M. Colajanni, P.S. Yu, D.M. Dias,
``Analysis of task assignment policies in scalable distributed Web-server system'',
IEEE Transaction on Parallel and Distributed Systems, vol. 9, no. 6,
A distributed multi-server Web site can provide the scalability
necessary to keep up
with growing client demand at popular sites. Load balancing of these
distributed Web-server systems, consisting of multiple Web servers for document
retrieval and a Domain name server (DNS) for address resolution, opens
interesting new problems. In this paper, we investigate the effects of
using a more active DNS which, as an atypical centralized
scheduler, applies some scheduling strategy in routing the requests to the
most suitable Web server.
Unlike traditional parallel/distributed systems in which a centralized
scheduler has full control of the system, the DNS controls only a very small
fraction of the requests reaching the
multi-server Web site. This peculiarity,
especially in the presence of highly skewed load, makes
it very difficult to achieve acceptable load balancing and avoid
overloading some Web server.
This paper adapts traditional scheduling algorithms to the DNS, proposes
new policies, and examines their impact under different scenarios.
Extensive simulation results show the advantage of strategies that make
scheduling decisions on the basis of the domain that originates
the client requests, and limited server state information
(e.g. whether a server is overloaded or not).
An initially unexpected result is that using detailed
server information, especially based on history,
does not seem useful in predicting the future load,
and can often lead to degraded performance.
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