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diff --git a/doc/core/grpc-cq.md b/doc/core/grpc-cq.md new file mode 100644 index 0000000000..b485c35456 --- /dev/null +++ b/doc/core/grpc-cq.md @@ -0,0 +1,64 @@ +# gRPC Completion Queue + +_Author: Sree Kuchibhotla (@sreecha) - Sep 2018_ + +Code: [completion_queue.cc](https://github.com/grpc/grpc/blob/v1.15.1/src/core/lib/surface/completion_queue.cc) + +This document gives an overview of completion queue architecture and focuses mainly on the interaction between completion queue and the Polling engine layer. + +## Completion queue attributes +Completion queue has two attributes + + - Completion_type: + - GRPC_CQ_NEXT: grpc_completion_queue_next() can be called (but not grpc_completion_queue_pluck()) + - GRPC_CQ_PLUCK: grpc_completion_queue_pluck() can be called (but not grpc_completion_queue_next()) + - GRPC_CQ_CALLBACK: The tags in the queue are function pointers to callbacks. Also, neither next() nor pluck() can be called on this + + - Polling_type: + - GRPC_CQ_NON_POLLING: Threads calling completion_queue_next/pluck do not do any polling + - GRPC_CQ_DEFAULT_POLLING: Threads calling completion_queue_next/pluck do polling + - GRPC_CQ_NON_LISTENING: Functionally similar to default polling except for a boolean attribute that states that the cq is non-listening. This is used by the grpc-server code to not associate any listening sockets with this completion-queue’s pollset + + +## Details + + + + +### **grpc\_completion\_queue\_next()** & **grpc_completion_queue_pluck()** APIS + + +``` C++ +grpc_completion_queue_next(cq, deadline)/pluck(cq, deadline, tag) { + while(true) { + \\ 1. If an event is queued in the completion queue, dequeue and return + \\ (in case of pluck() dequeue only if the tag is the one we are interested in) + + \\ 2. If completion queue shutdown return + + \\ 3. In case of pluck, add (tag, worker) pair to the tag<->worker map on the cq + + \\ 4. Call grpc_pollset_work(cq’s-pollset, deadline) to do polling + \\ Note that if this function found some fds to be readable/writable/error, + \\ it would have scheduled those closures (which may queue completion events + \\ on SOME completion queue - not necessarily this one) + } +} +``` + +### Queuing a completion event (i.e., "tag") + +``` C++ +grpc_cq_end_op(cq, tag) { + \\ 1. Queue the tag in the event queue + + \\ 2. Find the pollset corresponding to the completion queue + \\ (i) If the cq is of type GRPC_CQ_NEXT, then KICK ANY worker + \\ i.e., call grpc_pollset_kick(pollset, nullptr) + \\ (ii) If the cq is of type GRPC_CQ_PLUCK, then search the tag<->worker + \\ map on the completion queue to find the worker. Then specifically + \\ kick that worker i.e call grpc_pollset_kick(pollset, worker) +} + +``` + diff --git a/doc/images/grpc-cq.png b/doc/images/grpc-cq.png Binary files differnew file mode 100644 index 0000000000..2d9e095862 --- /dev/null +++ b/doc/images/grpc-cq.png |
