docstore/mongodocstore: pauses before getMore about go-cloud HOT 3 CLOSED

v0.34.0 shows the same behavior.

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I'd try adding some logging near here: https://github.com/google/go-cloud/blob/master/docstore/mongodocstore/query.go#L124 to see if you can determine where the pauses are coming from. I am also not familiar with the docstore/mongodocstore code, but it looks like we're just iterating calling cursor.Next and then decoding it.

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Thank you for the pointer. After some more debugging and some dead ends i think i understand whats happening. Especially the golang trace is helpful:

The green-pink-brown stacks are io operations (syscall.read from within go.mongodb.org/mongo-driver/mongo.(*Cursor).Next:133).
The light blue stacks are decode operations (gocloud.dev/docstore/mongodocstore.decodeDoc:125).

I would have expected the documents to be decoded from the moment the first element of a batch arrives. But it seems the mongo driver waits for the complete batch to arrive. Thereafter the gocloud decode operation is applied to all elements and only then the next batch is requested. That explains the above pattern in the io graph.

Not much that can be done about it. Using func (c *Cursor) TryNext(ctx context.Context) bool as a noneblocking alternative could be feasible. This would allow decoding to continue while the next batch is already in flight. But that would require at least parallelized code.

Thanks for having a look and helping me understand the issue.

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