Started f(a=1,)
Started f(a=2,)
Finished f(a=1,) = 0
Started f(a=3,)
Finished f(a=2,) = 0
Started f(a=4,)
Failed f(a=3,), 
ReleaseException: <metaopt.invoker.util.worker_provider.ReleaseException object at 0x52ce490>
Failed f(a=4,), 
ReleaseException: <metaopt.invoker.util.worker_provider.ReleaseException object at 0x4fd8750>
(a=1,)
