FAQ:
SPNP
Answers are in red.
The SPNP code is in blue
QUESTION 1:
From Vincent H Williams
<vhwillia@eos.ncsu.edu>
We
are working on an M/M/1/K project.
We tried to simulate but we couldn't get an output. We are sending you
our errors. Can you please help us? It is in postcript format.
You need to go
to the environment if you use the graphics and change the value for the length
of th simulation.
Click on
"Model Editor" then "Environment" and click on Simulation. I
you have typed your code with a textual environment, you need to define the
value
FOP_SIM_LENGTH
in the option function.
QUESTION 2:
From Srivatsan V sriv1@hotmail.com:
I have a doubt regarding
the "Length of each simulation iteration/batch" setting in "Model Editor->Environment->Simulation".
I understand that we need to set
"Required confidence interval for the simulation". I thought that
SPNP would run the simulation long enough for achieving that confidence. But, it requires us to enter
the length of simulation also. Am I missing something?
I would appreciate your
response
Note: I would like to
know if you could solve one example from the text book "Queueing Networks
and Markov Chains - Modeling and Performance Evaluation with Computer Science Applications"
- Page 88, Fig.2.15. Also, does SPNP have built-in support for non-preemptive and preemptive
priority systems or do I need to
write my own code for that
For the
steady-state measure, you should use Batch simulation. you should specify
FOP_SIM_ERROR for confidence tolerance and IOP_SIM_RUNS to -1 (Infinity).
For the
priority (preemptive and no-pre.), take a look at "policy" and
"affect" (or affected") keywords in SPNP manual.
QUESTION 3: inconsistent results of SPNP
From:
Yun Cui <ycui4@unity.ncsu.edu>
We tried to run the
SPNP, but have the inconsistent results under the same environment parameters.
Environment parameters:
simulation
method: Discrete event-simulation with batches number of batch: 1000
length
of each simulation iteration/batch: 1000
Model:
M/M/1 with arrival rate
0.25 and service rate 0.75
If we choose only one
analysis parameter: "probability that the given place is empty in
steady-state", we can get the correct result
NET:
==========================================================================
discrete places: 1
immediate transitions: 0
timed transitions: 2
constant input arcs: 1
constant output arcs: 1
constant inhibitor arcs:
0
variable input arcs: 0
variable output arcs: 0
variable inhibitor arcs:
0
==========================================================================
==========================================================================
TIME : 1.00E+03
==========================================================================
Simulation Analysis Result
Number of RUNS = 1000
==========================================================================
EXPECTED: Probability that the place
P0 is empty in steady-state(-1) =
6.4400e-01[6.0498e-01,6.8302e-01] @90%
If we choose more analysis parameters under the
same environment, results are different:
NET:
==========================================================================
discrete places: 1
immediate transitions: 0
timed transitions: 2
constant input arcs: 1
constant output arcs: 1
constant inhibitor arcs:
0
variable input arcs: 0
variable output arcs: 0
variable inhibitor arcs:
0
==========================================================================
==========================================================================
TIME : 1.00E+03
==========================================================================
Simulation Analysis Result
Number of RUNS = 1000
==========================================================================
EXPECTED: Expected # of tokens of
the place P0 in steady-state(-1) =
2.2120e+00[1.9001e+00,2.5239e+00] @90%
EXPECTED: Throughput of the
transition T0 in steady-state(-1)
=
1.0000e+00[1.0000e+00,1.0000e+00] @90%
EXPECTED: Throughput of the
transition T1 in steady-state(-1)
=
1.0680e+00[9.5093e-01,1.1851e+00] @90%
EXPECTED: Utilization for the
transition T0 in steady-state(-1) =
4.0000e+00[4.0000e+00,4.0000e+00] @90%
EXPECTED: Utilization for the
transition T1 in steady-state(-1) =
1.4240e+00[1.2679e+00,1.5801e+00] @90%
EXPECTED: Probability that the place
P0 is empty in steady-state(-1) =
2.5760e+00[2.4199e+00,2.7321e+00] @90%
It seems that the SPNP engine for LINUX
behaves not as I expected. The solution right now is to change the SPNP code
generated and have only ONE
Solve(INFINITY) in the AC_FINAL function
The probability of place empty changes from 0.64
to 2.57. What's the problem of this?
2. We are interested in the expected waiting time
of a priority queue, but there is no such analysis parameters in the tool. Do
you think it is
ok if we find the queue size and then divided it
by arrival rate, or add the output analysis parameters manually? What's your
recommendation
about it?
The first one is correct. Following the
Little’s law:
Response Time = Queue size / thoughput