tcpdump vs libpcap : CPU usage shooting high for two simultaneous captures on wireless monitor interfaces

[abhinav narain <abhinavnarain10 () gmail com>]

hi everyone,
I have written my own data collection tool, for custom needs using
libpcap for wireless interfaces(2.4,5 GHz) on a router.

I could not find any flag in tcpdump that i can collect only x
number of mgmt packets, y number of control packets and
the rest data packets.

The issue i face is this :
When I run two instances of tcpdump on each interface, it has 5-10% cpu
usage
on the router.
When I use my tool written using libpcap:
When I run it on one interface (say 2.4 GHz), I get 5-10% CPU usage (output
on top),
but when I run another instance of the tool on the other interface(5 GHz
lets say),
both the processes start eating 100% of CPU all the time.

Can someone explain this behavior ?
I was thinking, if both the process's recv() call was spinning on some
shared
resource (buffer)/some kernel thread ... but why does that not happen for
tcpdump ?
I have literally read tcpdump/pcap code before, but I hadn't notice anything
special.
Can someone help me debug this problem ?

-
Abhinav
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