Session hang because 'gc cr request' and 'cr request retry'

In 11g Rac, during datapump import, session hangs at step of importing indexes or constraints.

If you check v$session_wait for the sid, you may notice  'gc cr request' and 'cr request retry':

SQL> select event from v$session_wait where sid=405;

EVENT

----------------------------------------------------------------

gc cr request

SQL> select event from v$session_wait where sid=405;

EVENT

----------------------------------------------------------------

cr request retry

Mostly it's caused by Redo log IO performance or the interconnect performance.

You can try:
1) Improve the interconnect performance.
2) Improve the Redo log I/o performance.
3) Set undersore parameter "_cr_server_log_flush" =false.