How to monitor Postgres real memory usage

Hi All

  I am a Database DBA. I focus on PostgreSQL and DB2.

  Recently. I experience some memory issue. The postgres unable allocate memory. I don't know how to monitor Postgres memory usage.

 I try to search some  document. But not found any useful information.

 This server have 16G memory. On that time. The free command display only 3 G memory used. The share_buffers almost 6G.

 On that time. The server have 100 active applications.

 New connection failed. I have to kill some application by os command "kill -9"

The checkpoint command execute very slow. almost need 5-10 seconds.

  Is there any useful command to summary PostgreSQL memory usage ?

  How to analyse this memory issue ? Thanks for your help.

2022-05-23 17:42:51.541 CST,,,21731,,6288963b.54e3,8055,,2022-05-21 15:35:23 CST,,0,LOG,00000,"could not fork autovacuum worker process: Cannot allocate memory",,,,,,,,,""
2022-05-23 17:42:51.627 CST,,,21731,,6288963b.54e3,8056,,2022-05-21 15:35:23 CST,,0,LOG,00000,"could not fork new process for connection: Cannot allocate memory",,,,,,,,,""
2022-05-23 17:42:51.627 CST,,,21731,,6288963b.54e3,8057,,2022-05-21 15:35:23 CST,,0,LOG,00000,"could not fork new process for connection: Cannot allocate memory",,,,,,,,,""
2022-05-23 17:42:51.628 CST,,,21731,,6288963b.54e3,8058,,2022-05-21 15:35:23 CST,,0,LOG,00000,"could not fork new process for connection: Cannot allocate memory",,,,,,,,,""
2022-05-23 17:42:51.628 CST,,,21731,,6288963b.54e3,8059,,2022-05-21 15:35:23 CST,,0,LOG,00000,"could not fork new process for connection: Cannot allocate memory",,,,,,,,,""
2022-05-23 17:42:52.130 CST,,,21731,,6288963b.54e3,8060,,2022-05-21 15:35:23 CST,,0,LOG,00000,"could not fork new process for connection: Cannot allocate memory",,,,,,,,,""
2022-05-23 17:42:52.130 CST,,,21731,,6288963b.54e3,8061,,2022-05-21 15:35:23 CST,,0,LOG,00000,"could not fork new process for connection: Cannot allocate memory",,,,,,,,,""
2022-05-23 17:42:52.131 CST,,,21731,,6288963b.54e3,8062,,2022-05-21 15:35:23 CST,,0,LOG,00000,"could not fork new process for connection: Cannot allocate memory",,,,,,,,,""
2022-05-23 17:42:52.131 CST,,,21731,,6288963b.54e3,8063,,2022-05-21 15:35:23 CST,,0,LOG,00000,"could not fork new process for connection: Cannot allocate memory",,,,,,,,,""
2022-05-23 17:42:52.543 CST,,,21731,,6288963b.54e3,8064,,2022-05-21 15:35:23 CST,,0,LOG,00000,"could not fork autovacuum worker process: Cannot allocate memory",,,,,,,,,""