High Interrupt CPU load on 7204 (NPE-G2) with only 96kpps

Answered Question
Aug 5th, 2008

Rapid Explanation:

I have 96% of CPU Interrupt utilization with only 96kpps.

Need I replace my routers with more powerful router ?

Long Explanation:

My test bench is compose by 2 routers (c7200p-advsecurityk9-mz.124-15.T4.bin) connected together with ATM PA-A6-OC3-MM.

I'm using a Smartbits tester connected by 3 Fastethernet to each routeur. I send a total of 96kpps (trame size: 312 bytes) over its 3 Fastethernet interface connected to the first router, and receiving them on its other 3 Fastethernet connected to the second router.

With a light configuration (only one static route and interface IP addressing), I have this result:

Result for 96kpps (half in on direction, and half in the other direction):

Total CPU Utilization: 41%

Process Utilization: 1%

Interrupt Utilization: 40%

I have already 40% of Interrupt Utilization.

Now I use my complete configuration file: service-policy input on 2 Fastethernet, and service-policy output on ATM, OSPF for routing between, all interface Policy and CEF switched.

Result for 96kpps:

Total CPU Utilization: 94%

Process Utilization: 0%

Interrupt Utilization: 94%

=> Great, all packets are CEF switched (service policy managed by hardware) and no packet dropped

But, I'm using a 7204 with NPE-G2 that theoretically support 2Mpps…

How to explain that I'm already obtain 94% of Interrupt Utilization with only 96kpps ?


I have this problem too.
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rene.avi Tue, 08/05/2008 - 11:36

Hi Gugus,

from what I've learned on the c-nsp mailing list the CPU stats are not linear with the load. This meets with my results here.

Taking in bidirectional packetflows:

100kpps = 49%

200kpps = 60%

400kpps = 78%

600kpps = 97%

Cheers, /Rene

gugus Thu, 08/07/2008 - 07:43

I've use the Smartbits tester for other performance test on the NPE-G2 for trying to found what excatlly cause this low perf.

And here are the resuts for the first test on a single Cisco 7204 using the two Gigabit Ethernet port.

The smartbits send a bidirectionnal flow of 2.8Mpps (UDP packet of 64bytes size) across the router.

With the default configuration, the router can forward at 1,72Mpps... (CPU was at 99%)

Now I test the service-policy impact by sending a unidirectionnal flow (UDP packet of 64bytes size, dscp 0).

- Light service policy output (match dscp cs6 and bandwitdh percent 100 on this matched traffic). The generated traffic doesn't match

: forwanding rate is 940Kpps

- My customized Service policy output. The generated traffic doesn't match : forwanding rate is 413Kpps

- Light service policy input (extended ACL witch match dscp cs6 and remark dscp to af32). The generated traffic doesn't match: forwanding rate is 750Kpps

- Light Service policy input light (standard ACL standard which match non existant host and remank dscp to af32). The generated traffic doesn't match: forwanding rate is 788Kpps

- Light service policy input (standard ACL standard which match any and remank dscp to af32). The generated traffic match, forwanding rate is 740 Kpps

- My light Service policy input. The generated traffic doesn't match, forwanding rate is 780Kpps

- My light Service policy input AND my customized output: forwanding rate is 325Kpps.

Impact of using Service policy:

=> The router performance is half with a single light service policy... Enabling service policy have a big impact!

With my customized configuration, I reduce the performance with a factor of 5.

This mean that with my configuration, I can manage about 160kpps in production (for limiting my CPU to reach 70%).

...But 160kpps mean about 400Mb/s Ethernet (340 bytes size packet) and I want to use only an OC3 ATM interface, then it should be OK.

I continue my tests and use now two routers 7204 connected by ATM OC3 (155Mb/s ATM).

Performance drop to 96kpps (about 280 Mb/s ATM bidirectionnal)... The use of an ATM interface consume a lot of ressource to the NPE-G2.

For having an useable throughtput on the field, with my ATM and QoS configuration, I need to use only 70Kpps (CPU at 70%).

If I remove my service policy output and input then I can use it at 101kpps (about 297Mb/s ATM bidirectionnal) this is the maximum of this link with a CPU at 70%.

These tests permit me to found that the NPE-G2 is still too light for using "normal" QoS service-policy on an ATM OC3 interface.


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