Each yarn container only use 1 vcore even if taskmanager.numberOfTaskSlots is set

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Hi Experts,
        I am running Flink 1.7.1 program on Yarn 2.7, the 
taskmanager.numberOfTaskSlots is set to 4. The parallelism.default is set to 8. 
When the program is running, 3 yarn containers is launched, but each of them 
only use 1 vcore, I think by default the number of vcores is set to the number 
of slots per TaskManager.
        Because it can not get enough CPU resources, the CPU load may be high. 
So how can this problem be fixed? 
        Thanks a lot for your help.

Best
Henry