Re: running tcpdump seems to block disk operations on loaded systems

[Guy Harris <guy () alum mit edu>]

On Dec 17, 2011, at 12:37 PM, Nikola Ciprich wrote:

> When starting tcpdump (using either -i any or -i xxx) on system with
> higher disk load, it sometimes happens that system seems to hang almost immediately and 
> loadavg value shoots to  really bad numbers (almost 200 for example).

So that's CPU.  That means...

> Is there anything I should check, or any hint on how could I find the reason of this
> problem?

...that the first thing I'd do is run top to see what processes are chewing up the CPU, and if tcpdump is chewing up a 
significant amount of CPU time, possibly also run strace (you say "-i any", so this is presumably Linux, and your email 
address further supports that; thus, strace should be available, at least if you install it) against tcpdump to see if 
it's doing anything that involves system calls and what they are.

Unfortunately, both of those involve top or strace using the CPU, so they'd have to compete with whatever's burning up 
the processor.-
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