Dmitry Orlovsky created BEAM-14108:
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Summary: Legacy (SDF wrapper-based) KafkaIO read performance is
poor when Kafka poll latency is hign
Key: BEAM-14108
URL: https://issues.apache.org/jira/browse/BEAM-14108
Project: Beam
Issue Type: Improvement
Components: io-java-kafka
Reporter: Dmitry Orlovsky
Beam has two KafkaIO source implementations now:
* a modern one implemented as a Splittable DoFn (SDF), and
* a (deprecated) legacy one implemented as an SDF wrapper over an
UnboundedSource and KafkaUnboundedReader classes.
We found that the legacy KafkaIO source can not provide good throughput when
the latency of calls to Kafka
[Consumer.poll|https://github.com/apache/beam/blob/master/sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/KafkaUnboundedReader.java#L523]
becomes high. The degradation is very sharp: a pipeline that drops elements
immediately after reading them from source was only able to read about 100-1000
qps per Kafka partition. The Kafka cluster was overprovisioned but was in a
remote network and had poll latency about 30ms.
First problem that may be addressed in the scope of this bug is that there's
very little visibility into the Kafka source now. We had to add extra logging
to understand the issue with the pipeline above, or even see the poll latency.
We believe that the cause of throughput degradation is poor choice of the
[RECORDS_DEQUEUE_POLL_TIMEOUT and
RECORDS_ENQUEUE_POLL_TIMEOUT|[https://github.com/apache/beam/blob/master/sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/KafkaUnboundedReader.java#L334-L335|https://github.com/apache/beam/blob/master/sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/KafkaUnboundedReader.java#L334-L335],]][,|https://github.com/apache/beam/blob/master/sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/KafkaUnboundedReader.java#L334-L335],]
especially the former one which is now 10ms.
These are timeouts for popping and pushing elements from/to the
[availableRecordsQueue
|https://github.com/apache/beam/blob/master/sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/KafkaUnboundedReader.java#L343].
This is a synchronous queue (i.e. blocking, without buffering) used to hand
records fetched from Kafka between two loops:
* The
[consumerPollLoop|https://github.com/apache/beam/blob/master/sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/KafkaUnboundedReader.java#L515]
that polls data via a Kafka Consumer if there's no pending data already, and
offers it to the availableRecordsQueue otherwise. It also does offset
checkpointing but this is irrelevant to our case.
* The beam UnboundedSourceAsSDFWrapperFn message processing loop. It's a bit
complicated, but the important part is that it would call the [nextBatch
function|https://github.com/apache/beam/blob/master/sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/KafkaUnboundedReader.java#L573]
repeatedly until an attempt to [fetch an element from the
avaliableRecordsQueue|https://github.com/apache/beam/blob/master/sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/KafkaUnboundedReader.java#L580]
times out. After the timeout, it returns the control to the worker and it may
take relatively long time until the loop is scheduled again.
This is what we think is happening when the poll latency is high:
* consumerPollLoop fetches data bundle from Kafka via poll() and offers it to
the avaliableRecordsQueue
* message processing loop fetches bundle from avaliableRecordsQueue and
unblocks the avaliableRecordsQueue
* consumerPollLoop calls poll() again
* message processing loop completes processing the bundle BEFORE the poll()
call again completes, and tries to fetch next bundle from avaliableRecordsQueue.
* fetch from avaliableRecordsQueue has a very short timeout (10ms) and if it
expires before the pending poll() in the consumerPollLoop completes the message
processing loop will believe there's no fresh data in Kafka and exit.
Re-scheduling it is a wasted time.
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