Neil Conway wrote: > Tom Arthurs wrote: > >> Yes, shared buffers in postgres are not used for caching > > > Shared buffers in Postgres _are_ used for caching, they just form a > secondary cache on top of the kernel's IO cache. Postgres does IO > through the filesystem, which is then cached by the kernel. Increasing > shared_buffers means that less memory is available for the kernel to > cache IO -- increasing shared_buffers has been shown to be a net > performance loss beyond a certain point. Still, there is value in > shared_buffers as it means we can avoid a read() system call for hot > pages. We can also do better buffer replacement in the PG shared buffer > than the kernel can do (e.g. treating IO caused by VACUUM specially). >
As I recall, one of the performance problems with a large shared_buffers is that there are some commands which require looking at *all* of the shared buffer space. So the larger it gets, the longer those functions take. >> My biggest challenge with solaris/sparc is trying to reduce context >> switching. > > > It would be interesting to see if this is improved with current sources, > as Tom's bufmgr rewrite should have hopefully have reduced this problem. > These might be what was fixed with Tom's rewrite. I don't really know. John =:-> > -Neil > > ---------------------------(end of broadcast)--------------------------- > TIP 4: Don't 'kill -9' the postmaster
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