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UBC and page replacement

Hi, I've sent a similar message to netbsd-users@ list, but didn't get
any responses, maybe somebody on tech-kern@ knows the answer?

I've been looking at the following paper:

"UBC: An Efficient Unified I/O and Memory Caching Subsystem for NetBSD"

However I'm not clear about what strategy NetBSD uses for page
replacements. For example, a web server does mainly read operations
from filesystem, so the kernel will eventually fill up buffer cache.

When the buffer cache is full and there is no free RAM, the kernel
would need to replace some of the pages. How does it decide what pages
to replace? Does it look at the usage pattern or the underlying file
size? Does it try to steal pages from a single large (say 2GB) file,
rather than stealing them from many small files?

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