It is found that if there are too many concurrent statahead instances, then related statahead RPCs may accumulate on the client import (for MDT) RPC lists (imp_sending_list/imp_delayed_list/imp_unreplied_lis), as to seriously affect the efficiency of spin_lock under the case of MDT overloaded or in recovery. Be as the temporarily solution, it is necessary to control the concurrent statahead instances.