[SPARK-32506][TESTS] Flaky test: StreamingLinearRegressionWithTests

[huaxingao]

What changes were proposed in this pull request?

The test creates 10 batches of data to train the model and expects to see error on test data improves as model is trained. If the difference between the 2nd error and the 10th error is smaller than 2, the assertion fails:

FAIL: test_train_prediction (pyspark.mllib.tests.test_streaming_algorithms.StreamingLinearRegressionWithTests)
Test that error on test data improves as model is trained.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/spark/spark/python/pyspark/mllib/tests/test_streaming_algorithms.py", line 466, in test_train_prediction
    eventually(condition, timeout=180.0)
  File "/home/runner/work/spark/spark/python/pyspark/testing/utils.py", line 81, in eventually
    lastValue = condition()
  File "/home/runner/work/spark/spark/python/pyspark/mllib/tests/test_streaming_algorithms.py", line 461, in condition
    self.assertGreater(errors[1] - errors[-1], 2)
AssertionError: 1.672640157855923 not greater than 2

I saw this quite a few time on Jenkins but was not able to reproduce this on my local. These are the ten errors I got:

4.517395047937127
4.894265404350079
3.0392090466559876
1.8786361640757654
0.8973106042078115
0.3715780507684368
0.20815690742907672
0.17333033743125845
0.15686783249863873
0.12584413600569616

I am thinking of having 15 batches of data instead of 10, so the model can be trained for a longer time. Hopefully the 15th error - 2nd error will always be larger than 2 on Jenkins. These are the 15 errors I got on my local:

4.517395047937127
4.894265404350079
3.0392090466559876
1.8786361640757658
0.8973106042078115
0.3715780507684368
0.20815690742907672
0.17333033743125845
0.15686783249863873
0.12584413600569616
0.11883853835108477
0.09400261862100823
0.08887491447353497
0.05984929624986607
0.07583948141520978

Why are the changes needed?

Fix flaky test

Does this PR introduce any user-facing change?

No

How was this patch tested?

Manually tested

[huaxingao]

cc @srowen @viirya

[srowen]

Seems fine, or loosening the tolerances

[SparkQA]

Test build #127152 has finished for PR 29380 at commit df3f21e.

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

[viirya]

Ok for me too. If it's still flaky, simply loosening the tolerances like @srowen said.

[huaxingao]

@srowen @viirya Thanks for taking a look. I will merge to master. Does this need to go into 3.0.1 too?

[dongjoon-hyun]

+1. Looks reasonable.
Also, +1 for having this on branch-3.0.

[huaxingao]

Merged to master/3.0. Thanks everyone!

[HyukjinKwon]

Nice, @huaxingao!

[cloud-fan]

thanks for fixing it!