A very interesting article from a FreeBSD kernel developer. Learn what the kernel can do for you (and better than you):

Varnish allocate some virtual memory, it tells the operating system to back this memory with space from a disk file. When it needs to send the object to a client, it simply refers to that piece of virtual memory and leaves the rest to the kernel. [...] Varnish doesn't really try to control what is cached in RAM and what is not, the kernel has code and hardware support to do a good job at that, and it does a good job.