diff --git a/lib/fluent/plugin/out_forward.rb b/lib/fluent/plugin/out_forward.rb
index a674446442..064eb5b494 100644
--- a/lib/fluent/plugin/out_forward.rb
+++ b/lib/fluent/plugin/out_forward.rb
@@ -178,7 +178,7 @@ def start
       # But it should be overwritten by ack_response_timeout to rollback chunks after timeout
       if @ack_response_timeout && @delayed_commit_timeout != @ack_response_timeout
         log.info "delayed_commit_timeout is overwritten by ack_response_timeout"
-        @delayed_commit_timeout = @ack_response_timeout
+        @delayed_commit_timeout = @ack_response_timeout + 2 # minimum ack_reader IO.select interval is 1s
       end
 
       @rand_seed = Random.new.seed
@@ -214,22 +214,23 @@ def write(chunk)
       select_a_healthy_node{|node| node.send_data(tag, chunk) }
     end
 
-    ACKWaitingSockInfo = Struct.new(:sock, :chunk_id, :node, :time, :timeout) do
+    ACKWaitingSockInfo = Struct.new(:sock, :chunk_id, :chunk_id_base64, :node, :time, :timeout) do
       def expired?(now)
         time + timeout < now
       end
     end
 
     def try_write(chunk)
+      log.trace "writing a chunk to destination", chunk_id: dump_unique_id_hex(chunk.unique_id)
       if chunk.empty?
         commit_write(chunk.unique_id)
         return
       end
       tag = chunk.metadata.tag
       sock, node = select_a_healthy_node{|n| n.send_data(tag, chunk) }
-      chunk_id = Base64.encode64(chunk.unique_id)
+      chunk_id_base64 = Base64.encode64(chunk.unique_id)
       current_time = Process.clock_gettime(PROCESS_CLOCK_ID)
-      info = ACKWaitingSockInfo.new(sock, chunk_id, node, current_time, @ack_response_timeout)
+      info = ACKWaitingSockInfo.new(sock, chunk.unique_id, chunk_id_base64, node, current_time, @ack_response_timeout)
       @sock_ack_waiting_mutex.synchronize do
         @sock_ack_waiting << info
       end
@@ -341,7 +342,7 @@ def on_heartbeat(sockaddr, msg)
       end
     end
 
-    # return chunk id when succeeded for tests
+    # return chunk id to be committed
     def read_ack_from_sock(sock, unpacker)
       begin
         raw_data = sock.recv(@read_length)
@@ -359,11 +360,14 @@ def read_ack_from_sock(sock, unpacker)
       else
         unpacker.feed(raw_data)
         res = unpacker.read
-        if res['ack'] != info.chunk_id
+        log.trace "getting response from destination", host: info.node.host, port: info.node.port, chunk_id: dump_unique_id_hex(info.chunk_id), response: res
+        if res['ack'] != info.chunk_id_base64
           # Some errors may have occured when ack and chunk id is different, so send the chunk again.
-          log.warn "ack in response and chunk id in sent data are different", chunk_id: info.chunk_id, ack: res['ack']
+          log.warn "ack in response and chunk id in sent data are different", chunk_id: dump_unique_id_hex(info.chunk_id), ack: res['ack']
           rollback_write(info.chunk_id)
           return nil
+        else
+          log.trace "got a correct ack response", chunk_id: dump_unique_id_hex(info.chunk_id)
         end
         return info.chunk_id
       end
@@ -378,9 +382,9 @@ def read_ack_from_sock(sock, unpacker)
 
     def ack_reader
       select_interval = if @delayed_commit_timeout > 3
-                          2
+                          1
                         else
-                          @delayed_commit_timeout / 2.0
+                          @delayed_commit_timeout / 3.0
                         end
 
       unpacker = Fluent::Engine.msgpack_unpacker
@@ -388,30 +392,36 @@ def ack_reader
       while thread_current_running?
         now = Process.clock_gettime(PROCESS_CLOCK_ID)
         sockets = []
-        @sock_ack_waiting_mutex.synchronize do
-          new_list = []
-          @sock_ack_waiting.each do |info|
-            if info.expired?(now)
-              # There are 2 types of cases when no response has been received from socket:
-              # (1) the node does not support sending responses
-              # (2) the node does support sending response but responses have not arrived for some reasons.
-              log.warn "no response from node. regard it as unavailable.", host: info.node.host, port: info.node.port
-              info.node.disable!
-              info.sock.close rescue nil
-              rollback_write(info.chunk_id)
-            else
-              sockets << info.sock
-              new_list << info
+        begin
+          @sock_ack_waiting_mutex.synchronize do
+            new_list = []
+            @sock_ack_waiting.each do |info|
+              if info.expired?(now)
+                # There are 2 types of cases when no response has been received from socket:
+                # (1) the node does not support sending responses
+                # (2) the node does support sending response but responses have not arrived for some reasons.
+                log.warn "no response from node. regard it as unavailable.", host: info.node.host, port: info.node.port
+                info.node.disable!
+                info.sock.close rescue nil
+                rollback_write(info.chunk_id)
+              else
+                sockets << info.sock
+                new_list << info
+              end
             end
+            @sock_ack_waiting = new_list
           end
-          @sock_ack_waiting = new_list
-        end
 
