Why direct memory should be smaller than llap size

[邓志华]

when I use 'hive --service llap --name llap_service --size 4g --cache 4g' to 
generate the startup script for llap,  an exception throws:

java.lang.IllegalArgumentException: Cache size (4.00GB) has to be smaller than 
the container sizing (4.00GB)
        at 
com.google.common.base.Preconditions.checkArgument(Preconditions.java:92)
        at 
org.apache.hadoop.hive.llap.cli.LlapServiceDriver.run(LlapServiceDriver.java:207)
        at 
org.apache.hadoop.hive.llap.cli.LlapServiceDriver.main(LlapServiceDriver.java:104)

is there any reasons why should follow this rule?  

Thanks
Zhihua Deng