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Resources available on the main archive impose some limitations on the number of requests processed simultaneously: these resources are basically the number of Unix processes MARS can start and the number of drives available to read data from tapes. The cost of a request is evaluated on arrival in terms of number of fields requested and their location, either on disk or on tape. The more tapes a request has to access, the more likely that it will be queued. It is possible that requests will be queued if the MARS server reaches certain limit of concurrent requests.
The queueing queuing system handles priorities: the request with the highest priority will be chosen next for execution. This priority is computed from the age of a request since it entered the queue. An artificial mechanism of ageing requests allows to prioritise operational work or interactive users. You can view the MARS queue online.
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