checkpoint process use too much memory

OperationalError: (psycopg2.OperationalError) could not fork new process for connection: Cannot allocate memory

could not fork new process for connection: Cannot allocate memory

when I checked the database, I found checkpoint process used too much memory, htop showed

VIRT: 171GB

RES: 171GB

SHR:4176MB

MEM%: 70%

Command: Posgres: checkpointer process

The postgresq.conf is

Shared_buffers = 4GB

Work_mem = 128MB

Maintenance_work_mem = 2GB

Wal_level = replica

Wal_buffer = 16MB

Checkpoint_completion_target = 0.7

How can I troubleshooting this error? Why checkpoint process used so much memory? it sounds like a memory leak issue in checkpointer process. 

Thanks for your help!

James