A very interesting slide deck comparing sync and async IO

[deadalnix via Digitalmars-d]

On Friday, 4 March 2016 at 03:14:01 UTC, Ali Çehreli wrote:
> I imagine that lost cache is one of the biggest costs in thread 
> switching. It would be great if a thread could select a thread 
> with something like "I'm done, now please switch to my reader". 
> And that's exactly one of the benefits of fibers: two workers 
> ping pong back and forth, without much risk of losing their 
> cached data.
>
> Is my assumption correct?
>
> Ali

The minimal cost of a context switch is one TLB miss (~300), one 
cache miss (~300) and iret (~300). But then, you usually don't do 
context switch for nothing, so some work has to be done in the 
context switch. This is where it gets very dependent on which 
system call you use.

During its work, the system call would have evicted various cache 
lines and TLB entries to put its own data there. That means that 
after the context switch is done, part of your cache is gone, and 
you'll need some time to start again running full speed. You may 
think that it is the same with a library call, and to some extent 
it is, but much worse: as kernel and userspace do not share the 
same address space and access write, so you typically get way 
more trashing (you can't reuse TLB entries at all for instance).

Actual numbers will vary from one chip to the other, but the 
general idea remains.