Well, the advantage is it's already implemented. :)
It might be better to use median, since there are only as many numbers
to average as there are nodes connected. It wouldn't be any cheaper than
a geometric mean as it involves a sort (and a memory allocation if
you're not careful).
On Tue, Apr 18, 2006 at 01:34:14AM +0300, Jusa Saari wrote:
> Geometric mean is still vulnerable to this, just less so than arithmethic
> mean. Besides, it is likely much more expensive computationally - or do
> modern day chips come with instructions to calculate the Nth root (could
> be, I haven't really examined their capabilities) ?
>
> Is there any particular advantage geometric mean has over median ?
>
> On Mon, 10 Apr 2006 20:06:25 -0700, Ian Clarke wrote:
>
> > I believe another way to suppress the influence of outliers is to use a
> > geometric mean, ie. rather than adding N numbers and dividing my N, you
> > multiply the N numbers, and take the Nth root of the result.
> >
> > Ian.
> >
> > On 10 Apr 2006, at 18:02, John B?ckstrand wrote:
> >
> >> I was thinking about the mean ping times that are used for handling node
> >> load as I understood it, and had the idea to use a median instead.
> >>
> >> Using an average means one single node can artificially push the avg
> >> ping very high, unless you have more than 13 other active peers (the
> >> influence from a 30s ping (is this the max, btw?) with 13 peers is
> >> 2000ms, so even with ping=0 for the other 13 peers, the mean ping will
> >> be 2000ms. This could be a very easy way to DoS the network, no? Just
> >> create a very popular node (read: SkarX) and then answer pings/packets
> >> very slowly.
> >>
> >> A median suppresses outliers, and outliers in a latency calculation will
> >> mostly be, I suspect, those with high latency due to DoS:ing or just
> >> having very bad connections (or absurd overutilization). Another form of
> >> latency outliers is having local nodes, which could have very low
> >> latency not affected by internet connection load.
> >>
> >> Anyway, attached is a patch to print the median aswell. I had guessed
> >> that a few nodes would have "insanely" high ping and be one reason for
> >> overloads, and that seems to be the case atleast on my darknet node,
> >> which has more active connections. The difference isnt major or anythng
> >> though: mean is 353ms, and median is 311ms, caused mainly by one node
> >> with a latency of about 3000ms. I will try to graph these two aswell for
> >> some extended period of time for my node.
> >>
> >> ---
> >> John B?ckstrand
> >> Index: src/freenet/node/NodePinger.java
> >> =================================================================== ---
> >> src/freenet/node/NodePinger.java (revision 8505) +++
> >> src/freenet/node/NodePinger.java (working copy) @@ -1,5 +1,9 @@
> >> package freenet.node;
> >>
> >> +import java.util.ArrayList;
> >> +import java.util.Collections;
> >> +import java.lang.Double;
> >> +
> >> import freenet.support.Logger;
> >>
> >> public class NodePinger implements Runnable {
> >> @@ -30,20 +34,32 @@
> >> void recalculateMean(PeerNode[] peers) {
> >> int peerCount = 0;
> >> double total = 1.0;
> >> + ArrayList pingTimes = new ArrayList();
> >> for(int i=0;i<peers.length;i++) {
> >> PeerNode peer = peers[i];
> >> if(!peer.isConnected()) continue;
> >> peerCount++;
> >> double avgPingTime = peer.averagePingTime();
> >> + pingTimes.add(new Double(avgPingTime));
> >> double avgThrottledPacketSendTime =
> >> peer.throttledPacketSendAverage.currentValue();
> >> double value = Math.max(avgPingTime,
> >> avgThrottledPacketSendTime);
> >> Logger.minor(this, "Peer: "+peer.getPeer()+",
> >> avgPingTime:
> >> "+avgPingTime+", avg throttled send time: "+avgThrottledPacketSendTime);
> >> total *= value;
> >> }
> >> +
> >> + Collections.sort(pingTimes);
> >> +
> >> + for(int i=0; i<pingTimes.size(); i++){ +
> >> Logger.minor(this, "at " +
> >> i + " " + pingTimes.get(i)); + }
> >> +
> >> if(peerCount > 0) {
> >> total = Math.pow(total, 1.0 / peerCount); meanPing =
> >> total;
> >> - Logger.minor(this, "Mean ping: "+meanPing+"ms"); +
> >> + double median = ((Double)
> >> pingTimes.get((pingTimes.size()-1)/
> >> 2)).doubleValue();
> >> +
> >> + Logger.minor(this, "Mean ping: "+meanPing+"ms, median:
> >> " + median +
> >> "ms");
> >> }
> >> }
> >> }
> >> _______________________________________________ Devl mailing list
> >> Devl at freenetproject.org
> >> http://emu.freenetproject.org/cgi-bin/mailman/listinfo/devl
>
>
> _______________________________________________
> Devl mailing list
> Devl at freenetproject.org
> http://emu.freenetproject.org/cgi-bin/mailman/listinfo/devl
>
--
Matthew J Toseland - toad at amphibian.dyndns.org
Freenet Project Official Codemonkey - http://freenetproject.org/
ICTHUS - Nothing is impossible. Our Boss says so.
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