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Multicore timing solution
Solving the challenges of multicore timing analysis
Our unique solution to multicore timing analysis produces execution time evidence for multicore systems.
By following a V-model process, our engineers investigate multicore systems and produce evidence about multicore timing behavior.
Our industry leading tooling, including our unique RapiDaemon technology (which generates interference during tests), reduces analysis effort through automation.
Our approach has been designed to support projects within the CASTA-32A and ISO 26262 context.
Working with us
- We recognize that each test project is different, and work with you to meet your needs.
- We run testing activities on-site, at our headquarters in the UK, and at Rapita Systems Inc. in Novi, Michigan. We can support projects with UK / US EYES ONLY requirements.
- We can answer multicore timing questions and produce evidence for you, or implement a method and provide training so you can do so yourself.
RapiDaemons are specialized microbenchmark programs that generate contention on hardware resources such as buses, caches and GPUs.
They support multicore timing analysis by generating contention while multicore timing tests are run, allowing interference effects to be considered while performing the analysis.
Each RapiDaemon applies contention to a specific hardware resource on a specific hardware architecture, either matching a desired level of contention or maximizing contention on the resource.
RapiDaemons are built on the Barcelona Supercomputing Center's microbenchmark technology (MuBT).
When developing safety-critical applications to DO 178C (CAST32A) guidelines or ISO 26262 standards, there are special requirements for using multicore processors. Evidence must be produced to demonstrate that software operates within timing deadlines.
The goal of multicore timing analysis is to produce execution time evidence for these complex systems. In multicore processors, multiple cores compete for the same shared resources, resulting in potential interference channels that can affect execution time. Accounting for this interference and producing robust execution time evidence is a challenge addressed by Rapita’s Multicore Timing Solution.
By following a V-model process, our engineers investigate multicore systems and produce evidence about multicore timing behavior. Our approach has been designed to support projects within the DO 178C (CAST32A) and ISO 26262 context.
You can see an example workflow of Rapita’s Multicore Timing Solution in our Multicore White Paper.
Our multicore timing analysis solution comprises three components: a process, tool automation, and services.
Our multicore timing analysis process is a V-model process that we developed in line with DO-178 and CAST-32A. It follows a requirements-based testing approach that focuses on identifying and quantifying interference channels on multicore platforms.
The tools we have developed let us apply tests to multicore hardware (RapiTest) and collect timing data (RapiTime) and other metrics such as scheduling metrics (RapiTask) from them. We use RapiDaemons (developed by the Barcelona Supercomputing Center) to create a configurable degree of traffic on shared hardware resources during tests, so we can analyze the impact of this on the application’s timing behavior.
Our multicore timing analysis services include tool integration, porting RapiDaemons, performing timing analysis, identifying interference channels, and others depending on customer needs.
Rapita Systems are uniquely positioned to offer the combination of expertise and tools required to effectively perform multicore timing analysis.
Whilst the challenge of certifying multicore systems for safety-critical applications is a relatively new one for the industry as a whole, we have been researching this area for over a decade. Rapita are working with key industry stakeholders, including major chip-manufacturers like NXP, to support them in refining the evidence required to satisfy certification authorities.
Rapita have extensive experience in providing software verification solutions for some of the best-known aerospace and automotive companies in the world. For example, BAE Systems used RapiTime (one of the tools in our Multicore Timing Solution) to identify worst-case execution time optimizations for the Mission Control Computer on their Hawk Trainer jet.
See more of our Case Studies
In the CAST-32A position paper published by the FAA, an interference channel is defined as "a platform property that may cause interference between independent applications". This definition can be applied to a range of ‘platform properties’, including thermal factors etc.
Of these interference channels, interference caused by the sharing of certain resources in multicore systems is one of the most significant in terms of execution times. Interference based on shared resources may occur in multicore systems when multiple cores simultaneously compete for use of shared resources such as buses, caches and main memory.
Rapita’s Multicore Timing Solution analyzes the effects of this type of interference channel.
