. 0 1 2 3 4 5 6 7 8 9 10
malloc
st_table_entry
hash
key
record
next
Ruby 1.8, Ruby 1.9

17. :key => :key2 => :key3 => :key4 => :key5 => :key6 =>
value value2 value3 value4 value5 value6
st_table_entry st_table_entry st_table_entry
hash hash hash
key key key
record record record
next next next
Ruby 2.0 - first 6
elements

18. Hashes are
Arrays!

20. Time to add 1 element 10,000 times (ms) 14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Hash size

21. 6% improvement
0
Loading 10,000 small records

22. 2.6% improvement
Rails App: Ruby Execution Time

23. How many hashes?
describe "GET index" do
it "assigns all people as @people" do
person = Person.create! valid_attributes
get :index, {}, valid_session
assigns(:people).should eq([person])
end
end
5?

24. How many hashes?
describe "GET index" do
it "assigns all people as @people" do
person = Person.create! valid_attributes
get :index, {}, valid_session
assigns(:people).should eq([person])
end
end
5 50?

25. How many hashes?
describe "GET index" do
it "assigns all people as @people" do
person = Person.create! valid_attributes
get :index, {}, valid_session
assigns(:people).should eq([person])
end
end
50 409!

26. http://patshaughnessy.net/ruby-under-a-microscope