ReduceByKey OOME workaround: 'sample and subtract', but rdd.subtract does not subtract.

[Stanley Burnitt]

Sampled RDDs contain the number of expected elements, but RDD.subtract(sample) 
does nothing; the returned RDDs have the same # of elements as the originalRDD.

Here is the code...

    long count0 = resultRDD.count();

    JavaPairRDD<KeyBytesWritable, AggregationWritable> r1 = 
resultRDD.sample(false, 0.50, 7);
    JavaPairRDD<KeyBytesWritable, AggregationWritable> r2 = 
resultRDD.subtract(r1);

    long count1 = r1.count();
    long count2 = r2.count();    /* count0 == count2 ?!  */

    System.out.printf("RESULT-RDD.COUNT=%d ... COUNT1=%d ... COUNT2=%d  \n", 
count0, count1, count2);

The printf output shows nothing was subtracted from the original rdd.    Am I 
using the subtract method incorrectly?

The  goal is to split a very large Pair-RDD containing 1 partition into a 
Pair-RDD containing many smaller partitions in hopes that this will fix OOM 
errors during a reduceByKey task.
After much tweaking of jvm GC parameters there is improvement, but eventually, 
the job just stalls out - no OOM, but no progress (jstack and jmap shows 
workers' new-gen is maxed out during RDD de-serialization).

Does anyone have any suggestions?

Thanks,
Stan