31
after receiving the reply from the Web server, it waits for a certain interval based on the
Pareto distribution, before it generates the next request.
Table 3.3. Distribution Models Used in Simulation
Parameter
Description
Value
Incoming request interval
Pareto distribution
P (x) =  k
/x
 +1
,   = 1.1
-(ln x- )2
Distribution of objects
Lognormal distribution
P (x) =
1
  e
2 2
,
x  2 
(body)
  = 1.16,   = 0.9, x   10
Distribution of objects
Pareto distribution
P (x) =  k
  
/((1 - (
k 
p
)
)   x
 +1
)
(tail)
,x > k
, k
= 10 p = 2000,   = 1.1
3.4.2
System Validation
To validate our simulator, we have measured the round trip time in a 16 node
Linux cluster to compare with the simulator results. Figure 3.5 shows the latency com 
parisons from the simulator and the measurements. To test the round trip latency, we
used a pingpong application that exchanges a fixed size of data between two nodes, be 
cause the pingpong test represents the network latency correctly without incurring the
system overhead. In addition, we vary the number of pingpong processes by 1, 2 and 4.
In Figure 3.5, we show two latency results since VIA supports two communication modes:
non blocking and blocking. In both the Figures 3.5, the results from the simulator match
well with these from the real experiments.












  

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