Spark validatesPortableRunner test OOM

First ~50 tests run okay, and then this happens: 

org.apache.beam.sdk.transforms.CombineTest$WindowingTests > testGlobalCombineWithDefaultsAndTriggers FAILED
{{ java.lang.AssertionError at CombineTest.java:1209}}

org.apache.beam.sdk.transforms.CombineTest$WindowingTests > testCombineGloballyLambda FAILED
{{ java.lang.RuntimeException at CombineTest.java:1391}}
{{ Caused by: java.lang.RuntimeException at CombineTest.java:1391}}
{{ Caused by: java.util.concurrent.ExecutionException at CombineTest.java:1391}}
{{ Caused by: java.lang.OutOfMemoryError}}
{{}}

....

Most subsequent tests OOM after that.