presto-main/src/main/java/io/prestosql/SystemSessionProperties.java/SystemSessionProperties/getAggregationOperatorUnspillMemoryLimit
class SystemSessionProperties:    public static DataSize getAggregationOperatorUnspillMemoryLimit(Session session)
    {
        DataSize memoryLimitForMerge = session.getSystemProperty(AGGREGATION_OPERATOR_UNSPILL_MEMORY_LIMIT, DataSize.class);
        checkArgument(memoryLimitForMerge.toBytes() >= 0, "%s must be positive", AGGREGATION_OPERATOR_UNSPILL_MEMORY_LIMIT);
        return memoryLimitForMerge;
    }