[kentonv]

Probably the most common example is sendfile() for writing file contents out to a socket without reading them into userspace:

https://man7.org/linux/man-pages/man2/sendfile.2.html

[mrfox321]

Isn't that the opposite? That is, bypassing user space, not kernel space?

[kentonv]

Oh, hmm, yeah, perhaps OP meant something more like using raw sockets to get packets directly into userspace without relying on the kernel to arrange them into streams?

I'm not very familiar with that though.

[arter4]

Yes, I knew about sendfile() but I wasnt't aware of any web server using that (though I know Kafka uses it).

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.

[nh2]

Pretty much all modern Linux web servers support sendfile(). Examples:

* 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

[kentonv]

I'd expect most serious web servers support it. I've written one that does (workerd), it's not too hard.

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.

[loeg]

Sendfile isn’t kernel bypass.