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Cluster Scheduling for Explicitly-speculative Tasks (CMU-PDL-04-112)
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posted on 01.12.2004by David Patrou
A process scheduler on a shared cluster, grid, or supercomputer that is informed which submitted tasks are possibly unneeded speculative tasks can use this knowledge to better support increasingly prevalent user work habits, lowering user-visible response time, lowering user costs, and increasing resource provider revenue.
Large-scale computing often consists of many speculative tasks (tasks that may be canceled) to test hypotheses, search for insights, and review potentially finished products. For example, speculative tasks are issued by bioinformaticists comparing DNA sequences, computer graphics artists rendering scenes, and computer researchers studying caching. This behavior — exploratory searches and parameter studies, made more common by the cost effectiveness of cluster computing — on existing schedulers without speculative task support results in a mismatch of goals and suboptimal scheduling. Users wish to reduce their time waiting for needed task output and the amount they will be charged for unneeded speculation, making it unclear to the user how many speculative tasks they should submit.
This thesis introduces ‘batchactive’ scheduling (combining batch and interactive characteristics) to exploit the inherent speculation in common application scenarios. With a batchactive scheduler, users submit explicitly labeled batches of speculative tasks exploring ambitious lines of inquiry, and users interactively request task outputs when these outputs are found to be needed. After receiving and considering an output for some time, a user decides whether to request more outputs, cancel tasks, or disclose new speculative tasks. Users are encouraged to disclose more computation because batchactive scheduling intelligently prioritizes among speculative and non-speculative tasks, providing good wait-time-based metrics, and because batchactive scheduling employs an incentive pricing mechanism which charges for only requested task outputs (i.e., unneeded speculative tasks are not charged), providing better cost-based metrics for users. These aspects can lead to higher billed server utilization, encouraging batchactive adoption by resource providers organized as either cost- or profit-centers.
Not all tasks are equal — only tasks whose outputs users eventually desire matter — leading me to introduce the ‘visible response time’ metric which accrues for each task in a batch of potentially speculative tasks when the user needs its output, not when the entire batch was submitted, and the batchactive pricing mechanism of charging for only needed tasks, which encourages users to disclosure future work while remaining resilient to abuse. I argue that the existence of user think times, user away periods, and server idle time makes batchactive scheduling applicable to today’s systems.
I study the behavior of speculative and non-speculative scheduling using a highly-parameterizable discrete-event simulator of user and task behavior based on important application scenarios. I contribute this simulator to the community for further scheduling research.
For example, over a broad range of realistic simulated user behavior and task characteristics, I show that under a batchactive scheduler visible response time is improved by at least a factor of two for 20% of the simulations. A batchactive scheduler which favors users who historically have desired a greater fraction of tasks that they speculatively disclosed provides additional performance and is resilient to a denial-of-service. Another result is that visible response time can be improved while increasing the throughput of tasks whose outputs were desired. Under some situations, user costs decrease while server revenue increases. A related result is that more users can be supported and greater server revenue generated while achieving the same mean visible response time. A comparison against an impractical batchactive scheduler shows that the easily implementable two tiered batchactive schedulers, out of all batchactive schedulers, provide most of the potential performance gains. Finally, I demonstrate deployment feasibility by describing how to integrate a batchactive scheduler with a popular clustering system.