presto-main/src/main/java/io/prestosql/operator/MarkDistinctOperator.java/MarkDistinctOperator/updateMemoryReservation
class MarkDistinctOperator:    private boolean updateMemoryReservation()
    {
        // Operator/driver will be blocked on memory after we call localUserMemoryContext.setBytes().
        // If memory is not available, once we return, this operator will be blocked until memory is available.
        localUserMemoryContext.setBytes(markDistinctHash.getEstimatedSize());
        // If memory is not available, inform the caller that we cannot proceed for allocation.
        return operatorContext.isWaitingForMemory().isDone();
    }