golang mgo的mongo连接池设置：必须手动加上maxPoolSize

GlobalMgoSession, err := mgo.Dial(host)
 
func (m *MongoBaseDao) Get(tablename string, id string, result interface{}) interface{} {
    session := GlobalMgoSession.Clone()
    defer session.Close()
 
    collection := session.DB(globalMgoDbName).C(tablename)
    err := collection.FindId(bson.ObjectIdHex(id)).One(result)
 
    if err != nil {
        logkit.Logger.Error("mongo_base method:Get " + err.Error())
    }
    return result
}

// Clone works just like Copy, but also reuses the same socket as the original
// session, in case it had already reserved one due to its consistency
// guarantees.  This behavior ensures that writes performed in the old session
// are necessarily observed when using the new session, as long as it was a
// strong or monotonic session.  That said, it also means that long operations
// may cause other goroutines using the original session to wait.
func (s *Session) Clone() *Session {
    s.m.Lock()
    scopy := copySession(s, true)
    s.m.Unlock()
    return scopy
}
 
 
// Close terminates the session.  It's a runtime error to use a session
// after it has been closed.
func (s *Session) Close() {
    s.m.Lock()
    if s.cluster_ != nil {
        debugf("Closing session %p", s)
        s.unsetSocket()  //释放当前线程占用的socket 置为nil
        s.cluster_.Release()
        s.cluster_ = nil
    }
    s.m.Unlock()
}

func (s *Session) acquireSocket(slaveOk bool) (*mongoSocket, error) {
    // Read-only lock to check for previously reserved socket.
    s.m.RLock()
    // If there is a slave socket reserved and its use is acceptable, take it as long
    // as there isn't a master socket which would be preferred by the read preference mode.
    if s.slaveSocket != nil && s.slaveOk && slaveOk && (s.masterSocket == nil || s.consistency != PrimaryPreferred && s.consistency != Monotonic) {
        socket := s.slaveSocket
        socket.Acquire()
        s.m.RUnlock()
        logkit.Logger.Info("sgp_test 1 acquireSocket slave is ok!")
        return socket, nil
    }
    if s.masterSocket != nil {
        socket := s.masterSocket
        socket.Acquire()
        s.m.RUnlock()
        logkit.Logger.Info("sgp_test 1  acquireSocket master is ok!")
        return socket, nil
    }
 
    s.m.RUnlock()
 
    // No go.  We may have to request a new socket and change the session,
    // so try again but with an exclusive lock now.
    s.m.Lock()
    defer s.m.Unlock()
    if s.slaveSocket != nil && s.slaveOk && slaveOk && (s.masterSocket == nil || s.consistency != PrimaryPreferred && s.consistency != Monotonic) {
        s.slaveSocket.Acquire()
        logkit.Logger.Info("sgp_test 2  acquireSocket slave is ok!")
        return s.slaveSocket, nil
    }
    if s.masterSocket != nil {
        s.masterSocket.Acquire()
        logkit.Logger.Info("sgp_test 2  acquireSocket master is ok!")
        return s.masterSocket, nil
    }
 
    // Still not good.  We need a new socket.
    sock, err := s.cluster().AcquireSocket(s.consistency, slaveOk && s.slaveOk, s.syncTimeout, s.sockTimeout, s.queryConfig.op.serverTags, s.poolLimit)
 
......
    logkit.Logger.Info("sgp_test 3   acquireSocket cluster AcquireSocket is ok!")
    return sock, nil
 
}

Mar 25 09:46:40 dev02.com[12607]:  [info] sgp_test 1  acquireSocket master is ok!
Mar 25 09:46:40 dev02.com[12607]:  [info] sgp_test 1  acquireSocket master is ok!
Mar 25 09:46:41 dev02.com[12607]:  [info] sgp_test 1 acquireSocket slave is ok!
Mar 25 09:46:41 dev02.com[12607]:  [info] sgp_test 3   acquireSocket cluster AcquireSocket is ok!
Mar 25 09:46:41 dev02.com[12607]:  [info] sgp_test 3   acquireSocket cluster AcquireSocket is ok!
Mar 25 09:46:41 dev02.com[12607]:  [info] sgp_test 3   acquireSocket cluster AcquireSocket is ok!

src/gopkg.in/mgo.v2/cluster.go 
    s, abended, err := server.AcquireSocket(poolLimit, socketTimeout)
        if err == errPoolLimit {
            if !warnedLimit {
                warnedLimit = true
                logkit.Logger.Error("sgp_test WARNING: Per-server connection limit reached. " + err.Error())
                log("WARNING: Per-server connection limit reached.")
            }
            time.Sleep(100 * time.Millisecond)
            continue
        }
 
session.go:
// SetPoolLimit sets the maximum number of sockets in use in a single server
  // before this session will block waiting for a socket to be available.
  // The default limit is 4096.
  //
  // This limit must be set to cover more than any expected workload of the
  // application. It is a bad practice and an unsupported use case to use the
  // database driver to define the concurrency limit of an application. Prevent
  // such concurrency "at the door" instead, by properly restricting the amount
  // of used resources and number of goroutines before they are created.
  func (s *Session) SetPoolLimit(limit int) {
      s.m.Lock()
      s.poolLimit = limit
      s.m.Unlock()
  }