Performance of multiple nmaps

["Miguel Dilaj" <nekromancer () lycos com>]

Hi all,

New to the list here.
I've been trying to find information on system tuning (and/or nmap tuning) for running multiple instances of nmap.
So far I've found information on performance tuning or usage to run 1 or 2 nmaps on very big networks, but I'm in the 
opposite situation.
We need to run multiple instances (5-30) of nmap on small network ranges, in some cases with -p1-65535 (and -T0), and 
we found that after having the 5th or 6th nmap the system starts to slow down a lot, and the output of vmstat shows 
alarmingly high numbers on the "r" column.
lsof -i shows of course all the SYN-SENT, but in one particular system with 28 nmaps running we found that even that is 
being stopped from happening.
Of course if I use renice on all nmaps I can end with a pretty usable console, but I'm afraid that this can further 
impact nmap's performance. I also though about limits.conf, but it's not appropriate because we need to run nmap as 
root, and limits.conf doesn't really help to impose limits on processes from users with UID 0.
The boxes running the nmaps are not old crap, are quite new (2+GHz) Xeons, with SCSI disks and plenty of RAM.
ANY hints and recomendations are welcome.
TIA!
Regards,

Miguel


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