A very simple example of a shared resource interference channel is as follows:
In this simplified example, tasks running independently on the two cores may need to access main memory simultaneously via the memory controller. These accesses can interfere with each other, potentially degrading system performance.
A RapiDaemon is an application designed to create contention in a predictable and controlled manner on a specific shared resource in a multicore system. The effect of this contention can then be measured to identify interference channels and impacts on execution times.
Rapita have developed and optimized a set of standard RapiDaemons that target shared resources common to most multicore architectures, such as main memory. Some projects will require the creation of custom RapiDaemons for a specific architecture.
We can analyze almost all hardware architectures. To work with an architecture that is new to us, we first identify what metrics we can collect from the hardware, then adapt RapiDaemons for the architecture and implement a strategy to collect data from it.
For more on this, see the FAQ: ‘configuring and porting’.
This is part of the standard Multicore Timing Solution Workflow (detailed in our Multicore White Paper).
Rapita have been providing execution time analysis services and tooling since 2004.
RapiTime, part of the Rapita Verification Suite (RVS), is the timing analysis component of our Multicore Timing Solution. Our customers have qualified RapiTime on several DO178C DAL A projects where it has been successfully used to generate certification evidence by some of the most well-known aerospace companies in the world. See our Case Studies.
As well as providing a mature tool chain, we support the customer in ensuring that their test data is good enough, so that the timing information they generate from the target is reliable.
Our RapiDaemons are configured and tested (see the FAQ: ‘configuring and porting’) to ensure that they behave as expected on each specific customer platform.
We also assess available observability channels as part of a processor analysis. This primarily applies to the use of performance counters, where we assess their accuracy and usefulness for obtaining meaningful insights into the system under observation.
Before timing analysis can begin, it is essential to confirm that existing RapiDaemons operate as intended on a specific board. If they do not, it may be necessary to customize them to make them behave as they are designed to (a similar intention to a target integration, where we get the instrumented application software running on a specific target).
'Porting’ and ‘configuration’ of RapiDaemons are roughly equivalent terms in this context. For RapiDaemons to work as intended, they need to go through a configuration phase where their internal parameters are tuned as required. They’re then tested to ensure compatibility with the platform in question.
Each type of RapiDaemon may be implemented for a different instruction-set architecture and platform. While the main logic behind their behavior remains the same, they must be ported to execute correctly on each new platform.
RTOS vendors may provide partitioning mechanisms for their multicore processors, but these do not guarantee the complete elimination of interference. Instead, they are designed to provide an upper limit on the interference, sometimes at the expense of average-case performance.
In aerospace, these partitioning mechanisms may be referred to as ‘robust partitioning’. CAST-32A (the FAA’s position paper on multicore processors in avionics) identifies allowances for some of the objectives if you have robust partitioning in place – but it is still necessary to verify that the partitioning is indeed as robust as claimed.
From a certification standpoint, regardless of the methodology behind the RTOS vendor’s approach to eliminating interference, the effectiveness of the technology needs to be verified.
It is possible for companies to perform multicore timing analysis internally, but it is a highly complex undertaking which is very costly in terms of budget and effort. Anecdotally, one of our customers reported that it took them five years and a budget in the millions of dollars to analyze one specific platform.
Our Multicore Timing Solution is typically delivered as a turn-key solution, from initial system analysis and configuration all the way through to providing evidence for certification.
Some customers prefer to outsource only parts of the process to Rapita. For example, it is possible for a customer to purchase RapiDaemons under license and use them to gather and analyze their own data.
We’re completely flexible, and we understand that different customers have different needs. As such, you can purchase any component of our multicore timing analysis solution separately if that’s what you need. This includes, but is not limited to:
- Tool licenses (RapiTest, RapiTime, RapiTask)
- Services to integrate automation tools to work with your multicore system
- RTBx hardware to collect trace data from your multicore system
- Generic libraries of RapiDaemons
- Services to port RapiDaemons to your multicore system
- Services to perform multicore timing analysis
Yes: our approach can be used to get an in-depth understanding of how sensitive software can be to other software. For example:
- Task 1 executes acceptably in isolation and with most other tasks, but if it executes simultaneously with Task 127, its function X takes 10 times as long to return.
