Lucas Bradstreet created KAFKA-12330:
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Summary: FetchSessionCache may cause starvation for partitions
when FetchResponse is full
Key: KAFKA-12330
URL: https://issues.apache.org/jira/browse/KAFKA-12330
Project: Kafka
Issue Type: Bug
Reporter: Lucas Bradstreet
The incremental FetchSessionCache sessions deprioritizes partitions where a
response is returned. This may happen if log metadata such as log start offset,
hwm, etc is returned, or if data for that partition is returned.
When a fetch response fills to maxBytes, data may not be returned for
partitions where it's available. However, the fetch response will still contain
updates to metadata such as hwm if that metadata has changed. This can lead to
degenerate behavior where a partition's hwm or log start offset is updated
resulting in the next fetch being unnecessarily skipped for that partition. At
first this appeared to be worse, as hwm updates occur frequently, but
starvation should result in hwm movement becoming blocked, allowing a fetch to
go through and then becoming unstuck. However, it'll still require one more
fetch request than necessary to do so.
I believe we should only reorder the partition fetch priority if data is
actually returned for a partition.
{code:java}
{code}
{noformat}
private class PartitionIterator(val iter: FetchSession.RESP_MAP_ITER,
val updateFetchContextAndRemoveUnselected:
Boolean)
extends FetchSession.RESP_MAP_ITER {
var nextElement: util.Map.Entry[TopicPartition,
FetchResponse.PartitionData[Records]] = null
override def hasNext: Boolean = {
while ((nextElement == null) && iter.hasNext) {
val element = iter.next()
val topicPart = element.getKey
val respData = element.getValue
val cachedPart = session.partitionMap.find(new CachedPartition(topicPart))
val mustRespond = cachedPart.maybeUpdateResponseData(respData,
updateFetchContextAndRemoveUnselected)
if (mustRespond) {
nextElement = element
// Don't move partition to end of queue if we didn't actually fetch data
// This should help avoid starvation even when we are filling the fetch
response fully while returning metadata for these partitions
if (updateFetchContextAndRemoveUnselected && respData.records != null
&& respData.records.sizeInBytes > 0) {
session.partitionMap.remove(cachedPart)
session.partitionMap.mustAdd(cachedPart)
}
} else {
if (updateFetchContextAndRemoveUnselected) {
iter.remove()
}
}
}
nextElement != null
}{noformat}
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