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Kernel VS application file caching

As far as I know Unix kernels will transparently cache files into any 
available memory to speed up future I/O on those files.

For applications like Internet servers, which serve many static files from 
disk, is there any point in implementing file caching at application level? It 
seems like you would end up with 2 copies of the same data - one copy cached 
by kernel, another copy cached by application.

Another question is this: is it possible for userlevel applications to tell 
kernel not to cache certain files? For example application might want to keep 
frequently accesses .html .jpeg files in memory and not cache rarely accessed 
huge files, which would evict many other files when loaded into core.

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