[11gR2]: ORA-12801: error signaled in parallel query server Pxxx & ORA-01555: snapshot too old: roll

Hi,

I have implemented parallelism in Oracle 11gr2  by using following  commands at session-level

execute immediate 'alter session set PARALLEL_DEGREE_POLICY=AUTO';
execute immediate 'alter session force PARALLEL QUERY';

but this error is encountered:

ERROR: ORACLE execute error: ORA-12801: error signaled in parallel query server P015
ORA-01555: snapshot too old: rollback segment number string with name "string" too small

I have increased UNDO_RETENTION from 900 to 2400 but error still remain ... why ?

Thanks for help.

GD.