1) Not enough memory
2) Too slow disks
which as far as I can see aren't helped by sendfile. They also
say "CPU limitations are rarely encountered except in very large
caches". The same doc describes how to use clusters for large
caches.
But too slow disks and not enough memory are directly
related problems. If the kernel has frequently used
disk files in the buffer cache, it still needs to copy
the data to the user space server, then copy it back
out onto a socket. With sendfile(), it can directly
send data from the buffer cache without any user space
involvement. In addition, we no longer need user
space buffers in the server, cutting down on useless
"memory duplication", allowing for more buffer cache.
With sendfile and a length parameter, user space need
only get involved when it needs to inject data into or
modify data going through an fd stream, which
practically eliminates the need for any user space
buffers. This will help just about any task I can
think of.
Zach Amsden
amsden@andrew.cmu.edu
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