Glossary.md

@@ -73,6 +73,19 @@ Ping-Pong tests are a standard tool for network operators. They can thought of a
 
 Ping-pong testing is useful to measure network latency and bandwidth. It is also less susceptible to differences in speed in a given direction since only the time it took for the message to go in both directions is recorded.
 
+The central pingpong kernel of linktest between rank A and B is executed in the following way.   
+(num_warmup_messages = W, num_messages = N and size_messages = S)
+
+A sends W messages of size S to B  
+B sends W messages of size S to A  
+A takes the time t1  
+A sends N messages of size S to B  
+B sends N messages of size S to A  
+A takes the time t2 after the last receive finished  
+A writes average time (t2-t1)/2N to the sion file
+
+In the Matrix seen in Linktest reports this time corresponds to the entry in column A, ow B
+
 # Serial Testing
 By default Linktest tests as many connections as possible in parallel, this, however, can cause tests to interfere. This is sometimes desired, for example, if real-world network performance under a sustained network load is to be tested. In other cases peak performance without the interference of other parallel tests is desired. In this case serial testing is done, in which each connection between a pair of tasks is tested individually. This effectively serializes the test and will cause it to take significantly longer. Serial testing can be turned on in Linktest by using the `--serial-testing` command-line option.