04-07-2013 03:53 AM - edited 03-07-2019 12:41 PM
Hi,
I try to understand what packets do use the CPU at interrupt level.
I understand well what packets need to be processed level (in other words: software swiched), for example, packets destinated to the Sup720 itself.
Is this hardware switched packets which trigerred an interrupt on the CPU? Even if the packet is handled by ASIC, CPU still need to make a treatment?
What happens in case of card equipped with DFC: is the packet s 100% treated by a CPU on the card itself?
Thaks by advance by your help.
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04-13-2013 05:05 PM
Hi Pari,
High CPU utilization on an interrupt level is primarily caused by packets handled on interrupt level. Interrupts are generated any time a character is output from the console or auxiliary ports of a router.
Universal Asynchronous Receiver/Transmitters (UARTs) are slow compared to the processing speed of the router, so it is unlikely, though possible, that console or auxiliary interrupts can cause a high CPU utilization on the router (unless the router has a large number of tty lines in use).
There are several reasons for high CPU utilization due to interrupts:
To troubleshoot this potential issue, verify the following:
HTH
Regards
Inayath
*Plz rate all usefull posts.
04-10-2013 06:05 PM
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The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.
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Posting
See if the section Cisco Catalyst 6500: Day in the Life of a Packet, at the bottom of: http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/prod_white_paper0900aecd80673385.html, helps any.
The prior doesn't mention interrupts, but generally interrupts are generated by hardware that need the attention of the CPU. Often this attention is not per packet.
In Cisco routers, interrupt packet forwarding is a synonym for fast past forwarding in contrast to process forwarding.
04-10-2013 08:13 PM
It will depends on the ASIC, if it cant handle the packet it will be punted to the cpu, for example packets with IP options. Some other type of packets also need to be punted to the CPU like if ttl=1 or packets targeted to the CPU like routing protocols, ICMP.
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04-10-2013 09:34 PM
Hi,
1- Processes level vs Interrupt level
Processes level is the software switched.:
Proces switching means ARP/routing process/CDP process mainly IOS processes
Interupt level: CPU utlization due to interupt switching is caused due to packets which are not performed by the HW present on the data cards. Normal data pkts should not be processes towards the cpu.
2-
Is this hardware switched packets which trigerred an interrupt on the CPU? Even if the packet is handled by ASIC, CPU still need to make a treatment?
Once the Hardware table has been synchronized between the Sup SP card and DCEF cards on submodule then all the hardware forwarding will be direclty done by the module with dcef itself. The packets only will be punted to the cpu when there is no entry present on the cam/mac table entry.
3-
What happens in case of card equipped with DFC: is the packet s 100% treated by a CPU on the card itself?
I am not sure what you are looking for here. The L2 data will be pushed from the sup module to the DFC card hence all the layer 2 forwarding will be done direclty from the module itself. The packets will be punted to CPU only when it doesnt find any entry on the dfc card.
HTH
Regards
Inayath
04-13-2013 04:49 AM
Hi,
thanks for the answers.
Here is the current CPU usage on the C6500
CPU utilization for five seconds: 24%/9%
According to these values:
- 15% (24-9) are due to packets processed (ex: ARP, TTL 1, IP Option, etc ...)
- 9% of CPU is due to interrupts
I understand that these 9% are due to entries not in DFC or CFC (some of our cards have no DFC), and so are punted to CPU. However, I am a bit surprised that so many entries (according to the CPU usage) are not in the DFC.
04-13-2013 05:05 PM
Hi Pari,
High CPU utilization on an interrupt level is primarily caused by packets handled on interrupt level. Interrupts are generated any time a character is output from the console or auxiliary ports of a router.
Universal Asynchronous Receiver/Transmitters (UARTs) are slow compared to the processing speed of the router, so it is unlikely, though possible, that console or auxiliary interrupts can cause a high CPU utilization on the router (unless the router has a large number of tty lines in use).
There are several reasons for high CPU utilization due to interrupts:
To troubleshoot this potential issue, verify the following:
HTH
Regards
Inayath
*Plz rate all usefull posts.
04-14-2013 01:17 AM
Thanks, exactly what I looked for!
04-14-2013 06:08 AM
Disclaimer
The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.
Liability Disclaimer
In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.
Posting
CPU utilization for five seconds: 24%/9%According to these values:
- 15% (24-9) are due to packets processed (ex: ARP, TTL 1, IP Option, etc ...)
- 9% of CPU is due to interrupts
I understand that these 9% are due to entries not in DFC or CFC (some of our cards have no DFC), and so are punted to CPU. However, I am a bit surprised that so many entries (according to the CPU usage) are not in the DFC.
I don't believe your interpretation is correct.
The 9% should represent CPU involved with normal packet/frame forwarding using the main sup. Unlike a software based router, this CPU deals with the overhead of managing the hardware that performs the actual packet/frame forwarding. Basically, this CPU usage should reflect the Mpps being used for packet/frame forwarding.
Your 9% should indicate you're using that percentage of the 15 or 30 Mpps (NB: 15 vs. 30 depends on bus mode) forwarding capacity of your sup720.
As you mention you have some cards with DFCs, they too have CPU stats (have you looked at them?), and whatever they do locally would be off-loaded from the main sup.
The 15% is the sum of all other CPU processes, including, but not limited to, software based forwarding. I.e. some or all of this might be for packets "punted". The show CPU proc will show where this CPU is being used.
Now that I understand, your question about understanding interrupts seem to be more about investigation of your sup's CPU stats, you might also want to review: http://www.cisco.com/en/US/products/hw/switches/ps708/products_tech_note09186a00804916e0.shtml
Again, as you have DFCs, the DFC should off load some this kind of CPU loading.
04-28-2013 02:18 PM
Hi Joseph,
thanks for these informations.
I indeed noticed the CPU on DFC cards. Look at one of the DFCs, here is the output of process using the most of CPU:
253 19045216 139994087 136 3.51% 1.32% 1.35% 0 fw_lcp process
283 6198900 693729 8935 3.35% 0.67% 0.50% 0 L2 MAC oob sync
245 22710796 743782 30534 1.59% 1.61% 1.61% 0 Vlan Statistics
308 2538192 1572974 1613 0.87% 0.35% 0.22% 0 XDR LC Backgroun
355 5427248 2215600 2449 0.39% 0.40% 0.39% 0 CEF: IPv4 proces
It seems that some processes are linked to DFC itself (L2 MAC oob sync, perhaps fw_lcp too), for synchronization. Other processes like CEF: IPv4 process seems indeed permit to offload CPU on the suprvisor.
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