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Author Topic: ICMP and the kernel  (Read 6321 times)

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OdO

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ICMP and the kernel
« on: August 18, 2011, 10:39:20 PM »
Hi All,

Just a little question  ;D

I'm not developper, I want to understand when AIX system is in Hang state (blackout, no TTY, no network access) ... why icmp still work. Why, after take a dump,  the stack for all processor not display icmp.

ICMP is in a Layer 3 (network) ... does kernel execute icmp thread reply ???
what is the mechanism to thread icmp protocol   ???

Best regards,
Little Odo 

OdO

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Re: ICMP and the kernel
« Reply #1 on: August 21, 2011, 12:51:12 AM »
Hi All,

From Infocenter:
ICMP is embedded in the kernel, and no application programming interface (API) is provided to this protocol.

I don't understand why my server respond to "ping" and was in HANG state.
the icmp reply must be executed by processor ?

Best regards,
Little Odo 

Michael

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Re: ICMP and the kernel
« Reply #2 on: September 06, 2011, 09:21:15 PM »
There are several levels of "hanging". The easiest way to get a UNIX server to hang is to have root authority and use the setpri() call to set your process priority to a value less than 40.

The kernel operates with priorities at less than 40, and can go quite low.

ICMP is handled by the IP stack processing in the kernel - and to pick a number - lets say priority 15. I do not have access to source code, so it is just a number I choose. Using 15 as an example, what ever priority your "hanging" application was using, it was greater than "15", so the interrupt could be processed and ICMP responded.

So, you computer was not hanging the in the "PC" sense of the word, but for some reason you were not getting any processor cycles scheduled to regular processes.

AIX has a mechanism to help with "hanging".

Read up on "Managing System Hang" --> http://publib.boulder.ibm.com/infocenter/aix/v6r1/topic/com.ibm.aix.baseadmn/doc/baseadmndita/syshang.htm?resultof=%22%73%68%64%61%65%6d%6f%6e%22%20

Mark Taylor

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Re: ICMP and the kernel
« Reply #3 on: September 27, 2011, 03:12:13 PM »
If you can ping a system, then it is not hung, its a performance issue .. collect perfpmr and send to IBM ..

From the dump, you can work out all sorts of things, cpu starvation, paging space contention, physical memory contention, kernel memory exhaustion, network memory contention, locks, incomplete i/o etc etc ...

HTH
Mark Taylor