netdev - ss -p is much too slow

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Date:	Wed, 9 Jun 2010 11:42:38 -0700
From:	Steve Fink <sphink@...il.com>
To:	netdev@...r.kernel.org
Subject: ss -p is much too slow

I was recently working with a monitoring system, and one check
involved displaying what process(es) were listening on specific ports.
I was using a single call to ss -lntp and processing the output, but
it would often take several minutes to respond. netstat -lntp, on the
other hand, was immediate. I had a few thousand processes running on
the machine, and it could take 7 minutes or so.

On closer inspection, it appears that ss -p does a quadratic scan. It
rescans every entry in /proc/*/fd/* repeatedly (once per listening
port? per process? I don't remember what I figured out.)

I humbly suggest that this is not a good idea.

I am currently running iproute-2.6.27-2.fc10.i386, though my real test
case was on a different machine. I don't have a newer version to check
right now.

% strace ss -lntp |& fgrep /proc/3768/fd/10
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
readlink("/proc/3768/fd/10", "socket:[16214]"..., 63) = 14
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