chore: add SimpleChannel::NotifyProducer function #314
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| @@ -81,6 +81,10 @@ template <typename T, typename Queue = folly::ProducerConsumerQueue<T>> class Si | |||
| return q_.capacity(); | |||
| } | |||
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| // Advanced control - tries to wake up the producer, blocked on a full queue. | |||
| void NotifyProducer() { | |||
| push_ec_.notify(); | |||
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We need to support cancellation of the producer when it is stack in a loop trying to push to the queue but its slots are full. This will solve the bug we saw with debug populate and replication in df. The problem is not that the producer is sleeping, the problem is that the producer infinitely tries to push on a channel that's never empty and never drained. If we support cancellation, the producer will back off, skip the push and continue with its thing
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I am unaware of this bug, nor I understand how a producer can be stuck if a channel is not full. it should succeed with the push.
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I am talking about this one. dragonflydb/dragonfly#3649
how a producer can be stuck if a channel is not full.
That's the bug. The producer is stuck trying to push because the channel is full and it's blocked. Then the replication is cancelled, the fiber that handles full sync drains the channel but before the cancellation process (another fiber) kicks in the channel gets filled again. Then cancellation fiber kicks in and tries to unregister from the journal but the fiber that is trying to write to the journal is blocked because the channel is full and we deadlock. What solves this cycle is a cancel flag. I even wrote it internally on google chat 😄
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It get's worse when the replica has more flows (because it has more proactors than master) because then multiple threads push to the same channel which gets filled even quicker
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This PR is unrelated to the bug.
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Currently SimpleChannel wakes up producer only when the queue has been depleted. This can be suboptimal in some situations, specifically in Dragonfly, consumer can pop an element and block on writing it into disk, an meanwhile producer can not progress. This function provides more advanced control, allowing to wake up producer earlier