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question about "show proc cpu"

Ray Han
Level 1
Level 1

Hi all,

I have a question about the output of "show proce cpu"

Router#sh proc c
CPU utilization for five seconds: 8%/3%; one minute: 3%; five minutes: 2%

  1         128       193        663  0.00%  0.00%  0.00%   0 Chunk Manager   
   2        4400    586915          7  0.00%  0.00%  0.00%   0 Load Meter      
   3      382504   7811772         48  0.00%  0.00%  0.00%   0 OSPF Hello   

in above output

first line : five seconds: 8%/3%

The first number indicates the total,

the second number indicates the percent of CPU time spent at the interrupt level.

I don't know about the percent of cpu time spent for interrupt level exactly.

how the router use a cpu for interrupt level?

I have no idea about interrupt level?

as i known,

when router receive a packet in some interface,

the interface interrupt a cpu to process the packet.

cpu usage for interrupt is in proportion to received packets?

is there anyone explain about this concept more detail?

Thanks

1 Accepted Solution

Accepted Solutions

vvasisth
Level 1
Level 1

Interrupt Driven CPU Utilization

•Examples
- CEF and fast packet switching
- ACL processing
- QoS processing
- Alignment or spurious memory access errors on MIPS and PPC based platforms (not on 68K based platforms).  Check the command show align
- Spurious Interrupts, check the output of the show stacks command.
•Often a function of traffic rate and configured features in the switching path.
•Get more information with CPU profiling.

6500 - Interrupt Driven CPU utilization
Main causes:

Incorrect / Inappropriate switching path.
---CEF Disabled – globally or on specific interface.
---TCAM overflow.
---ACL Merge Algorithms / logging.


Router / Switch overloaded with traffic.

Memory issues like Spurious access, alignment errors etc.

regards,

Varun

View solution in original post

4 Replies 4

Calin C.
Level 5
Level 5

Hard question you have there

I don't have very detailed knowledge  about how the CPU interrupt is generated, and I also didn't found any  documentation in regard to this, but look what I know:

as i known,

when router receive a packet in some interface,

the  interface interrupt a cpu to process the packet.

This is true, but not in every cases. If in the routing process the switching part is the one called "process switched" then yes, for every packet arriving on the interface an interruption is sent to the CPU. If it's direct proportional with the number of packets arriving on the interface, I would have to say yes. In this case, if you have a lot of traffic, then you are stuck with 100% CPU utilisation. That's why process switched method is not so common this days.

So, this method will involve the CPU all the time.

Next method is fast-switching which still uses the CPU, but only for the first packet that arrive on the interface with a certain destination. After this packet has been forwarded the information how to reach that destination is stored in the fast-switching cache, so all other packets with the same destination will be forwarded based on the cached information. No interruption to CPU is needed anymore. In this case I believe that there is no direct proportion between packets entering the interface and interrupts sent to the CPU.

Then it is CEF, which builts the FIB table in which it stores all the information needed to forward packets, so no interrupt to the CPU is needed. It will consume more memory (due to FIB table), compared with the previous method, but less CPU. This the most common method used today. Totally no proportion between packets entering on the interface and CPU interrupts.

Of course there are other processes on the router that can request CPU interrupts, like in any PC, but I believe that the answer above is related to your question.

I hope this helps!

Thanks for your reply.

as i known,

if router's interface receive a packet

the interface media controller(or asic or relative h/w) interrupts a cpu it receives a packet firstly.

(for every packet it received)

in other words,

the switching method(process, fast, cef switching) will be determinded after interrupt process.

above my opinion is not exact and may be wrong. i'm searching about this.

if i find the key, will post it.

Thanks.

vvasisth
Level 1
Level 1

Interrupt Driven CPU Utilization

•Examples
- CEF and fast packet switching
- ACL processing
- QoS processing
- Alignment or spurious memory access errors on MIPS and PPC based platforms (not on 68K based platforms).  Check the command show align
- Spurious Interrupts, check the output of the show stacks command.
•Often a function of traffic rate and configured features in the switching path.
•Get more information with CPU profiling.

6500 - Interrupt Driven CPU utilization
Main causes:

Incorrect / Inappropriate switching path.
---CEF Disabled – globally or on specific interface.
---TCAM overflow.
---ACL Merge Algorithms / logging.


Router / Switch overloaded with traffic.

Memory issues like Spurious access, alignment errors etc.

regards,

Varun

Ray Han
Level 1
Level 1

Hi Varun

Thanks for your reply.

it's a good help to understand it.

Best regards

Sanghee

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