[c-nsp] CPU utilization and performance - 7500

[Rodney Dunn]

Well, I wish it was that simple.

Let me try to explain it this way.

On any software based forwarding CPU you can
only do things so fast.  Sure you can optimize
the code and be as efficient as possible but there
must be a limit at some point.

The confusion on the 75xx comes from the fact that
a VIP cpu can be at 100% CPU when in reality the
VIP isn't that busy doing what I call real work
(ie: switching packets).  That's the entire conversation
about rx-side buffering.

http://www.cisco.com/en/US/products/hw/modules/ps3088/products_tech_note09186a0080094345.shtml

Now, let's assume you are doing WFQ or some fancy queueing
on all outbound interfaces such that no rx-side buffering is
happening.  Then in theory your CPU is a pretty close representation
of what the CPU is actually doing switching packets.

It's not a great idea to have it running at 90+% because
you still have some background work that has to be done.

What you will see people talk about is in lab testing if
they are trying to confirm a NDR (no drop rate) then ignore
what the CPU says.  Just measure that what goes in does
come out irregardless of what the CPU is at.

That's not practical in the real world because if you run
the CPU that high you will have control plane problems most
likely.

I usually start to watch pretty closely when the CPU starts
to get up there around the 60-70% range on a constant basis.
People ask me why that value.  I respond with 6 years
of TAC experience. :)

Rodney



On Mon, Oct 04, 2004 at 09:19:40AM +0530, Amol Sapkal wrote:
> While reading one of the pages posted on the list, I came across this:
> 
> ?Total CPU utilization shouldn't be used as a measure of the ability
> of the router to switch more packets. On Cisco 7500 routers, Versatile
> Interface Processors (VIP) and Route/Switch Processors (RSP) don't
> report linear CPU utilization. Close to half of the switching
> packet-per-second power comes after 90 - 95 percent CPU utilization.
> 
> 
> Can anyone explain what does the above mean? Does it mean that a 7500
> can perform well even after a cpu load of 95%?
> 
> Source: http://www.ciscodata.com/troubleshooting-high-cpu-utilization-on-cisco-routers.asp#symptons
> 
> (taken from previous post)
> 
> -- 
> Warm Regds,
> 
> Amol Sapkal
> 
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