Auto DOP can't work,no idea why

Table test is very big,about 45G,

following SQL takes very long time to finish if running in serial:

select avg(a.line_location_id+b.line_location_id) from test a,test b

the workload of the database server is very low:

top - 15:51:42 up 29 days,  2:38,  1 user,  load average: 0.42, 0.50, 0.47

Tasks: 1377 total,   1 running, 1373 sleeping,   0 stopped,   3 zombie

Cpu(s):  2.6%us,  1.1%sy,  0.0%ni, 95.7%id,  0.6%wa,  0.0%hi,  0.0%si,  0.0%st

Mem:  264331892k total, 58181952k used, 206149940k free,  1244216k buffers

Swap: 20479992k total,        0k used, 20479992k free, 28706808k cached

and I configured auto DOP as following:

SQL> show parameter parallel

NAME                                 TYPE        VALUE

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