[c-nsp] Varying CPU usage on 881

[Pelle]

Hi.

I'm doing some performance testing on a Cisco 881, but am a bit
puzzled over a varying CPU usage. I don't see this behavior in for
example a 1841. Have somebody got similar results? And (hopefully) an
explanation of the variation? The variation in it self is no problem,
but it makes it harder to do a performance evaluation.

In the tests, the same constant bidirectional bitstreams [1] are
applied on both routers, and the routers are running the same
configuration [2].

"show process cpu history" [3] on the 881 running 12.4(20)T [4]:

    333222221111133333222222222222222333332222222222444442222222
    555999999999944444999991111188888777771111133333000001111155
100
 90
 80
 70
 60
 50
 40 ***                              *****          *****
 30 ********     **********     **********          *****     **
 20 ************************************************************
 10 ************************************************************
   0....5....1....1....2....2....3....3....4....4....5....5....6
             0    5    0    5    0    5    0    5    0    5    0
               CPU% per second (last 60 seconds)

Here the CPU load varies between 19 and 40 percent.


The 1841 running 12.4(20)T:

    555555555555555555555555555555555555555555555555555555555555
    444443333344444444444444444444444443333344444333333333333333
100
 90
 80
 70
 60
 50 ************************************************************
 40 ************************************************************
 30 ************************************************************
 20 ************************************************************
 10 ************************************************************
   0....5....1....1....2....2....3....3....4....4....5....5....6
             0    5    0    5    0    5    0    5    0    5    0
               CPU% per second (last 60 seconds)

The 1841 is rock stable with an CPU load of 53-54 percent [5].


[1] Traffic consists of:
- 8.0 Mbps @ 64 Byte/pkt downstream (14285 pps)
- 8.8 Mbps in 3 classes upstream (2825 pps total)

[2] Configuration:
- Downstream: just routing
- Upstream: H-QoS with parent shaper (8.0M), 3 classes in child (1 LLQ)

[3] I know the "show process cpu history" shows both interrupt based
load (traffic) and "other" load. Monitoring the SNMP variables
cpmCPUTotalMonIntervalValue and cpmCPUInterruptMonIntervalValue shows
that the "other" load never is more than 2-3 percent.

[4] The same results are achieved with 12.4(20)T1

[5] It's a bit surprising that the 881 outperforms the 1841, but
nevertheless welcome :)

-- 
Pelle