-        readable_sockets, _, _ = IO.select(sockets, nil, nil, select_interval)
-        next unless readable_sockets
+          readable_sockets, _, _ = IO.select(sockets, nil, nil, select_interval)
+          next unless readable_sockets
 
-        readable_sockets.each do |sock|
-          read_ack_from_sock(sock, unpacker)
+          readable_sockets.each do |sock|
+            chunk_id = read_ack_from_sock(sock, unpacker)
+            commit_write(chunk_id)
+          end
+        rescue => e
+          log.error "unexpected error while receiving ack", error: e
+          log.error_backtrace
         end
       end
     end
diff --git a/test/plugin/test_out_forward.rb b/test/plugin/test_out_forward.rb
index 9e02dc5de7..cad5fd566e 100644
--- a/test/plugin/test_out_forward.rb
+++ b/test/plugin/test_out_forward.rb
@@ -35,16 +35,25 @@ def teardown
 
   def create_driver(conf=CONFIG)
     Fluent::Test::Driver::Output.new(Fluent::Plugin::ForwardOutput) {
-      attr_reader :responses, :exceptions
+      attr_reader :response_chunk_ids, :exceptions, :sent_chunk_ids
 
       def initialize
         super
-        @responses = []
+        @sent_chunk_ids = []
+        @response_chunk_ids = []
         @exceptions = []
       end
 
+      def try_write(chunk)
+        retval = super
+        @sent_chunk_ids << chunk.unique_id
+        retval
+      end
+
       def read_ack_from_sock(sock, unpacker)
-        @responses << super
+        retval = super
+        @response_chunk_ids << retval
+        retval
       rescue => e
         @exceptions << e
         raise e
@@ -302,7 +311,7 @@ def read_ack_from_sock(sock, unpacker)
     assert_equal ['test', time, records[0]], events[0]
     assert_equal ['test', time, records[1]], events[1]
 
-    assert_empty d.instance.responses # not attempt to receive responses, so it's empty
+    assert_empty d.instance.response_chunk_ids # not attempt to receive responses, so it's empty
     assert_empty d.instance.exceptions
   end
 
@@ -329,7 +338,7 @@ def read_ack_from_sock(sock, unpacker)
     assert_equal ['test', time, records[0]], events[0]
     assert_equal ['test', time, records[1]], events[1]
 
-    assert_empty d.instance.responses # not attempt to receive responses, so it's empty
+    assert_empty d.instance.response_chunk_ids # not attempt to receive responses, so it's empty
     assert_empty d.instance.exceptions
   end
 
@@ -354,7 +363,7 @@ def read_ack_from_sock(sock, unpacker)
       {"a" => 2}
     ]
     target_input_driver.run(expect_records: 2) do
-      d.end_if{ d.instance.responses.length > 0 }
+      d.end_if{ d.instance.response_chunk_ids.length > 0 }
       d.run(default_tag: 'test', wait_flush_completion: false, shutdown: false) do
         d.feed([[time, records[0]], [time,records[1]]])
       end
@@ -364,7 +373,8 @@ def read_ack_from_sock(sock, unpacker)
     assert_equal ['test', time, records[0]], events[0]
     assert_equal ['test', time, records[1]], events[1]
 
-    assert_equal 1, d.instance.responses.length
+    assert_equal 1, d.instance.response_chunk_ids.size
+    assert_equal d.instance.sent_chunk_ids.first, d.instance.response_chunk_ids.first
     assert_empty d.instance.exceptions
   end
 
@@ -400,7 +410,7 @@ def read_ack_from_sock(sock, unpacker)
       end
     end
 
-    assert_equal 1, delayed_commit_timeout_value
+    assert_equal (1 + 2), delayed_commit_timeout_value
 
     events = target_input_driver.events
     assert_equal ['test', time, records[0]], events[0]
@@ -409,7 +419,6 @@ def read_ack_from_sock(sock, unpacker)
     assert{ d.instance.rollback_count > 0 }
 
     logs = d.instance.log.logs
-    assert{ logs.any?{|log| log.include?("failed to flush the buffer chunk, timeout to commit.") } }
     assert{ logs.any?{|log| log.include?("no response from node. regard it as unavailable.") } }
   end
 
@@ -445,7 +454,7 @@ def read_ack_from_sock(sock, unpacker)
       end
     end
 
-    assert_equal 5, delayed_commit_timeout_value
+    assert_equal (5 + 2), delayed_commit_timeout_value
 
     events = target_input_driver.events
     assert_equal ['test', time, records[0]], events[0]
@@ -454,7 +463,6 @@ def read_ack_from_sock(sock, unpacker)
     assert{ d.instance.rollback_count > 0 }
 
     logs = d.instance.log.logs
-    assert{ logs.any?{|log| log.include?("failed to flush the buffer chunk, timeout to commit.") } }
     assert{ logs.any?{|log| log.include?("no response from node. regard it as unavailable.") } }
   end