bzr commit into mysql-5.1-bugteam branch (davi:3456) Bug#54790

[Davi Arnaut]

# At a local mysql-5.1-bugteam repository of davi

 3456 Davi Arnaut	2010-07-02
      Bug#54790: Use of non-blocking mode for sockets limits performance
      
      The problem was that a optimization for the case when the
      server uses alarms for timeouts could cause a slowdown when
      socket timeouts are used instead.
      
      In case alarms are used for timeouts, a non-blocking read
      is attempted first in order to avoid the cost of setting up
      a alarm. If the non-blocking read fails, the socket mode is
      changed to blocking and a alarm is armed.
      
      If socket timeout is used, there is no point in attempting a
      non-blocking read first as the timeout will be automatically
      armed by the OS. Yet the server would attempt a non-blocking
      read first and later switch the socket to blocking mode. This
      could inadvertently impact performance as switching the blocking
      mode of a socket requires at least two calls into the kernel,
      apart from problems inherited by the scalability of fcntl.
      
      The solution is to not attempt a non-blocking read first if
      built-in socket timeouts are being used. Also, in this case,
      spurious interruptions with a errno value of EINTR should
      be ignored.
     @ sql/net_serv.cc
        Do not set socket to non-blocking if not using alarms.
        
        Assert that the socket mode is blocking if using std
        socket timeouts. Also, ignore spurious interruptions
        in this mode.

    modified:
      sql/net_serv.cc
=== modified file 'sql/net_serv.cc'
--- a/sql/net_serv.cc	2010-06-09 08:29:27 +0000
+++ b/sql/net_serv.cc	2010-07-02 13:13:43 +0000
@@ -139,7 +139,7 @@ my_bool my_net_init(NET *net, Vio* vio)
   if (vio != 0)					/* If real connection */
   {
     net->fd  = vio_fd(vio);			/* For perl DBI/DBD */
-#if defined(MYSQL_SERVER) && !defined(__WIN__)
+#if defined(MYSQL_SERVER) && !defined(__WIN__) && !defined(NO_ALARM)
     if (!(test_flags & TEST_BLOCKING))
     {
       my_bool old_mode;
@@ -805,6 +805,7 @@ my_real_read(NET *net, size_t *complen)
     thr_alarm(&alarmed,net->read_timeout,&alarm_buff);
 #else
   /* Read timeout is set in my_net_set_read_timeout */
+  DBUG_ASSERT(net_blocking);
 #endif /* NO_ALARM */
 
     pos = net->buff + net->where_b;		/* net->packet -4 */
@@ -815,10 +816,21 @@ my_real_read(NET *net, size_t *complen)
 	/* First read is done with non blocking mode */
         if ((long) (length= vio_read(net->vio, pos, remain)) <= 0L)
         {
-          my_bool interrupted = vio_should_retry(net->vio);
+          my_bool interrupted;
 
 	  DBUG_PRINT("info",("vio_read returned %ld  errno: %d",
 			     (long) length, vio_errno(net->vio)));
+
+#ifdef NO_ALARM
+          if (vio_errno(net->vio) == SOCKET_EINTR)
+          {
+            DBUG_PRINT("warning",("Interrupted read. Retrying..."));
+            continue;
+          }
+#endif
+
+          interrupted= vio_should_retry(net->vio);
+
 #if !defined(__WIN__) || defined(MYSQL_SERVER)
 	  /*
 	    We got an error that there was no data on the socket. We now set up


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