Eric,

If it takes twice as long to ping 600 things as it does 300 things, isn't that to be expected? After all, you are pinging twice as many items. I don't think this is "geometric" but linear. I don't know how many parallel paths fping has, but I suspect it is far less than either 300 or 600, so some queuing is going to occur.

But it is hard to calculate, too, because some of the IP addresses do not respond. If a host responds, then the pinger is free to move to the next one in the list. Otherwise it has to go through the "time out and retry" dance. So a non-responsive host could cause 5 or 6 seconds in delay until the pinger decides it is down and moves on. Since Fping runs things in parallel, it is the luck of the draw regarding which stream might get bogged down.

What is your ping interval? Pinging 1400 IP's in 40 seconds sounds pretty good to me -- you have a lot of room for growth. (Big Brother can't do this without some help) That is about 35 IPs per second. Round down to 30 per second, multiply by 300, and you could possibly monitor 9,000 IP's with this one server in a five minute window! After all, you only need to get to them all in the cycle before circling around and hitting them again. Some of the pay ware management systems actually try to space out their activity through the polling cycle so they don't hog the network themselves.

GLH

-----Original Message----- From: Schwimmer, Eric E *HS [mailto:EES2Y at hscmail.mcc.virginia.edu] Sent: Wednesday, April 26, 2006 4:50 PM To: hobbit at hswn.dk Subject: RE: [hobbit] fping tuning

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We're monitoring 1420 IPs in hobbit, and it takes fping ~40 seconds to go through them all:

Is that a number you get from the "bbtest" status or from running fping by hand?

Both. The values are fairly consistent, falling between somewhere in the 39-42 range.

Are you doing other network tests in Hobbit than just ping? Hobbit does the ping tests in parallel with the other tests.

We are doing other tests, but not many. Here's the relevent lines from our servers bbtest report:

TIME SPENT Event Starttime Duration TCP tests completed 1146086231.293585 1.211963 PING test completed (1434 hosts) 1146086271.488185 40.194600 PING test results sent 1146086271.523332 0.035147 TIME TOTAL 41.549643

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<snip> [root at hobbit fping]# fping -i5 -b12 -f ips -r1 -t250 -B2 -q -s

Are you using those parameters also on the FPING command in hobbitserver.cfg? Or is it just for your testing ?

This is just what I've been using for testing (the -f flag is root only and wouldn't work very well when used from within hobbit). The value of my FPING envvar in hobbitserver.cfg is "/usr/sbin/fping -i10 -b12". However the average difference in polling time betweeh the two is only 1 or 2 seconds.

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Now, this seems a bit lengthy to me. I mean, if the avg round trip time is 5.83 ms, and there are 1430 hosts, should the total time in transit for all hosts should be 8336ms, or 8 seconds... right?

No, it should be less - because fping pings several hosts in parallel.

You have "-i5" which causes a 5 ms delay between each ping. So that's (5/1000)*1430 = 7.15 seconds where it does nothing. The default setting is "-i25" - i.e. 5 times higher - which would actually match your ~40 seconds nicely.

Using the default delay interval (i.e. not specifying the -i flag when calling fping) causes the test to take much longer, on the order of 60 - 70 seconds. However, values of 15 or less passed to -i don't make much of a difference in polling time. (FWIW, fping doesn't let you specific a value for -i less than 10 unless you are root. I hacked the fping code to get around this so I could run it under hobbit with -i1, but I saw no difference in polling times using -i1 vs -i15).

Don't forget that there is probably also some time spent doing ARP lookups for all of these IP's. Unless you have "testip" on all of the entries in bb-hosts (or run bbtest-net with "--dns=ip"),

you'll also spend some time on DNS lookups (hint: use a local caching DNS server on the Hobbit server).

Yep, I have --dns=ip in the bbtest-net stanza of my hobbitlaunch.cfg (that makes a BIG difference), so I don't think it's a DNS resolution problem. In the testing fping command above, the -f flag specifies a file that is a list of all the IP addresses from my bb-host file, with not DNS names included, so I don't think it's a DNS problem. I feel like its some sort of concurrency issue within fping, since I can reproduce this latency completely outside of hobbit.

