doc/tutorials/tcp_tunning: note about network buffers size

- they may need tunning when dealing with large data

(cherry picked from commit 38a696f)
(cherry picked from commit d44cf21)
(cherry picked from commit bd6dffa)

doc/tutorials/tcp_tunning.txt

@@ -19,22 +19,22 @@ For now it deals only with Linux specific optimizations.
 
 net.core.somaxconn             -  limit of the listen() backlog, default 128
 net.ipv4.tcp_max_syn_backlog   -  default 1024 or 128
-net.ipv4.tcp_timestamps        - default on., should be on (along with 
+net.ipv4.tcp_timestamps        - default on., should be on (along with
                                  tcp_tw_recycle and timestamp supporting
                                  peers allows for fast connections rates)
 
-1.2 connection in close_wait
+1.2 Connection in close_wait
 
- Connection should stay as little as possible
- in close_wait to quickly free the fd/resources for new connections attempts
- WARNING: this could break normal TCP use, use it only if you know what you are
+  Connection should stay as little as possible
+  in close_wait to quickly free the fd/resources for new connections attempts
+  WARNING: this could break normal TCP use, use it only if you know what you are
   doing
- 
+
 net.ipv4.tcp_max_tw_buckets - maximum number of timewait sockets
                               (the default seems to be ok)
 net.ipv4.tcp_tw_recycle     - enables fast time wait sockets recycling (default
                               off), should be enabled if you have lots of short
-                              lived connections 
+                              lived connections
                               WARNING: see the above warning
 net.ipv4.tcp_tw_reuse       - allows reusing of time-wait sockets (default off)
                               WARNING: see above
@@ -49,10 +49,26 @@ net.ipv4.ip_local_port_range - should be increased (e.g. 4096-65534)
 1.4 Open file descriptors
 
 fs.file-max                 - maximum number of fds that will be allocated
-                              (you probably need to increase it, default 
+                              (you probably need to increase it, default
                                depends on installed memory)
 
-1.5 other sysctl that might affect tcp connection rate or the maximum number
+1.5 Buffers size
+
+  When dealing with large data, check the values for network sockets buffers
+  to be sufficient, among them:
+
+net.core.rmem_max
+net.core.wmem_max
+net.core.rmem_default
+net.core.wmem_default
+net.ipv4.tcp_mem
+net.ipv4.tcp_rmem
+net.ipv4.tcp_wmem
+
+  Note: sysctl -w net.ipv4.route.flush=1 - enusre that immediatly subsequent
+  connections use the values
+
+1.6 Other sysctl that might affect tcp connection rate or the maximum number
     of open connections
 
 fs.epoll.max_user_instances - maximum number of devices - per user (2.6.27.8+)