To SMP or not to SMP

[Barney Cordoba]

--- On Tue, 1/8/13, Mark Atkinson <atkin901 at gmail.com> wrote:

> From: Mark Atkinson <atkin901 at gmail.com>
> Subject: Re: To SMP or not to SMP
> To: freebsd-net at freebsd.org
> Date: Tuesday, January 8, 2013, 11:29 AM
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> On 01/07/2013 18:25, Barney Cordoba wrote:
> > I have a situation where I have to run 9.1 on an old
> single core
> > box. Does anyone have a handle on whether it's better
> to build a
> > non SMP kernel or to just use a standard SMP build with
> just the
> > one core? Thanks.
> 
> You can build a SMP kernel, but you'll get better
> performance (in my
> experience) with SCHED_4BSD on single cpu than with ULE.
> 

I've tested the 2 schedulers on an SMP kernel with 1 core. I don't have
a 1 core system to test with so I'm using an E5520 with  1 core enabled.

Bridging a controlled test (curl-loader doing a web-load test with 100 
users that consistently generates 870Mb/s and 77Kpps, I see the following:

top -SH

ULE:

idle: 74.85%
kernel {em1 que} 17.68%
kernel {em0 que} 5.86%
httpd: .49%  

4BSD:

idle: 70.95%
kernel {em1 que} 18.07%
kernel {em0 que} 4.44%
httpd: .93%

Note that the https is a monitor I'm running.

so it appears that theres 7% of usage missing (all other apps show 0%
usage). 

If i had to guess just looking at the numbers, it seems that 4BSD might 
do better with the interrupt level stuff, and not as good with user 
level context switching. I think they're close enough to stick with ULE
so I can just use a stock kernel.

One thing that bothers me is the idle sits at 100% when other tasks are 
registering values under light loads, so it's certainly not all that 
accurate.

BC