Leveraging Transactional Memory for a Predictable Execution of Applications Composed of Hard Real-Time and Best-Effort Tasks

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Leveraging Transactional Memory for a Predictable Execution of Applications Composed of Hard Real-Time and Best-Effort Tasks Stefan Metzlaff, Sebastian Weis, and Theo Ungerer Department of Computer Science, University of Augsburg, Germany International Conference on Real-Time Networks and Systems October 16, 2013 RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 1

Motivation Real-Time and Multi-Cores I Safety-critical embedded systems I Increasing parallelism I Interferences at shared resources I I I Access on shared data structures Real-time capable concurrency, Predictable execution in multi-cores RTNS 2013 A380, [1] Interferences in parallel applications I I Access to bus, memory, and I/O Predictable arbitration with bandwidth guarantees (e.g. TDMA) Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks Google Driverless Car, [2] 2

Outline Transactional Memory for HRT systems HRT-TM - Enhancement for Non Real-Time Evaluation and Conclusion RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 3

Transactional Memory for HRT systems Derived from database transactions Optimistic concurrency control Simplifies parallel programming Requirements for hard real-time (HRT) TM Predictable worst-case behaviour of TM primitives Commit guarantee for each transaction Calculable number of transaction aborts RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 4

HRT-TM Design Overview (1) Predictable worst-case behaviour of TM primitives TM-Begin, TM-End, TM-Abort, (Rollback+Retry) Implemented in predictable hardware (TDMA bus/no cache) Read set/write set accesses upper-bounded Commit guarantee for each transaction FIFO transaction commit queue Registering transactions on transaction begin Core 1 Core 2 HTM Ext. Working Set Buffer Memory Bus HTM Ext. Working Set Buffer I/O Device HTM Mgmt. TX Status Info. HRT FIFO Queue RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 5

HRT-TM Design Overview (2) Calculable number of transaction aborts HRT contention manager (commit serialisation) Lazy versioning HRT manager supports lazy/eager conflict detection Core 1 Core 2 HTM Ext. Working Set Buffer Memory Bounded maximum transaction delay Conclusion Allows calculation of WCET bounds (the set of concurrent transactions must be known) Bus HTM Ext. Working Set Buffer I/O Device HTM Mgmt. TX Status Info. HRT FIFO Queue RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 6

Lazy Conflict Detection WCET Estimation Worst-case scenario for a transaction 1. Worst-Case Conflicts All concurrently executed transactions have conflicting working sets 2. Worst-Case Commit Delay The transaction has to be at the back position of the commit queue 3. Worst-Case Release Time All transactions are released at the same time 4. Worst-Case Transaction Order The shortest transaction is in the front position, followed by the longest RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 7

Lazy Conflict Detection RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 8

Lazy Conflict Detection RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 8

Lazy Conflict Detection RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 8

Lazy Conflict Detection RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 8

Lazy Conflict Detection 1 2 3 4 Commit Queue 1 2 3 4 2 3 4 3 4 4 t Execute Corrupted Wait Commit Abort RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 8

Eager Conflict Detection WCET Estimation Worst-case scenario for a transaction 1. Worst-Case Conflicts All concurrently executed transactions have conflicting working sets 2. Worst-Case Commit Delay The transaction has to be at the back position of the commit queue 3. Worst-Case Release Time All transactions are released at the same time No Worst-Case Transaction Order WCET is independent from the transaction ordering RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 9

Eager Conflict Detection RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 10

Eager Conflict Detection RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 10

Eager Conflict Detection RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 10

Eager Conflict Detection RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 10

Eager Conflict Detection 1 2 3 4 Commit Queue 1 2 3 4 2 3 4 3 4 4 t Execute Wait Commit Abort RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 10

Lazy vs. Eager CD Lazy Fewer rollback in case of conflicts Global progress stall depends on ratio of the largest and smallest transaction Eager No global progress stall Transaction Order is irrelevant RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 11

Lazy vs. Eager CD Lazy Fewer rollback in case of conflicts Global progress stall depends on ratio of the largest and smallest transaction Eager No global progress stall Transaction Order is irrelevant Conclusion With increasing heterogeneity of the transactions the choice of the conflict detection becomes more important RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 11

HRT-TM Enhancement for Non Real-Time Applications with tasks of different RT requirements e.g.: Advanced Driver Assistance System Hard real-time (HRT): collision avoidance Soft real-time (SRT): night vision Best-effort (BE): traffic sign recognition Data sharing among applications Limiting interference of non-hrt tasks Collision Avoidance, HRT, [3] Night Vision, SRT, [4] Traffic Sign Recognition, BE, [5] RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 12

Extension to Parallel HRT/BE Applications TM manager with second queue for BE tasks BE tasks commit only when no HRT task is running Core 1 Core 2 HTM Ext. Working Set Buffer Memory Bus HTM Ext. Working Set Buffer I/O Device HTM Mgmt. TX Status Info. HRT FIFO Queue BE FIFO Queue Bounding possible interferences of BE transactions RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 13

Possible Overlaps of HRT and BE Transactions HRT starts while BE commits BE HRT HRT BE Wait Commit Abort t RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 14