As a complete aside, we caching server for things outside of hobbit, and I've written a little script that monitors the bb-hosts file ( and all filed included from bb-hosts) and when it detects any changes, it will write a bind9 zone file to somewhere on disk. Its handy for making sure your bb-hosts is synced with your DNS. If anybody is interested in it, drop me a line (I'll have to 'pretty it up' first) and I'll posted it on my hobbit tools page for people to use.

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Even when I remove the hosts that aren't responding, the results on are par with those above.

Our polling interval is once every 60 seconds (which we want to maintain, because we like to know ASAP when something drops even one

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ping), so it's not a problem yet. We add hosts on a daily basis, however, so it will be a problem some time in the future and I'd like to fix it before it becomes a problem.

Well, the good news is that it probably won't become a problem. Because fping pings multiple hosts in parallel, the runtime doesn't change very much when you add more hosts.

Ah, so you would think ;) However, our graph in our bbtest column says otherwise; it has been climbing slowly but steadily since it started graphing data. You can also reproduce this by using a newline delimited list of IP addresses in a file, like I did above, and feeding it to fping. As you increase the number of IPs in the file, the poll time increases geometrically. For instance, when I poll 300 hosts:

<snip> [root at hobbit fpingtest]# fping -i5 -b12 -f 300ips -r1 -t250 -B2 -q -s

 300 targets
 298 alive
   2 unreachable
   0 unknown addresses

  12 timeouts (waiting for response)
 310 ICMP Echos sent
 298 ICMP Echo Replies received
   0 other ICMP received

0.24 ms (min round trip time) 2.38 ms (avg round trip time) 101 ms (max round trip time) 8.856 sec (elapsed real time) </snip>

vs when I poll 600 hosts: <snip> [root at hobbit fpingtest]# fping -i5 -b12 -f 600ips -r1 -t250 -B2 -q -s

 600 targets
 597 alive
   3 unreachable
   0 unknown addresses

  14 timeouts (waiting for response)
 611 ICMP Echos sent
 597 ICMP Echo Replies received
   0 other ICMP received

0.21 ms (min round trip time) 2.48 ms (avg round trip time) 100 ms (max round trip time) 16.144 sec (elapsed real time) </snip>

You can see that the ping time roughly doubles. This is bad :(

If it does become an issue, spread the load. Setup an extra server to do half the network tests, and configure your bb-hosts file with "NET:net-a" and "NET:net-b" tags on the hosts. Then you set BBLOCATION="net-a" on one box, and "BBLOCATION=net-b" on the other. Then they'll only test those hosts where the NET:... setting matches. Unless it's an OS limitation, you could probably do that on a single box and just have two instances of the [bbnet] task in hobbitlaunch.cfg - instead of running bbtest-net directly, they would run a shell-script which sets the BBLOCATION environment just before running bbtest-net.

I was thinking of doing something along these lines, however the bb-hosts file is maintained mostly by the (non-unix-savvy) staff here, using the bb-hostedit CGI script, and I'd rather not have them have to keep track of which host needed which NET tag, etc.

I've tested the network capabilities of this box using iperf as well as several concurrent ping floods, and it can send upwards of 10000+ ICMP packets per second (with successful replies from another host on the same 1000bT switch).

So this leads me to believe that it is a problem solely with fping; if they had a public forum or a mailing list, I'd be whining there instead of here. :) I can't say that I was expecting to find the 'magic bullet' for this problem here, but I was hoping that there might be some fping wizard out there some magic bullets to spare. Anywho, thanks for your thoughts, Henrik. I'll poke some more at the fping code and see if I can figure out whats going on (I doubt it); if not, I'll start working towards hacking together a load balancing script that will auto-add NET: tags to bb-hosts entry, or something along those lines.

Thanks, -Eric

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