Bug #24373 High CPU utilization when no idle connection

Description:
MySqlPool.GetConnection runs a very tight loop until there's either a timeout or a connection is retrieved from the pool.

Specifically:
while (driver == null && (Environment.TickCount - startTime) < timeOut)
{
   driver = GetPooledConnection();
}

private Driver GetPooledConnection()
{
   while (idlePool.Count <= 0)
   {
      if (inUsePool.Count == maxSize)
      {
         return null;
      }
      CreateNewPooledConnection();
   }
   return CheckoutConnection();
}

 
If there are no idle connections in idlePool and we are using the maximum amount of connections, we'll just spin our wheels in the loop.

This can have a pretty harsh performance impact on a system which is pushing up against it's connection limit.

How to repeat:
This code will reproduce the problem each time. Using a profiler, perfmon or just task-manager, we can see the impact on performance this code will have.  I used snippet compiler, so WL and RL just map to Console.WriteLine and Console.ReadLine .

private const string _connectionString = "XXXXXX;Maximum Pool Size=1";
public static void Main()
{
   MySqlConnection connection1 = null;      
   MySqlConnection connection2 = null;   
   try
   {
      connection1 = GetConnection();
      connection2 = GetConnection();
   }
   catch(Exception ex)
   {
      WL(ex);
   }
   finally
   {
      if (connection1 != null)
      {
         connection1.Dispose();
      }
      if (connection2 != null)
      {
         connection2.Dispose();
      }
   }
     
   WL("Done");
   RL();
}

private static MySqlConnection GetConnection()
{
   MySqlConnection connection = new MySqlConnection(_connectionString);
   connection.Open();
   return connection;
}

Suggested fix:
I don't know what people think about sleeping inside the while loop...

Ideally it'd be nice to see this block and wait for a signal that a connection has become available.

From a performance standpoint, it'd be nice if CreateNewPooledConnection() actually Checkedout and returned the connection directly. As is, 1 thread can keep creating new pool connections while other threads sneak in and steal them.

Sorry, the last point about "CreateNewPooledConnection" was just an unrelated remark...

Karl, thanks for your excellent problem report.
Verified as described by reporter with latest sources. Until timeout expires, CPU is locked at 100%:

Connecting to Mysql DB
error connecting: Timeout expired.  The timeout period elapsed prior to obtainin
g a connection from the pool.  This may have occurred because all pooled connect
ions were in use and max pool size was reached.

Test case

Attachment: Program.cs (text/plain), 1.50 KiB.

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/20413

Fixed in 1.0.10 and 5.0.4

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/20420

A note has been added to the 5.0.4 and 1.0.10 changelogs.