【ubuntu 10.04LTS】PostgreSQL のエラー

昨日、突然こんなエラーが。

2011-05-09 11:36:44 JST FATAL: ??????????????????: Invalid argument
2011-05-09 11:36:44 JST ??: ????????????shmget(key=5432001, size=37879808, 03600)????
2011-05-09 11:36:44 JST ???: ?????????PostgreSQL?????????????????????
SHMMAX??????????????????????????????????
?????????SHMMAX????????????????????????(?
?37879808???)??????PostgreSQL?shared_buffers?????(??4096)?
max_connections(??103)??????????????????
???????????SHMMIN???????????????????????
??????????????????SHMIN????????????????
???????????????PostgreSQL?????????????????
2011-05-09 11:36:50 JST FATAL: ??????????????????: Invalid argument

/etc/postgresql/8.4/main/postgresql.conf
の

shared_buffers = 32MB

をコメントアウトしたら、治った。なんだ？