本节内容来源于PGer的一个问题: Q: 为什么checkpointer后台进程使用这么多内存? ``` [pg12@localhost ~]$ ps -aux|grep postgres pg12 1506 0.0 1.8 753156 34704 ? Ss 10:30 0:00 /appdb/xdb/pg12beta1/bin/postgres pg12 1507 0.0 0.0 187304 1472 ? Ss 10:30 0:00 postgres: logger pg12 1509 0.0 24.1 754120 452672 ? Ss 10:30 0:04 postgres: checkpointer pg12 1510 0.0 28.4 753292 533184 ? Ss 10:30 0:08 postgres: background writer pg12 1511 0.0 0.9 753260 18012 ? Ss 10:30 0:11 postgres: walwriter pg12 1513 0.0 0.0 189420 1448 ? Ss 10:30 0:00 postgres: archiver last was 00000001000000020000005C pg12 1514 0.0 0.0 189556 1324 ? Ss 10:30 0:00 postgres: stats collector pg12 1515 0.0 0.1 753688 1984 ? Ss 10:30 0:00 postgres: logical replication launcher pg12 2281 0.0 0.0 112660 972 pts/3 S+ 16:38 0:00 grep --color=auto postgres [pg12@localhost ~]$ grep 'shared' $PGDATA/postgresql.conf shared_buffers = 512MB # min 128kB #shared_memory_type = mmap # the default is the first option dynamic_shared_memory_type = posix # the default is the first option #wal_buffers = -1 # min 32kB, -1 sets based on shared_buffers #shared_preload_libraries = '' # (change requires restart) ``` 比如上例,共享内存设置为512MB,在2G内存的机器上,checkpointer和background writer进程占用了差不多25%的内存大小. A: > resident" RAM of a postgres subprocess is often just be the fraction of shared_buffers it's read/written. checkpointer must necessarily read all dirty pages from s-b and write out to disk (by way of page cache), so that's why its RSS is nearly 32GB. And the recovery process is continuously writing into s-b. 原因是checkpointer和bg writer需要从shared memory中读取数据(从s-b到page cache)并刷到磁盘上,因此给人占用大量内存的假象. **参考资料** [Re: checkpoint and recovering process use too much memory](https://www.postgresql.org/message-id/MWHPR13MB138955AB55FAAE0004A5AC5CAA500%40MWHPR13MB1389.namprd13.prod.outlook.com)