Anna Minaeva, who recently received her PhD degree, just had a journal article entitled “Efficient Heuristic and Exact Approach for Control Performance Optimization in Time-Triggered Periodic Scheduling” accepted for publication in IEEE Transactions on Computers. This article is the result of a HiPEAC collaboration grant that Anna was awarded back in 2016 to visit the group of Samarjit Chakraborty at TU Munich. I am very happy to see that this grant resulted in a joint publication in a prestigious journal and hope to collaborate with Samarjit again in the future.
The article addresses the problem of generating a time-triggered schedule for a number of independently developed automotive applications on a number of shared resources, such that their control performance only suffers minimal degradation. The three main contributions are: 1) a constraint programming model that solves the problem optimally, exploiting properties of the problem to reduce the computation time; 2) a fast heuristic called Flexi that only has a minor impact on the optimality of the solution; and 3) an experimental evaluation of the scalability and efficiency of the proposed approaches on a case study, in addition to several synthetic datasets. The results show that the heuristic provides a solution on average 5 times faster, finding a feasible solution in 31% more problem instances than the optimal approach within a time limit, while only sacrificing 0.5% of the control performance quality for the largest dataset.
Our collaboration with CISTER has been extremely fruitful this year, as yet another paper in our research line on mixed-criticality scheduling has been accepted. This latest paper is entitled “Techniques and Analysis for Mixed-criticality Scheduling with Mode-dependent Server Execution Budgets” and has been accepted at the International Conference on Embedded Software (EMSOFT).
The goal of this work is, like many other in this research line, is to reduce cost of mixed-criticality systems. This time, we achieve this by addressing the limitation that a server only has a single execution budget in all modes, despite that their computational requirements may vary significantly. More specifically, the three main contributions of the paper are: 1) a scheduling arrangement for uni-processor systems employing fixed-priority scheduling within periodic servers, whose budgets are dynamically adjusted at run-time in the event of a mode change, 2) a new schedulability analysis for such systems, and 3) heuristic algorithms for assigning budgets to servers in different modes and ordering the execution of the servers. Experiments with synthetic task sets demonstrate considerable improvements (up to 52.8%)
A paper “Memory Bandwidth Regulation for Multiframe Task Sets” has been accepted at RTCSA 2018. This paper aims to reduce cost of real-time systems where the worst-case execution times of tasks vary from job to job, according to known patterns. This kind of execution behavior can be captured by the multi-frame task model. However, this model is optimistic and unsafe for multi-cores with shared memory controllers, since it ignores memory contention, and existing approaches to stall analysis based on memory regulation are very pessimistic if straight-forwardly applied.
This paper remedies this by adapting existing stall analyses for memory-regulated systems to the multi-frame model. Experimental evaluations with synthetic task sets show up to 85% higher scheduling success ratio for our analysis, compared to the frame-agnostic analysis, enabling higher platform utilization without compromising safety. We also explore implementation aspects, such as how to speed up the analysis and how to trade off accuracy with tractability.
We are pleased to announce that our paper “Worst-case Stall Analysis for Multicore Architectures with Two Memory Controllers” has been accepted at ECRTS 2018. This paper represents another successful collaboration with my former colleagues from CISTER.
The paper addresses the problem that increasing bandwidth requirements have resulted in platform architectures with multiple memory controllers, for which existing analyses to compute worst-case memory stall time are not safe. This work presents a new worst-case memory stall analysis for a memory-regulated multi-core architecture with two memory controllers. This stall analysis can be integrated into the schedulability analysis of systems under fixed-priority partitioned scheduling. Five heuristics for assigning tasks and memory budgets to cores in a stall-cognisant manner are also proposed. We experimentally quantify the cost in terms of extra stall for letting all cores benefit from the memory space offered by both controllers, and also evaluate the five heuristics for different system characteristics.
Another paper written with my former colleagues at CISTER has been accepted. The paper is entitled “Mixed-criticality Scheduling with Memory Bandwith Regulation” and appear at DATE 2018. The paper considers the problem that existing schedulability analyses for mixed-criticality multi-core systems do not consider task interference in shared platform resources, such as memories, potentially making them optimistic and unsafe. We address this issue by formulating a schedulability analysis for mixed-criticality fixed-priority-scheduled multi-core systems using per-core memory access regulation. We also propose multiple heuristics for memory bandwidth allocation and task-to-core assignment. The analysis and heuristics are implemented in a tool and evaluated through extensive experiments.
