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Current instruction fetch policies in SMT processors are oriented towards optimization of overall throughput and/or fairness. However, they provide no control over how individual threads are executed, leading to performance unpredictability, since the IPC of a thread depends on the workload it is executed in and on the fetch policy used.From the point of view of the Operating System (OS), it is the job scheduler that determines how jobs are executed. However, when the OS runs on an SMT processor, the job scheduler cannot guarantee execution time constraints of any job due to this performance unpredictability.In this paper we propose a novel kind of collaboration between the OS and the SMT hardware that enables the OS to enforce that a high priority thread runs at a specific fraction of its full speed. We present an extensive evaluation using many different workloads, that shows that this mechanism gives the …