Håkan Sundell, Chalmers University of Technology 1 Evaluating the performance of wait-free snapshots in real-time systems Björn Allvin Andreas Ermedahl Hans Hansson Marina Papatriantafilou Håkan Sundell Philippas Tsigas

Håkan Sundell, Chalmers University of Technology 2 Schedule What are we evaluating How did we do the evaluation Evaluation platform Analysis Experiments Conclusions Future work

Håkan Sundell, Chalmers University of Technology 3 Snapshot –A consistent momentous state of a set of several shared variables –One reader Reads the whole set of variables in one atomic step –Many writers Writes to only one variable each time What are we evaluating

Håkan Sundell, Chalmers University of Technology 4 What are we evaluating Synchronization methods –Lock Uses semaphores, spinning, disabling interrupts Negative –Blocking –Priority inversion –Risk of deadlock Positive –Execution time guarantees easy to do Take lock... do operation... Release lock

Håkan Sundell, Chalmers University of Technology 5 What are we evaluating Synchronization methods –Lock-free Retries until not interfered by other operations Usually uses some kind of shared flag variable Write flag... do operation... Check flag and maybe retry

Håkan Sundell, Chalmers University of Technology 6 What are we evaluating Synchronization methods –Lock-free Negative –No execution time guarantees, can continue forever - thus can cause starvation Positive –Avoids blocking and priority inversion –Avoids deadlock –Fast execution when low contention

Håkan Sundell, Chalmers University of Technology 7 What are we evaluating Synchronization methods –Wait-free Uses atomic synchronization primitives Uses shared memory Negative –Complex algorithms –Memory consuming Test&SetCompare&SwapCopyingHelpingAnnouncingSplitoperation???

Håkan Sundell, Chalmers University of Technology 8 What are we evaluating Synchronization methods –Wait-free Positive –Execution time guarantees –Fast execution –Avoids blocking and priority inversion –Avoids deadlock –Avoids starvation –Same implementation on both single- and multiprocessor systems

Håkan Sundell, Chalmers University of Technology 9 What are we evaluating Wait-free snapshot algorithm –Several register copies –Uses Test&Set for synchronization Used by writer Used by reader

Håkan Sundell, Chalmers University of Technology 10 How did we do the evaluation Analytically –Single-processor system –Measured schedulability

Håkan Sundell, Chalmers University of Technology 11 How did we do the evaluation Experimentally –Single-processor system Measured schedulability –Multi-processor system with CAN-bus Measured snapshot response time

Håkan Sundell, Chalmers University of Technology 12 Evaluation platform Hardware –CAN-bus, 1 Mhz –Nodes 1 processor, Motorola 68020, 20 MHz Single wait-state memory CAN-controller

Håkan Sundell, Chalmers University of Technology 13 Evaluation platform Software –RTOS –Device tasks (interrupts) –Updater tasks –Local snapshot tasks –System snapshot task

Håkan Sundell, Chalmers University of Technology 14 Evaluation platform Additional parameters –Snapshot algorithm timings from WCET analysis Cross-compiling Cycle counting –CAN-bus timing from estimation

Håkan Sundell, Chalmers University of Technology 15 Analysis Parameters –10 tasks –15 devices –Snapshot of 5 components

Håkan Sundell, Chalmers University of Technology 16 Analysis Response time formulas RiRiRiRi CiCiCiCi  CjCjCjCj RiRiRiRi TjTjTjTj = j  hp(i) +......

Håkan Sundell, Chalmers University of Technology 17 Analysis Generation of 100 random scenarios for each of 31 different CPU loads Each scenario schedulable without any synchronization method

Håkan Sundell, Chalmers University of Technology 18 Analysis Results

Håkan Sundell, Chalmers University of Technology 19 Experiments Simulation –RT-simulator in Erlang Fixed priority preemptive scheduler Execution time Semaphores Memory Messages

Håkan Sundell, Chalmers University of Technology 20 Experiments Single-node –Parameters 10 tasks 15 devices Snapshot of 5 components

Håkan Sundell, Chalmers University of Technology 21 Experiments Single-node –Simulator programming Using subset of scenarios used for the analysis Lock –Execution and semaphores Lock-free –Execution and shared memory Wait-free –Execution

Håkan Sundell, Chalmers University of Technology 22 Experiments Single-node –Results

Håkan Sundell, Chalmers University of Technology 23 Experiments Multi-node –CAN-simulation Sending –Execution Receiving –Execution of highest priority task –Execution

Håkan Sundell, Chalmers University of Technology 24 Experiments Multi-node –Parameters 10 nodes 10 tasks on each node 15 devices on each node Local snapshot of 24 components on each node 1 super snapshot task on last node

Håkan Sundell, Chalmers University of Technology 25 Experiments Multi-node –Simulator programming Using subset of scenarios used for the analysis Lock –Execution, semaphores and messages Lock-free –Execution, shared memory and messages Wait-free –Execution and messages

Håkan Sundell, Chalmers University of Technology 26 Experiments Multi-node –Results

Håkan Sundell, Chalmers University of Technology 27 Experiments Multi-node –Results

Håkan Sundell, Chalmers University of Technology 28 Conclusions Wait-free is always better than lock Lock-free mostly performs better than lock –Single-node Lock-free performs best in practice Wait-free performs very good –Multi-node Lock-free performs poor Wait-free performs best

Håkan Sundell, Chalmers University of Technology 29 Future work Investigations of other wait-free synchronization methods Implementations in RTOS kernels, by Enea and NRTG WARPing project -