eejbyfeldt opened a new pull request, #38427:
URL: https://github.com/apache/spark/pull/38427
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### What changes were proposed in this pull request?
In FetchBlockRequest we currently store a `Seq[FetchBlockInfo]` as part of
the function `isRemoteAddressMaxedOut` (probably other places as well, but this
is the function that showd up in my profileling) we use the length of this Seq.
In scala 2.12 `Seq` is an alias for `scala.collection.Seq` but in 2.13 it an
alias for `scala.collection.immutable.Seq`. This means that in when for example
we call `toSeq` on a `ArrayBuffer` in 2.12 we do nothing and the `blocks` in
the `FetchRequest` will be backed by something with a cheap `length` but in
2.13 we end up copying the data to a `List`.
This PR solves this changing the `Seq` to and `IndexedSeq` and therefore
making the expectation of a cheap length function explicit. This means that we
some places will do an extra copy in scala 2.13 compared to 2.12 (was also the
case before this PR). If we wanted to avoid this copy we should instead change
it to use `scala.collection.IndexedSeq` so we would have the same types in both
2.13 and 2.12.
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### Why are the changes needed?
The performance for ShuffleBlockFetcherIterator is much worse on Scala 2.13
than 2.12. Have seen cases were the overhead of repeatedly calculating the
length is as much as 20% of cpu time (and could probably be even worse for
larger shuffles).
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### Does this PR introduce _any_ user-facing change?
No. I think the interface changes are only on private classes.
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### How was this patch tested?
Existing specs.
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