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
>
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