Add comparison of ets vs. gen server

Author: devonestes

README.md

@@ -246,6 +246,42 @@ sort/2           4.93 K - 1.05x slower
 sort_by/2        4.81 K - 1.08x slower
 ```
 
+#### Retrieving state from ets tables vs. Gen Servers [code](code/general/ets_vs_gen_server.exs)
+
+There are many differences between Gen Servers and ets tables, but many people
+have often praised ets tables for being extremely fast. For the simple case of
+retrieving information from a key-value store, the ets table is indeed much
+faster for lookups. For more complicated use cases, and for comparisons of
+writes instead of reads, further benchmarks are needed, but so far ets lives up
+to its reputation for speed.
+
+```
+Operating System: macOS
+CPU Information: Intel(R) Core(TM) i5-4260U CPU @ 1.40GHz
+Number of Available Cores: 4
+Available memory: 8.589934592 GB
+Elixir 1.7.0-dev
+Erlang 20.2
+Benchmark suite executing with the following configuration:
+warmup: 2.00 s
+time: 10.00 s
+parallel: 1
+inputs: none specified
+Estimated total run time: 24.00 s
+
+
+Benchmarking ets table...
+Benchmarking gen server...
+
+Name                 ips        average  deviation         median
+ets table         9.18 M       0.109 μs   ±555.09%       0.100 μs
+gen server        0.34 M        2.97 μs  ±1954.50%        3.00 μs
+
+Comparison:
+ets table         9.18 M
+gen server        0.34 M - 27.24x slower
+```
+
 ## Something went wrong
 
 Something look wrong to you? :cry: Have a better example? :heart_eyes: Excellent!

code/general/ets_vs_gen_server.exs

@@ -0,0 +1,42 @@
+defmodule RetrieveState.Fast do
+  def get_state(ets_pid) do
+    :ets.lookup(ets_pid, :stored_state)
+  end
+end
+
+defmodule StateHolder do
+  use GenServer
+
+  def init(_), do: {:ok, %{stored_state: :returned_state}}
+
+  def start_link(state \\ []), do: GenServer.start_link(__MODULE__, state, name: __MODULE__)
+
+  def get_state(key), do: GenServer.call(__MODULE__, {:get_state, key})
+
+  def handle_call({:get_state, key}, _from, state), do: {:reply, state[key], state}
+end
+
+defmodule RetrieveState.Slow do
+  def get_state do
+    StateHolder.get_state(:stored_state)
+  end
+end
+
+defmodule RetrieveState.Benchmark do
+  def benchmark do
+    ets_pid = :ets.new(:state_store, [:set, :public])
+    :ets.insert(ets_pid, {:stored_state, :returned_state})
+    StateHolder.start_link()
+
+    Benchee.run(
+      %{
+        "ets table" => fn -> RetrieveState.Fast.get_state(ets_pid) end,
+        "gen server" => fn -> RetrieveState.Slow.get_state() end
+      },
+      time: 10,
+      print: [fast_warning: false]
+    )
+  end
+end
+
+RetrieveState.Benchmark.benchmark()