[SPARK-26264][CORE]It is better to add @transient to field 'locs' for class ResultTask.

[10110346]

What changes were proposed in this pull request?

The field 'locs' is only used in driver side for class ResultTask, so it is not needed to serialize when sending the ResultTask to executor.
Although it's not very big, it's very frequent, so we can add transient for it like ShuffleMapTask.

How was this patch tested?

Existed unit tests

[SparkQA]

Test build #99660 has finished for PR 23216 at commit b3ede8b.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[wangyum]

retest this please

[SparkQA]

Test build #99674 has finished for PR 23216 at commit b3ede8b.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[srowen]

Are you sure it's even a field in the class? it looks like it's only used to define this:

  @transient private[this] val preferredLocs: Seq[TaskLocation] = {
    if (locs == null) Nil else locs.toSet.toSeq
  }

I'd expect Scala would not generate a field. Indeed the thing it is used to make is transient.

[HyukjinKwon]

why is it var BTW?

[10110346]

Are you sure it's even a field in the class? it looks like it's only used to define this:

  @transient private[this] val preferredLocs: Seq[TaskLocation] = {
    if (locs == null) Nil else locs.toSet.toSeq
  }

I'd expect Scala would not generate a field. Indeed the thing it is used to make is transient.

Yeah, it would not generate a field, thanks @srowen
By the way, is it better to remove transient for ShuffleMapTask?

[srowen]

I think just leave it. The @transient in ShuffleMapTasks's locs is just superfluous here, not sure it's worth changing.

[10110346]

Ok, I will close this PR, thank you very much