Details
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Bug
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Status: Resolved
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Major
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Resolution: Fixed
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None
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None
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None
Description
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 even if the fetch offset is lower than the fetch upper bound. 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. Consumers may be affected more than replica fetchers, however they often remove partitions with fetched data from the next fetch request and this may be helping prevent starvation.
I believe we should only reorder the partition fetch priority if data is actually returned for a partition.
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
// Example POC change:
// 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
}
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