Load Adaptation: Options for Basic Services Vance Maverick ADAPT Bologna Feb. 13, 2003

Problem statement Client and service are separated Client does not have exclusive access to service Service performance varies unpredictably depending on load How can the system adapt? –Specifically, how can a CS adapt to variations in load on a service it uses?

Overview Goal: discussion between WP1 and other groups Review options in architecture draft –Load reporting –Dynamic adjustment of support level –Capacity reservation Conclusion and open questions

Load reporting Service tells client how busy it is –Information may be cached in an intermediate node Scenarios –Client chooses between competing services –Client predicts its own computation time Goals of the two parties –Client wants to know how fast its request will be served –Service wants to protect its current clients from overload

How to represent load? Percentage load (artificial, unhelpful) Client counts –Number currently connected –Estimated number of further clients that can be handled without degradation Average recent response times –Affected by complexity of computation, network between client and server –May not be available if service just came up

Dynamic support adjustment After connection, client may request “more support” from service –Representation? Not explicit number of replicas Does not provide any guarantee to client or service May lead to waste –Client of slow service will demand more replicas, even if it doesn’t help Realistic?

Capacity reservation QoS contract between client and service If service is overloaded, then service can’t begin Advantages to both parties –Client receives guarantee of performance –Service can protect its other clients Clear relationship to billing (i.e. institutional contracts) Can be extended to dynamic negotiation

How to represent capacity? Explicit number of allocated replicas –requires client to understand system arch Multiples of fictitious reference system (“horsepower”) –simplistic: relationship between two systems not linear Requests per second –ignores computation complexity Trust

Conclusion First two options not very useful –Load reporting alone –Dynamic adjustment of service level Capacity reservation –QoS representation is an open question –Is this in the scope of ADAPT?