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