Anna Minaeva had an article entitled “Time-Triggered Co-Scheduling of Computation and Communication with Jitter Requirements” accepted in IEEE Transactions on Computers. The article considers the problem of efficiently co-scheduling task execution and communication in multi-core automotive platforms. Most existing works typically deal with zero-jitter scheduling, which results in lower resource utilization, but has lower memory requirements. In contrast, this article focuses on jitter-constrained scheduling that puts constraints on the tasks jitter, increasing schedulability over zero-jitter scheduling.
The contributions of this article are: 1) Integer Linear Programming and Satisfiability Modulo Theory model exploiting problem-specific information to reduce the formulations complexity to schedule small applications. 2) A heuristic approach, employing three levels of scheduling scaling to real-world use-cases with 10000 tasks and messages. 3) An experimental evaluation of the proposed approaches on a case-study and on synthetic data sets showing the efficiency of both zero-jitter and jitter-constrained scheduling. It shows that up to 28% higher resource utilization can be achieved by having up to 10 times longer computation time with relaxed jitter requirements.
Our paper “Mixed-criticality Scheduling with Dynamic Redistribution of Shared Cache” has been accepted at ECRTS 2017, marking the end of yet another succesful collaboration with my former colleagues at CISTER. The paper proposes an extension of Vestal’s model for mixed-criticality multi-core systems that 1) accounts for the per-task partitioning of the last-level cache, and 2) supports dynamic reassignment of cache portions initially reserved for lower-criticality tasks to the higher-criticality tasks when switching to high-criticality mode. A schedulability analysis based on partitioned EDF is presented that is aware of the cache resources assigned to each task and leverages the dynamic reconfiguration to improve schedulability. We also propose heuristics for partitioning the cache in low- and high-criticality mode. Experimental result indicate tangible improvements in schedulability compared to a baseline cache-aware arrangement where there is no redistribution of cache resources from low- to high-criticality tasks in the event of a mode change.
We just received the good news that Hazem’s article “ Reducing the Complexity of Dataflow Graphs using Slack-based Merging” has been accepted for publication in ACM Transactions on Design Automation of Electronic Systems (TODAES). The article addresses an important problem when working with synchronous data-flow (SDF) graphs, namely that the size of the graph explodes when transforming it to its equivalent homogeneous (HSDF) representation, which prevents any design or analysis algorithms requiring this transformation as a first step from scaling to larger graphs. In the scope of Hazem’s work, this has caused problems when converting an SDF graph into a set of independent periodic real-time tasks.
This article proposes a heuristic algorithm to reduce the size of the resulting HSDF graph prior to analysis by merging actors in the graph, thereby speeding up analysis algorithms using the resulting graph. Three key properties of the algorithm are: 1) it cannot violate the latency or throughput requirements of the original graph, 2) it cannot cause deadlock in the resulting merged graph, and 3) only HSDF actors corresponding to firings of the same SDF actor can be merged to enable the resulting merged graph to be efficiently used by mapping algorithms. The behavior of the algorithm is evaluated with applications from the SDF3 benchmark suite and it is compared to results of an optimal exhaustive merging algorithm for smaller graphs.
The spree of accepted journal articles continues as Sven Goossens’ article entitled “Power/Performance Trade-offs in Real-Time SDRAM Command Scheduling” was accepted for publication in IEEE Transactions on Computers. The article contains a detailed discussion about the trade-offs between bandwidth, execution time, and power when DRAM requests are scheduled by a real-time memory controller under a close-page policy. The results cover a wide range of memories ranging from DDR2/3/4 to LPDDR1/2/3 for different request sizes and amounts of bank parallelism. Other key contributions of the article are: 1) publicly available heuristic and optimal algorithms for generation of memory patters that covers all aforementioned memories, 2) a simple abstraction that quickly captures the differences between the different DRAM generations allowing algorithms and analyses to be easily adapted to cover all of them, and 3) a pairwise bank-group interleaving scheme for DDR4 that exploits bank grouping for improved performance.
We just had a paper accepted at the Real-Time and Embedded Technology and Applications Symposium (RTAS) in Seattle. The paper is entitled “An Efficient Configuration Methodology for Time-Division Multiplexed Single Resources” and presents an ILP-based methodology to allocate TDM slots to resource clients, such that bandwidth and latency constraints are satisfied while resource utilization is minimized. A heuristic algorithm is furthermore proposed to determine the number of TDM slots in the schedule. This paper is a collaboration both with colleagues here at CTU Prague and with Andrew Nelson from Eindhoven University of Technology.
For the camera-ready version of the paper, please click here.