strategy – Routing to the server with the
smallest value of (m * n1
i
+ n2
i
), which is the
normalized number of requests. Here, n1
i
and
n2
i
represent the number of class-1 and class-2
requests in the server i (including the arriving
one).
The performance measures of practical interest
of these strategies, such as the loss probability, mean
waiting time in the service waiting queue, and mean
sojourn time are evaluated via simulation, and
compared with the values in case of the following
conventional strategies:
(c) IT strategy – Routing to each server in turn
according to a fixed order.
(d) RAND strategy – Routing to each server
randomly, with the same probability.
Based on the evaluation results, we discerned the
most suitable routing strategies for the loss system
and waiting system.
Under the PS rule, when a request either arrives
at or departs from the system, the remaining sojourn
time of other requests will be extended or reduced,
respectively. This extension or reduction of the
sojourn time is calculated using the number of
requests of each class and the priority ratio.
Employing a simulation program to execute these
events and calculations enables us to analyze the
performance of the prioritized limited multi-server
PS rule, which is realistic in a time-sharing system
(TSS) with a sufficiently small time slot.
In the simulation program, the arrival timer or
service timer of each request controls the simulation
clock. In each while loop of the simulation program,
one of these timers expire, and the abovementioned
arrival processing or service end processing is
executed. Simultaneously, the time duration until the
expiry of the next timer is pushed forward in order
to skip the insignificant simulation clock, thereby
shortening the total simulation time.
The PS rule, an idealization of quantum-based
RR scheduling at the limit where quantum size
becomes infinitesimal, has been the subject of many
papers (Kleinrock, 1967) (Fayolle and Mitrani,
1980) (Altman, Avrachenkov and Ayesta, 2006)
(Haviv and Val, 2008). A limited PS system and a
prioritized limited PS system, in which the number
of requests receiving service is restricted to a fixed
value, have been proposed as well. Further, the
performance of these systems has been analyzed
(Yamazaki and Sakasegawa, 1987) (Shikata,
Katagiri and Takahashi, 2011). Moreover, load-
balancing strategies for multi-class multi-server PS
systems with a Poisson input stream, and
heterogeneous service rates have been investigated
(Chen, Marden and Wierman, 2009) (Gupta, Balter,
Sigman, and Whitt, 2007) (Altman, Ayesta, and
Prabhu 2011). However, routing strategies in a
prioritized limited multi-server PS system have not
been investigated. Thus, the most suitable routing
strategy in the loss system or waiting system
remains to be discerned. Moreover, the influence
that the service facility capacity may have on the
mean sojourn time, the mean waiting time in the
service waiting queue, and the loss probability in the
prioritized limited multi-server PS system have not
been investigated.
2 PRIORITIZED LIMITED
MULTI-SERVER PS SYSTEM
In the prioritized limited multi-server PS system, an
arriving request enters the dispatcher, which routes
this request to each prioritized limited PS server
according to a predetermined strategy. Suppose that
there are two classes, and an arriving (class-1 or
class-2) request, which is routed to server i,
encounters n1
i
class-1 and n2
i
class-2 requests.
According to the prioritized limited multi-server PS
rule, if (m * n1
i
+ n2
i
≤ C
i
), class-1 requests
individually and simultaneously receive m / (m * n1
i
+ n2
i
) of the service facility capacity of server i,
whereas class-2 requests receive 1 / (m * n1
i
+ n2
i
)
of it. When a server meeting this condition (m * n1
i
+ n2
i
≤ C
i
) does not exist, the arriving request will
be queued in the corresponding class waiting room
prepared in the dispatcher or rejected. Here, m (1)
denotes the priority ratio, and C
i
(∞), the service
facility capacity of server i.
2.1 Routing Strategies
The evaluation model is shown in Figure 1. When a
request arrives at the dispatcher, first, it is checked
whether there exists a server that satisfies the
condition (m * n1
i
+ n2
i
) ≤ C
i
. Otherwise, the
arriving request is rejected or is placed in the service
waiting queue for each request prepared in the
dispatcher. If there are one or more servers in which
the value of (m * n1
i
+ n2
i
) is less than the C
i
, the
arriving request is routed to one of these servers
according to the predetermined routing strategy. In
the waiting system, when the service for a request
ends in one of the servers, another request is taken
from the service waiting queue and is routed to this
server. The following four routing strategies are
considered, and their performances are compared.
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