I believe that you are seeing this from a weird perspective. Personal opinion's aside, maybe reading pregel's paper could be of great benefit, because Giraph is based on it.

Regarding to your questions:
Workers stop when all vertex are halt, and every worker has a partition of vertex. Applicacion Manager (master) halt when all workers are done.

A worker will wake up, if one of its vertex is active again, for example, because it's receiving a message.

2015-12-22 4:30 GMT-03:00 Kaushik Baruah <kaushikbaruaa@gmail.com>:
Hi Experts,

Can you please let me know, if master compute halts automatically when all the workers halt.

1) If it is so, how can we know from master compute if all workers have halted or is there a way to keep the master compute running even though all the workers have halted.

2) Is there a way to wake up the workers from master compute if they have halted.