Possible Overlaps of HRT and BE Transactions HRT starts while BE commits BE HRT HRT BE Wait Commit Abort Interference (but bounded by t e + t a of HRT) t RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 14

Possible Overlaps of HRT and BE Transactions HRT starts while BE commits BE HRT HRT BE Wait Commit Abort Interference (but bounded by t e + t a of HRT) HRT waits until BE commits BE HRT HRT BE Wait Commit Abort t t RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 14

Possible Overlaps of HRT and BE Transactions HRT starts while BE commits BE HRT HRT BE Wait Commit Abort Interference (but bounded by t e + t a of HRT) HRT waits until BE commits BE HRT HRT BE Wait Commit Abort Interference (but bounded by t c of BE) t t RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 14

Conclusion on Prioritised TM Contention Manager BE tasks interfere with HRT tasks on TM commit Commit duration depends on the transaction write set Analysis requires profiling of BE write sets only Worst-Case Interference with HRT task: Bounded by max i TXBE (t ci ) RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 15

Evaluation Methodology System Model MERASA quad-core with TDMA bus Benchmark Synchrobench Linked List with 50 entries Table: Characteristics of the Linked List Operation Tasks Task Max. Max. WCET Bound RS-Size WS-Size in Isolation add (a) 400 bytes 12 bytes 1977 cycles remove (r) 400 bytes 4 bytes 1806 cycles normalise (n) 400 bytes 200 bytes 2755 cycles RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 16

Evaluation Results Case study: static WCET analysis of linked list operations Lazy conflict detection WCET estimate of HRT remove (r) executed in parallel with HRT add (a) and HRT/BE normalise (n) 5 Normalised WCET Estimate 4 3 2 1 0 r r+a r+a+n HRT r+a+n BE Executed Workload Minimal impact of BE task on WCET bounds of HRT tasks RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 17

Conclusions An analysable TM system for HRT tasks Commit guarantee for each transaction Calculable number of transaction aborts Supports eager and lazy conflict detection RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 18

Conclusions An analysable TM system for HRT tasks Commit guarantee for each transaction Calculable number of transaction aborts Supports eager and lazy conflict detection Enhancement for non real-time HRT contention manager with a two-level commit scheduling Interferences depend on commit time of BE tasks Low impact of BE tasks on WCET of HRT tasks RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 18

Conclusions An analysable TM system for HRT tasks Commit guarantee for each transaction Calculable number of transaction aborts Supports eager and lazy conflict detection Enhancement for non real-time HRT contention manager with a two-level commit scheduling Interferences depend on commit time of BE tasks Low impact of BE tasks on WCET of HRT tasks Our approach allows the independent development of BE tasks with limited interferences on the HRT tasks RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 18

Conclusions An analysable TM system for HRT tasks Commit guarantee for each transaction Calculable number of transaction aborts Supports eager and lazy conflict detection Enhancement for non real-time HRT contention manager with a two-level commit scheduling Interferences depend on commit time of BE tasks Low impact of BE tasks on WCET of HRT tasks Our approach allows the independent development of BE tasks with limited interferences on the HRT tasks Future work: Extend the approach to soft real-time tasks RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 18

Questions? RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 19

Backup Slides RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 20

Parallel HRT/BE Applications and Optimistic Concurrency Control worker task 0 worker task 1 shared data system monitor task worker task 2 Conventional HRT system Worker and Monitoring tasks are HRT tasks All tasks must be analysable HRT/BE TM system Worker tasks are HRT tasks and must be analysable Only working set of BE Monitoring task must be known RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 21

Possible Overlaps of HRT and BE Transactions HRT starts before BE BE HRT t HRT BE Wait Commit Abort HRT starts while BE running No interference BE HRT HRT BE Wait Commit Abort t No interference RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 22

Evaluation of NRT & HRT Tasks No. of Iterations No. of Iterations Core Type w. Delay of Type w. Delay of 0 600 1,800 0 600 1,800 1 HRT 50 50 50 HRT 50 50 50 2 HRT 45 46 48 BE 14 23 40 3 HRT 44 45 48 BE 13 23 39 4 HRT 44 44 48 BE 13 22 38 Table: Number of Iterations for the add Operation on a Shared Linked List in System with HRT and BE Tasks RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 23

HRT-TM Proposal WCET Estimation Method Transaction 1. Program analysis per task 2. TX extraction & characterisation 3. Exchange of TX characteristics among all tasks 4. WCET estimation of each TX including interferences of any concurrent TX 5. TX reduction per task 6. WCET calculation per task Transaction Predictable concurrency control in shared memory systems RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 24

Images [1] http://www.flickr.com/photos/8313254@n08/496320750/ [2] http://www.flickr.com/photos/jurvetson/5499949739/ [3] http://www.flickr.com/photos/13524418@n07/2921138655/ [4] http://www.flickr.com/photos/jurvetson/22226826/ [5] Eichner, M.L.; Breckon, T.P., Integrated speed limit detection and recognition from real-time video, Intelligent Vehicles Symposium, pp.626-631, 2008, IEEE RTNS 2013 Metzlaff, Weis, and Ungerer / TM for Applications Composed of HRT & BE Tasks 25

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