Hi all :
I'm learning the polling engine of grpc core by reading this doc :
https://github.com/grpc/grpc/blob/master/doc/core/epoll-polling-engine.md.
I have some questions:
1. How could the "Thundering Herds" phenomenon happened ?
I know if we use epoll with ET properly , actually only one
thread will awoke from epoll_wait even multiple thread are blocking on it
at the moment when some fds are ready, consulting from the epoll man page
<http://man7.org/linux/man-pages/man7/epoll.7.html>. Whereas I don't find
poll has such capabilities .
So the reason for "Thundering Herds" is just because the core
using poll rather than epoll ?
- If the answer is Yes, why using poll then ? For the
compatibility on *nix platforms ?
- If the answer is No, what's the true reason?
2. For this paragraph :
For example, in Figure 1, if fd1 becomes readable, any one of the
threads i.e Threads 1 to Threads K or Thread P, might be woken up. Let's
say Thread P was calling a grpc_completion_queue_pluck() and was actually
interested in the event on fd1 but Thread 1 woke up. In this case, Thread 1
executes the callbacks and finally kicks Thread P by signalling event_fd_P.
Thread P wakes up, realizes that there is a new completion event for it and
returns from grpc_completion_queue_pluck() to its caller.
1> what's the meaning of "actually interested in the event"?
For my understanding , all the threads which are calling '
grpc_completion_queue_pluck' or 'grpc_completion_queue_next' are symmetric
, they are all the threads from the upper application layer, and they will
do the corresponding work related to the event it got , share
nothing difference. Why one could distinguishing from the other by "interested
in the event or not"?
2> Regardless of question 1>.
If "*Thread 1 finishes** executing the callbacks"* , why it still
have to *"kicks **Thread P by signalling event_fd_P",*hasn't the task been
finished due to Thread 1's execution? Why thread 1 need to kick thread P ?
I'm lack of the knowledge of the interaction details between
completionQueue and epoll_set , so the above questions may seems silly, I'm
very appreciated for patient answers. Thanks!
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