A very interesting slide deck comparing sync and async IO

[Ola Fosheim Grøstad via Digitalmars-d]

On Sunday, 6 March 2016 at 13:21:41 UTC, Shachar Shemesh wrote:
> You have an iret whether you switched or not. Going into the 
> kernel and coming back has that, whether we switched or not. In 
> particular, since you need a system call in order to call 
> "read", "write" and "accept", these are there regardless.

Yes, rule-of-thumb seems to be that regular syscall can be 
expected to have 200-1000 cycles overhead, on a virtualized setup 
2000+ cycles.

But some low level interfaces allow you to submit multiple 
i/o-ops in a single system call.