strange ping response times...
Luigi Rizzo
rizzo at iet.unipi.it
Wed Apr 11 10:41:18 UTC 2012
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
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