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 Bäckstrand 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 Bäckstrand
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");
}
}
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