How is the memory usage of containers controlled?

[Yuzhang Han]

Hi,
I am curious about how YARN containers control their memory usage. Say, 
I have a MR job, and I configure that every map task should be assigned 
a 1 GB container, and every reduce task a 1.5 GB one. So, when YARN runs 
the containers, how is it ensured that all map containers use less than 
1 GB memory and all reduce containers less than 1.5 GB? Is container 
memory control realized via -Xmx arguments in the scripts generated by 
YARN to invoke the MR program in the containers?

Thank you.