Hello everybody, I went through most of the documentation I could find for Giraph and also most of the messages in this email list, but still I have not figured out precisely what a "worker" really is. I would really appreciate it if you could help me understand how the framework works.
At first I thought that a worker has a one-to-one correspondence to a map task. Apparently this is not exactly the case, since I have noticed that if I ask for x workers, the job finishes after having used x+1 map tasks. What is this extra task for? I have been trying out the example SSSP application on a single node with 12 cores. Giving an input graph of ~400MB and using 1 worker, around 10 GBs of memory are used during execution. What intrigues me is that if I use 2 workers for the same input (and without limiting memory per map task), double the memory will be used. Furthermore, there will be no improvement in performance. I rather notice a slowdown. Are these observations normal? Might it be the case that 1 and 2 workers are very few and I should go to the 30-100 range that is the proposed number of mappers for a conventional MapReduce job? Finally, a last observation. Even though I use only 1 worker, I see that there are significant periods during execution where up to 90% of the 12 cores computing power is consumed, that is, almost 10 cores are used in parallel. Does each worker spawn multiple threads and dynamically balances the load to utilize the available hardware? Thanks a lot in advance! Best, Alexandros
