Scalability based Admission Control of Real Time Channels

This paper reports our continuing e orts and initial results with the problem of admission control in real time networks This problem was rst addressed by the Tenet group and their approach was based on the assumption that the link level scheduling was EDD Earliest Due Date based Our work departs from this assumption by addressing the problem in the context of any arbit rary dynamic xed priority link level scheduling Our approach is based on extending a result we have derived in a di erent context viz Task Scalability It involves assessing the current capacity of a link in terms of its ability to accommodate scale to new channels This assessment called the admittance measure is then heur istically compared against the tra c requirements of the newly requested channel to decide its admissibility A simulation study was performed to study the e ective ness of our approach in improving both utilization of the link and admissibility of channels Further we demon strate the relevance of our heuristic by observing that it reduces to the Tenet schedulability test for the case of EDD Background and Introduction Admission control is the mechanism by which mul tiple real time connections can simultaneously share the resources of a packet switching network without result ing in congestion The connections require guaranteed quality of service QoS that is initially at connection set up agreed upon Admission control comes into play when a new real time connection is being requested A real time connection request is accompanied with a QoS list that describes its requirements Popular QoS require ments in the literature of distributed real time systems are throughput latency or deadline packet loss tol erance etc This work is partly funded by a grant from NASA NAG A popular model used to characterize a real time con nection is a real time RT channel An RT channel i is characterized by a source and destination travers ing multiple links and such parameters as packet inter generation time period gi packet size message size mi and end to end deadline di Derivation of the route associated with the connection involves considering both static network topology and dynamic already existing channels information In this paper we assume that the route is given we are currently investigation the routing problem also The mechanism used to determine the admissibility of a real time channel involves verifying at each intermediate link along the route in turn whether the RT channel s QoS requirements can be guaranteed If a channel s requirements can be met at each of the inter mediate links then we can accept the channel If however the channel s requirements cannot be met at any of the intermediate link then we can reject the channel In fact the rst such link that deems the channel inadmissible is su cient to con rm that the channel would not be admissible Of all the QoS metrics the latency deadline metric bears the most relevance to real time systems We therefore restrict ourselves to this metric In order to test whether a channel s requirements will be met at an intermediate link we have to know its dead line and its period at that link Finding the period is straightforward according to the phase adjustment mechanism Phase adjustment is a mechanism which allows us to extend the end to end period given by the inter packet generation time directly to the individual links Therefore For a given RT channel its frequency of arrival at an intermediate link is the same as its fre quency of occurrence at the source Deadline derivation unlike period derivation is a tougher issue Since the service time of the channel on each of the links is the same one way to derive the deadlines would be to divide the slack given by the di erence between the end to end deadline and the total transmission time of the message of the RT channel equally among the intermediate links However if one wishes one can use a more sophistic ated heuristic to derive these deadlines We are presently also investigating other heuristics Now the problem of nding the admissibility of an RT channel is equivalent to solving the admissibility at each of the in termediate links Therefore from here onwards when we refer to the admissibility of an RT channel we mean its admissibility at an intermediate link The question of admissibility at a link can be described by the following test Admissibility Test Does the addition of the new channel to the already established channels using this link cause either the new channel or one of the already established channels to miss their deadline Di erent approaches to the admission control prob lem in real time systems will di er in the way the above question is answered Therefore a study in admission control reduces to the study of this test Any answer to this question must consider i The scheduling mech anism used at the link and ii Is preemption allowed The Tenet approach assumes the local scheduling mech anism for messages to be based on the Earliest Due Date EDD While a dynamic scheduling mechanism such as EDD gives good performance its both costly and also results in more preemption The problem of schedulabil ity of messages is analogous to the problem of schedulab ility of real time tasks in which context EDD was rst derived However unlike processors where saving the state and restoring it on being re enabled of a preemp ted process is simple the same does not hold in the con text of messages To the extent possible therefore any approach to message scheduling must minimize preemp tion Having said that the above schedulability test is ana logous to task schedulability we make the following ob servations regarding task schedulability Schedulability analysis of a task set is expensive time wise The only exception being EDD which has a simple computation that involves checking if the resultant on addition of the new channel util ization is less than or equal to For static xed priority schedulers analyzing the schedulability of a task set involves verifying for each task in the order of priority whether it meets its deadline The rate monotonic scheduler RMS a static xed priority scheduler has a simple schedulability test It involves checking if the resultant utilization is less than or equal to n n where n is the number of tasks This condition is su cient but not necessary for schedulability when deadlines are allowed to be less than or equal to task periods The cost involved in doing a precise schedulability test as described by Lehoczky in is unacceptable in the case of message scheduling This is primarily due to the fact that such a test has to be performed in real time while the channel is being established Therefore any question of admissibility has to be answered in a reasonably short time