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Old 04-16-2007, 12:04 AM   #1
Registered: Jan 2002
Distribution: Ubuntu 12.04.2 (Precise)
Posts: 89

Rep: Reputation: 16
Smokeping shows latency increase with new kernel!

Hello helpers.

I've got a problem that I can't quite place. Let me set the scene:

On Tuesday last week at ~9pm I upgraded the kernel on a Red Hat Linux 9 (Shrike) box from 2.4.20-30.9 to 2.6.19-2. (In so doing, I managed to break RPM and now I can no longer use the RPM tool - but that will have to wait for another thread.)

Anyway, I normally run Smokeping on this server and what I noticed after the kernel upgrade is rather peculiar - see the image below:

Can anyone explain what may have brought this about? The process I followed when installing the new kernel was as follows:

make mrproper
make menuconfig
- enabled all iptables related items
- enabled all QoS related modules
- ensured that ext3 was not compiled as a module
make bzImage && make modules && make modules_install
make install
vi /etc/lilo.conf
- ensured that previous kernel was still the default
lilo -R new-kernel

Everything works as expected, but for this peculiar smokeping behaviour.

I then run the following command:

ping -i 0.000000000001 -c 50

and got the following output:

64 bytes from icmp_seq=1 ttl=128 time=0.520 ms
64 bytes from icmp_seq=2 ttl=128 time=1.20 ms
64 bytes from icmp_seq=3 ttl=128 time=0.342 ms
64 bytes from icmp_seq=4 ttl=128 time=1.19 ms
64 bytes from icmp_seq=5 ttl=128 time=0.473 ms
64 bytes from icmp_seq=6 ttl=128 time=1.20 ms
64 bytes from icmp_seq=7 ttl=128 time=0.439 ms
64 bytes from icmp_seq=8 ttl=128 time=1.19 ms
64 bytes from icmp_seq=9 ttl=128 time=0.432 ms

As you can see, the latency between each pair of responses alternates between a high and low reading - isn't this rather peculiar behaviour??? I've not seen anything like it before and would appreciate any help.

I am not willing to go to previous kernel as I upgraded so that I could make use of the PPTP helpers for NAT'ing PPTP packets to a Windows 2K3 server.

Old 04-16-2007, 01:21 AM   #2
LQ Guru
Registered: Nov 2004
Location: San Jose, CA
Distribution: Ubuntu
Posts: 8,507

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It might be something to do with interrupt handling or some other issue. Out of curiosity, is there any practical performance impact?
Old 04-16-2007, 02:34 AM   #3
Registered: Jan 2002
Distribution: Ubuntu 12.04.2 (Precise)
Posts: 89

Original Poster
Rep: Reputation: 16
Thanks Matir.

I have not seen any negative impact on performance yet - though I might mention that I have not been looking. I've had tunnel vision on this one issue wondering what could have caused it!

If you could provide pointers on what I need to check to verify that it is indeed interrupts that are behind this issue, please let me know. I'm even willing to consider going back to previous kernel (not for more than 2hrs) if only to get to the bottom of it.



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