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"""MMC.py
An M/M/c queue
Jobs arrive at random into a c-server queue with
exponential service-time distribution. Simulate to
determine the average number in the system and
the average time jobs spend in the system.
- c = Number of servers = 3
- rate = Arrival rate = 2.0
- stime = mean service time = 1.0
"""
from SimPy.Simulation import *
from random import expovariate,seed
## Model components ------------------------
class Generator(Process):
""" generates Jobs at random """
def execute(self,maxNumber,rate,stime):
''' generate Jobs at exponential intervals '''
for i in range(maxNumber):
L = Job("Job "+`i`)
activate(L,L.execute(stime),delay=0)
yield hold,self,expovariate(rate)
class Job(Process):
''' Jobs request a gatekeeper and hold
it for an exponential time '''
NoInSystem=0
def execute(self,stime):
arrTime=now()
self.trace("Hello World")
Job.NoInSystem +=1
m.observe(Job.NoInSystem)
yield request,self,server
self.trace("At last ")
t = expovariate(1.0/stime)
msT.observe(t)
yield hold,self,t
yield release,self,server
Job.NoInSystem -=1
m.observe(Job.NoInSystem)
mT.observe(now()-arrTime)
self.trace("Geronimo ")
def trace(self,message):
FMT="%7.4f %6s %10s (%2d)"
if TRACING:
print FMT%(now(),self.name,message,Job.NoInSystem)
## Experiment data -------------------------
TRACING = False
c = 3 ## number of servers in M/M/c system
stime = 1.0 ## mean service time
rate = 2.0 ## mean arrival rate
maxNumber= 1000
m = Monitor() ## monitor for the number of jobs
mT = Monitor() ## monitor for the time in system
msT = Monitor() ## monitor for the generated service times
seed(333555777) ## seed for random numbers
server=Resource(capacity=c,name='Gatekeeper')
## Model/Experiment ------------------------------
initialize()
g = Generator('gen')
activate(g,g.execute(maxNumber=maxNumber,
rate=rate, stime=stime))
m.observe(0) ## number in system is 0 at the start
simulate(until=3000.0)
## Analysis/output -------------------------
print 'MMC'
print "%2d servers, %6.4f arrival rate,%6.4f mean service time"%(c,rate,stime)
print "Average number in the system is %6.4f"%(m.timeAverage(),)
print "Average time in the system is %6.4f"%(mT.mean(),)
print "Actual average service-time is %6.4f"%(msT.mean(),)
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