You notice it when web servers are doing kernel bypass to for zero-copy, low-latency networking, or database engines throw away the kernel's page cache to implement their own file buffer.
With mmap, you get to avoid thinking about how much data to buffer at once, caching data to speed up repeated access, or shedding that cache when memory pressure is high. The kernel does all that. It may not do it in the absolute ideal way for your program but the benefit is you don't have to think about these logistics.
But if you're already writing intense systems code then you can probably do a better job than the kernel by optimizing for your use case.
Then I found out Apache supports it via the EnableSendfile directive. Nice.
>This directive controls whether httpd may use the sendfile support from the kernel to transmit file contents to the client. By default, when the handling of a request requires no access to the data within a file -- for example, when delivering a static file -- Apache httpd uses sendfile to deliver the file contents without ever reading the file if the OS supports it.
That said, it's tricky to use if the server also does TLS termination... then you need kTLS, which is a much bigger can of worms.
* nginx: [1] * Haskell webserver module: [2] * caddy: [3]
[1]: https://nginx.org/en/docs/http/ngx_http_core_module.html#sen... [2]: https://hackage.haskell.org/package/warp-3.3.28/docs/Network... [3]: https://github.com/caddyserver/caddy/pull/5022
I'm not very familiar with that though.
You'll find DPDK mentioned a lot in the networking/HPC/data center literature. An example of a backend framework that uses DPDK is the seastar framework [2]. Also, I recently stumbled upon a paper for efficient RPC networks in data centers [3].
If you want to learn more, the p99 conference has tons of speakers talking about some interesting challenges in that space.
I do wonder what trend is going to win: bypass the kernel or embrace the kernel for everything?
The way I see it, latency decreases either way (as long as you don't have to switch back and forth between kernel and user space), but userspace seems better from a security standpoint.
Then again, everyone is doing eBPF, so probably the "embrace the kernel" approach is going to win. Who knows.
That said, I'm not sure many people write webservers in DPDK, since the Kernel is pretty well suited to webservers (sendfile, etc.). Most applications that use kernel-bypass are more specialized.
That may be acceptable for your purposes, or it may not.