- This intelligence can feed into system integration activities to ensure that function X can never execute at the same time as Task 127.
The information from this type of analysis can also provide insights into potential improvements to the implementation of the two tasks. Sensitive tasks are not always the guilty party: other tasks can be overly aggressive and cause delays in the rest of the system.
For safety reasons, WCET will always be somewhat pessimistic. However, techniques that work well for single-core systems risk generating a WCET that is unreasonably large when applied to multicore systems, because the effects of contention can become disproportionate. The objective, therefore, is to calculate a value that is plausible and useful, without being optimistic. Optimism in relation to WCET is inherently unsafe.
It is not enough to identify how sensitive an application’s tasks are to different types and levels of interference; it is also necessary to understand what degree of interference a task may suffer in reality. It is possible to lessen the pessimism in WCET analysis by viewing the processor under observation through this paradigm.
The degree to which we can reduce pessimism is dependent on how effectively we can analyze the system. Factors influencing this include:
- The overhead of the tracing mechanism (which affects depth of instrumentation)
- The availability and reliability of performance counters
- The availability of information regarding other tasks executing on the system
- The quality of tests that exercise the code
Cache partitioning is all about predictability, not performance. Your code might execute faster on average without cache partitioning, but it’s probably not as predictable and can be quite sensitive to whatever executes in parallel.
Cache partitioning aims to remove all the sensitivity to other tasks sharing the caches, thus making your task more predictable – but potentially at the expense of overall performance. In critical systems, predictability is of far greater importance than performance.
Rapita’s Multicore Timing Solution can be used to exercise cache partitioning mechanisms by analyzing any shared – and usually undocumented – structures internal to the caches.
To analyze how a specific task is affected by contention on a specific resource, we need to be able to synchronize the execution of the task with the execution of RapiDaemons (the applications that generate contention on the resource).
Usually it is highly desirable to have RTOS/HV support for enabling user-level access to performance counters. Additionally, context switch information is very valuable when performing timing analysis.
Yes. Our solution makes it easy to specify the core on which you run your tests, and the level of resource contention to apply from each other core in the system.
We can also analyze systems that use non-synchronized clocks such as those often present in AMP platforms, by using the RTBx to timestamp data.
The maximum number of metrics we can collect depends on the performance monitoring unit(s) (or equivalent) on the hardware. An ARM A53, for example, lets us collect at least 30 metrics, but only access 6 in a single test. By running tests multiple times, however, we could collect all 30 metrics.
Developing a one-button tool solution for multicore timing analysis would be impossible. This is because interference, which can have a huge impact on a task’s execution time, must be taken into account when analyzing multicore timing behavior.
Analyzing interference effects is a difficult challenge that cannot be automatically solved through a software-only solution. Using approaches developed for timing analysis of single-core systems would result in a high level of pessimism, as it would assume that the highest level of interference possible is feasible, while this is almost never the case.
It is possible to collect a range of metrics by instrumenting your source code with the Rapita Verification Suite (RVS), including a range of execution time metrics:
- RapiTime: high-water mark and maximum execution times
- RapiTask: scheduling metrics such as periodicity, separation, fragmentation and core migration
It is also possible to collect information on events in your hardware using performance counters. The information we can collect depends on the performance monitoring unit(s) (or equivalent) of your system, but typically includes events such as L2 cache accesses, bus accesses, memory accesses and instructions executed. We can also collect information about operating system activity such as task switching and interrupt handling via event tracing or hooks.
Yes. We develop and test RapiDaemons against appropriate requirements, e.g. RapiDaemon M should access resource R when run N times.
Yes, we formally test and assess the accuracy of performance counters to ensure the validity of results we collect for the software under analysis.
Ada-Europe & DASIA 2019
17 Jun 2019
Multicore processor testing and validation
03 May 2019
Highlights from Embedded World 2019
06 Mar 2019
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