From owner-freebsd-current@FreeBSD.ORG  Wed Apr 11 10:41:18 2012
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Date: Wed, 11 Apr 2012 13:00:36 +0200
From: Luigi Rizzo <rizzo@iet.unipi.it>
To: Andre Oppermann <andre@freebsd.org>
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Cc: Barney Wolff <barney@databus.com>, current@freebsd.org, net@freebsd.org
Subject: Re: strange ping response times...
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On Wed, Apr 11, 2012 at 12:35:10PM +0200, Andre Oppermann wrote:
> On 11.04.2012 01:32, Luigi Rizzo wrote:
> >On Tue, Apr 10, 2012 at 07:05:00PM -0400, Barney Wolff wrote:
> >>CPU cache?
> >>Cx states?
> >>powerd?
> >
> >powerd is disabled, and i am going down to C1 at most
> >	>  sysctl -a | grep cx
> >	hw.acpi.cpu.cx_lowest: C1
> >	dev.cpu.0.cx_supported: C1/1 C2/80 C3/104
> >
> >which shouldn't take so much. Sure, cache matters, but the
> >fact is, icmp processing on loopback should occur inline.
> >
> >unless there is a forced descheduling on a select with timeout>  0
> >which would explain the extra few microseconds (and makes me worry
> >on how expensive is a scheduling decision...)
> 
> Things going through loopback go through a NETISR and may
> end up queued to avoid LOR situations.  In addition per-cpu
> queues with hash-distribution for affinity may cause your
> packet to be processed by a different core.  Hence the additional
> delay.

so you suggest that the (de)scheduling is costing several microseconds ?

Do we have something like yield() to measure how expensive is the
scheduler ? I ran some tests in a distant past and i remember numbers
of a few microseconds, but that was almost two gigahertz ago...

cheers
luigi