diff --git a/Makefile b/Makefile
index d780bde..ccc1d41 100644
--- a/Makefile
+++ b/Makefile
@@ -4,6 +4,6 @@ test:
 	$(CRYSTAL_BIN) spec
 
 benchmark:
-	$$(mkdir tmp -p)
+	$$(mkdir -p tmp)
 	$(CRYSTAL_BIN) build --release -o tmp/benchmark ./benchmark.cr $(CRFLAGS)
 	tmp/benchmark
diff --git a/benchmark.cr b/benchmark.cr
index 8fb4df5..34bda09 100644
--- a/benchmark.cr
+++ b/benchmark.cr
@@ -3,17 +3,25 @@ require "./src/kiwi/file_store"
 require "./src/kiwi/redis_store"
 require "./src/kiwi/leveldb_store"
 require "./src/kiwi/memcached_store"
-
-N = 100_000
-
-def benchmark(name)
-  start_time = Time.now
-  yield
-  writing_time = Time.now - start_time
-  speed = (N.to_f / writing_time.to_f)
-  rounded_speed = ((speed / 1000).round * 1000).to_i
-  puts "  #{name}: #{rounded_speed} ops/sec"
-  rounded_speed
+require "benchmark"
+X = 50000
+N =  5
+LIMITER = Time::Span.new(seconds: 5)
+
+def benchmark(name, &block)
+  r = Time::Span.new
+  i = 0
+  loop do
+    puts "N=%i / %i" % [i, N]
+    r += Benchmark.realtime do
+      X.times do
+        block.call
+      end
+    end
+    i += X
+    break if r >  LIMITER
+  end
+  (i.to_f/r.seconds.to_f).round.to_i
 end
 
 def gen_data
@@ -113,9 +121,10 @@ puts "Initializing stores..."
 stores = Array(Kiwi::Store).new
 
 stores << Kiwi::MemoryStore.new
+stores << Kiwi::MemcachedStore.new(Memcached::Client.new)
+
 stores << Kiwi::LevelDBStore.new(LevelDB::DB.new("/tmp/kiwi_benchmark_leveldb"))
 stores << Kiwi::RedisStore.new(Redis.new)
-stores << Kiwi::MemcachedStore.new(Memcached::Client.new)
 stores << Kiwi::FileStore.new("/tmp/kiwi_benchmark_file")
 
 result = measure(stores)
diff --git a/src/kiwi/file_store.cr b/src/kiwi/file_store.cr
index 123da1e..da789a4 100644
--- a/src/kiwi/file_store.cr
+++ b/src/kiwi/file_store.cr
@@ -9,12 +9,12 @@ module Kiwi
       create_dir
     end
 
-    def get(key)
+    def get(key) : String | Nil
       file = file_for_key(key)
       File.exists?(file) ? File.read(file) : nil
     end
 
-    def set(key, val)
+    def set(key, val) : String
       create_dir unless dir_created?
 
       file = file_for_key(key)
@@ -22,7 +22,7 @@ module Kiwi
       val
     end
 
-    def delete(key)
+    def delete(key) : String
       create_dir unless dir_created?
 
       file = file_for_key(key)
@@ -31,11 +31,11 @@ module Kiwi
         File.delete(file)
         value
       else
-        nil
+        ""
       end
     end
 
-    def clear
+    def clear : Store
       remove_dir
       self
     end
diff --git a/src/kiwi/leveldb_store.cr b/src/kiwi/leveldb_store.cr
index 501d1c1..fdfc190 100644
--- a/src/kiwi/leveldb_store.cr
+++ b/src/kiwi/leveldb_store.cr
@@ -6,22 +6,22 @@ module Kiwi
     def initialize(@leveldb : ::LevelDB::DB)
     end
 
-    def set(key, val)
+    def set(key, val) : String
       @leveldb.put(key, val)
       val
     end
 
-    def get(key)
+    def get(key) : String | Nil
       @leveldb.get(key)
     end
 
-    def delete(key)
+    def delete(key) : String
       val = get(key)
       @leveldb.delete(key)
-      val
+      val || ""
     end
 
-    def clear
+    def clear : Store
       @leveldb.clear
       self
     end
diff --git a/src/kiwi/memcached_store.cr b/src/kiwi/memcached_store.cr
index 7bb50d1..359a4d8 100644
--- a/src/kiwi/memcached_store.cr
+++ b/src/kiwi/memcached_store.cr
@@ -6,22 +6,22 @@ module Kiwi
     def initialize(@memcached : Memcached::Client)
     end
 
-    def set(key, val)
+    def set(key, val) : String
       @memcached.set(key, val)
       val
     end
 
-    def get(key)
+    def get(key) : String | Nil
       @memcached.get(key)
     end
 
-    def delete(key)
+    def delete(key) : String
       val = get(key)
       @memcached.delete(key)
-      val
+      val || ""
     end
 
-    def clear
+    def clear : Store
       @memcached.flush
       self
     end
diff --git a/src/kiwi/memory_store.cr b/src/kiwi/memory_store.cr
index 9e97a06..4dca65f 100644
--- a/src/kiwi/memory_store.cr
+++ b/src/kiwi/memory_store.cr
@@ -6,19 +6,19 @@ module Kiwi
       @mem = Hash(String, String).new
     end
 
-    def get(key)
+    def get(key) : String | Nil
       @mem[key]?
     end
 
-    def set(key, val)
+    def set(key, val) : String
       @mem[key] = val
     end
 
-    def delete(key)
-      @mem.delete(key)
+    def delete(key) : String
+      @mem.delete(key) || ""
     end
 
-    def clear
+    def clear : Store
       @mem.clear
       self
     end
diff --git a/src/kiwi/redis_store.cr b/src/kiwi/redis_store.cr
index 78d359e..e74f1ba 100644
--- a/src/kiwi/redis_store.cr
+++ b/src/kiwi/redis_store.cr
@@ -6,22 +6,22 @@ module Kiwi
     def initialize(@redis : Redis)
     end
 
-    def set(key, val)
+    def set(key, val) : String
       @redis.set(key, val)
       val
     end
 
-    def get(key)
+    def get(key) : String | Nil
       @redis.get(key)
     end
 
-    def delete(key)
+    def delete(key) : String
       val = get(key)
       @redis.del(key)
-      val
+      val || ""
     end
 
-    def clear
+    def clear : Store
       @redis.flushdb
       self
     end