About extra memory on yarn mode

[Sea]

Hi all:
I have a question about why spark on yarn will need extra memory
I apply for 10 executors, executor memory 6g,  I find that it will allocate 1g 
more for 1 executor, totally 7g for 1 executor.
I try to set spark.yarn.executor.memoryOverhead, but it did not help.
1g for 1 executor is too much, who knows how to adjust its size?