perf: streamline recursion in mapAsyncPartitioned #32031
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leviramsey
commented
Aug 10, 2023
| @@ -127,7 +127,13 @@ private[akka] final case class MapAsyncPartitioned[In, Out, Partition]( | |||
| // dropped from per-partition queue, to be able to execute next, but result is kept in the | |||
| // main linear buffer for when out is ready | |||
| buffer.dropOnlyPartitionHead(partition) | |||
| dropCompletedThenPushIfPossible(partition) | |||
|
|
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| if (perPartition > 1 && buffer.usedInPartition(partition) > perPartition) { | |||
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The special case for perPartition == 1 does mean that we assume that any future started as a consequence of the dropOnlyPartitionHead above will wait until that async callback triggers.
Not only do we save the cost of introspecting the partition buffer's used, we also save the cost of an almost certainly pointless iteration of peeking and checking for completion.
patriknw
approved these changes
Aug 14, 2023
johanandren
approved these changes
Aug 14, 2023
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In
mapAsyncPartitionedwhenever a future in a given partition completes we dequeue the longest prefix of the partition's queue which has completed before we potentially emit elements downstream and demand more elements from upstream.When
perPartitionis low (say 1 or 2), this doesn't delay demanding more elements too long and if there are unstarted futures we get a head start. However there can be situations with largerperPartitionvalues where this process materially delays demanding new elements. Additionally, in the situation where there are no unstarted futures in a partition, we still eagerly dequeue more elements even though there's no gain from starting them earlier.This change introspects the partition queue: if after dequeuing, there are no unstarted futures for the partition, the stage will move into emission mode rather than attempt to dequeue more.