Re: [9fans] cfs

[erik quanstrom <quanstro@quanstro.net>]

> Why is cfs in the kernel and not a userlevel program?

i don't know.  but by putting cfs in the kernel, you can use the kernel's
built-in concurrency.  cfs predates the thread library.

> I guess there might be performance issues also, though these would
> be swamped by the performance improvment of plugging in CFS in the
> first place (IMHO).

why guess?  we can get a good approximation.  the difference
is a round trip to user space which has bandwidth and latency
costs.  i used dd and ramfs to simulate.

		userspace			kernel
		b/w		latency		b/w		latency
i7 2666		460 MB/s	1680µs		3900 MB/s	118µs
Xeon5000 1859	156 MB/s	1205		868		265
AMD64 2604 be	142 MB/s	2511µs		2600		332
AMD64 2004	135 MB/s	2510		2600		324

if you're limited by 1gbe, the bandwidth would be good
but the latency is spoiled.  if you're limited by 10gbe,
the vodka's weak, too.

- erik