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A Taxonomy of Out-of-Order Instruction Commit
Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Computer Systems.
Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Computer Systems.
Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Computer Systems.
2017 (English)In: 2017 Ieee International Symposium On Performance Analysis Of Systems And Software (Ispass), Los Alamitos: IEEE Computer Society, 2017, p. 135-136Conference paper, Published paper (Refereed)
Abstract [en]

While in-order instruction commit has its advantages, such as providing precise interrupts and avoiding complications with the memory consistency model, it requires the core to hold on to resources (reorder buffer entries, load/store queue entries, registers) until they are released in program order. In contrast, out-of-order commit releases resources much earlier, yielding improved performance without the need for additional hardware resources. In this paper, we revisit out-of-order commit from a different perspective, not by proposing another hardware technique, but by introducing a taxonomy and evaluating three different micro-architectures that have this technique enabled. We show how smaller processors can benefit from simple out-oforder commit strategies, but that larger, aggressive cores require more aggressive strategies to improve performance.

Place, publisher, year, edition, pages
Los Alamitos: IEEE Computer Society, 2017. p. 135-136
National Category
Computer Systems Computer Sciences
Identifiers
URN: urn:nbn:se:uu:diva-352938DOI: 10.1109/ISPASS.2017.7975283ISI: 000426905600020ISBN: 978-1-5386-3890-3 (electronic)ISBN: 978-1-5386-3891-0 (print)ISBN: 978-1-5386-3889-7 (print)OAI: oai:DiVA.org:uu-352938DiVA, id: diva2:1216769
Conference
2017 Ieee International Symposium On Performance Analysis Of Systems And Software (Ispass), Santa Rosa, CA, USA.
Available from: 2018-06-12 Created: 2018-06-12 Last updated: 2020-02-02Bibliographically approved
In thesis
1. Rethinking Dynamic Instruction Scheduling and Retirement for Efficient Microarchitectures
Open this publication in new window or tab >>Rethinking Dynamic Instruction Scheduling and Retirement for Efficient Microarchitectures
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Out-of-order execution is one of the main micro-architectural techniques used to improve the performance of both single- and multi-threaded processors. The application of such a processor varies from mobile devices to server computers. This technique achieves higher performance by finding independent instructions and hiding execution latency and uses the cycles which otherwise would be wasted or caused a CPU stall. To accomplish this, it uses scheduling resources including the ROB, IQ, LSQ and physical registers, to store and prioritize instructions.

The pipeline of an out-of-order processor has three macro-stages: the front-end, the scheduler, and the back-end. The front-end fetches instructions, places them in the out-of-order resources, and analyzes them to prepare for their execution. The scheduler identifies which instructions are ready for execution and prioritizes them for scheduling. The back-end updates the processor state with the results of the oldest completed instructions, deallocates the resources and commits the instructions in the program order to maintain correct execution.

Since out-of-order execution needs to be able to choose any available instructions for execution, its scheduling resources must have complex circuits for identifying and prioritizing instructions, which makes them very expansive, therefore, limited. Due to their cost, the scheduling resources are constrained in size. This limited size leads to two stall points respectively at the front-end and the back-end of the pipeline. The front-end can stall due to fully allocated resources and therefore no more new instructions can be placed in the scheduler. The back-end can stall due to the unfinished execution of an instruction at the head of the ROB which prevents other resources from being deallocated, preventing new instructions from being inserted into the pipeline.

To address these two stalls, this thesis focuses on reducing the time instructions occupy the scheduling resources. Our front-end technique tackles IQ pressure while our back-end approach considers the rest of the resources. To reduce front-end stalls we reduce the pressure on the IQ for both storing (depth) and issuing (width) instructions by bypassing them to cheaper storage structures. To reduce back-end stalls, we explore how we can retire instructions earlier, and out-of-order, to reduce the pressure on the out-of-order resource.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2020. p. 76
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1902
Keywords
Out-of-Order Processors, Energy-Efficient, High-Performance, Instruction Scheduling
National Category
Computer Sciences
Research subject
Computer Science
Identifiers
urn:nbn:se:uu:diva-403675 (URN)978-91-513-0868-5 (ISBN)
Opponent
Supervisors
Available from: 2020-02-27 Created: 2020-02-02 Last updated: 2020-05-19Bibliographically approved

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Alipour, MehdiCarlson, Trevor E.Kaxiras, Stefanos

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