checkpointer is a PostgreSQL server process which automatically executes checkpoints.
See mail thread "Separating bgwriter and checkpointer" for background information on why the
checkpointer was created.
The checkpointer process code is in src/backend/postmaster/checkpointer.c.
The function which actually performs the checkpoint is
CreateCheckPoint(), or if the PostgreSQL instance is in recovery,
CreateRestartPoint(). Both functions are located in src/backend/access/transam/xlog.c.