Performance Evaluation of UML2-Modeled Embedded Streaming Applications with System-Level Simulation
© Tero Arpinen et al. 2009
Received: 27 February 2009
Accepted: 21 July 2009
Published: 13 September 2009
This article presents an efficient method to capture abstract performance model of streaming data real-time embedded systems (RTESs). Unified Modeling Language version 2 (UML2) is used for the performance modeling and as a front-end for a tool framework that enables simulation-based performance evaluation and design-space exploration. The adopted application meta-model in UML resembles the Kahn Process Network (KPN) model and it is targeted at simulation-based performance evaluation. The application workload modeling is done using UML2 activity diagrams, and platform is described with structural UML2 diagrams and model elements. These concepts are defined using a subset of the profile for Modeling and Analysis of Realtime and Embedded (MARTE) systems from OMG and custom stereotype extensions. The goal of the performance modeling and simulation is to achieve early estimates on task response times, processing element, memory, and on-chip network utilizations, among other information that is used for design-space exploration. As a case study, a video codec application on multiple processors is modeled, evaluated, and explored. In comparison to related work, this is the first proposal that defines transformation between UML activity diagrams and streaming data application workload meta models and successfully adopts it for RTES performance evaluation.
Multiprocessor System-on-Chip (SoC) offers high performance, yet energy-efficient, and programmable platform for modern embedded devices. However, parallelism and increasing complexity of applications necessitate efficient and automated design methods. Model-driven development (MDD) aims to shorten the design time using abstraction, gradual refinement, and automated analysis with transformation of models. The key idea is to utilize models to highlight certain aspects of the system (behavior, structure, timing, power consumption models, etc.) without an implementation.
Unified Modeling Language version 2 (UML2)  is a standard language for MDD. In embedded system domain, its adoption is seen promising for several purposes: requirements specification, behavioral and architectural modeling, test bench generation, and IP integration . However, it should be noted that UML2 has had also criticism on its suitability in MDD [3, 4]. UML2 offers a rich set of diagrams for modeling and also expansion and tailoring methods to derive domain-specific languages. For example, several UML profiles targeted at embedded system design have been developed [5–7].
SoC complexity requires efficient performance evaluation and design-space exploration methods. These methods are often utilized at the system level to make early design decisions. Such decisions include, for instance, choosing the number and type of processors, and determining the mapping and scheduling of application tasks. Design-space exploration seeks to find optimum solution for a given application (domain) and boundary constraints. Design space, that is, the number of possible system configurations, is practically always so large that it becomes intractable not only for manual design but also for brute force optimization. Hence, efficient methods are needed, for example, optimization heuristics, tool frameworks, and models .
Our focus is on modeling streaming data applications. It is characteristic to streaming applications that a long sequence of data items flows through a stable set of computation steps (tasks) with only occasional control messaging and branching. Each task waits for the data items, processes them, and outputs the results to the next task. The adopted application metamodel has been formulated based on this assumption and it resembles Kahn Process Network (KPN)  model.
A proprietary UML2 profile for capturing performance characteristics of an application and platform is defined. The profile definition is based on a well-defined metamodel and reusing suitable modeling concepts from the profile for Modeling and Analysis of Realtime and Embedded systems (MARTE) . MARTE is a standard profile promoted by the Object Management Group (OMG) and it is a promising extension for general-purpose embedded system modeling. It has been intended to replace the UML Profile for Schedulability, Performance and Time (SPT) . MARTE is methodology-independent and it offers a common set of standard notations and semantics for a designer to choose from while still allowing to add custom extensions. This means that the profile defined in this article is a specialized instance of the MARTE profile that is dedicated for our performance evaluation methodology.
It should be noted that the performance models defined in this work can be and have been used together with a custom UML profile for embedded systems, called TUT-Profile [7, 11]. However, this article illustrates the models using the concepts of MARTE because the adoption of standards promotes commonly known notations and semantics between designers and interoperability between tools.
Further, the article presents how performance values can be specified on UML models with expressions using MARTE Value Specification Language (VSL). This allows effective parameterization of system performance model representation according to application-specific variables and reduces the amount of time consuming and error-prone manual work.
The presented modeling methods are utilized in a tool framework targeted at simulation-based design-space exploration and performance evaluation. The exploration is based on collecting performance statistics from simulation to optimize the platform and mapping according to a predefined cost-function.
An execution-monitoring tool provides visualization and monitoring the system performance during the simulation. As a case study, a video codec system is modeled with the presented modeling methods and performance evaluation and exploration is carried out using the tool framework.
The rest of the article is organized as follows. Section 2 analyses the methods and concepts used in RTES performance evaluation. Section 3 presents the metamodel utilized in this work for system performance characterization. UML2 and MARTE for RTES modeling are discussed in Section 4. Section 5 presents the UML2 specification of the utilized performance metamodel. Section 6 presents our performance evaluation tool framework. The video codec case study is covered in Section 7. After final discussion on our proposal in Section 8, Section 9 concludes the article.
2. Analysis of Methods and Concepts Used in RTES Performance Evaluation
In this section the methods and concepts used in RTES performance evaluation are covered. This comprises an introduction to design Y-chart in RTES performance evaluation, phases of a model-based RTES performance evaluation process, discussion on modeling language and tool development, and a short introduction to RTES timing analysis concepts. Finally, the related work on UML in RTES performance evaluation is examined.
2.1. Design Y-Chart and RTES Modeling
There are several possible abstraction levels for describing the application and platform for performance evaluation. One possibility is to utilize abstract specifications. This means that application workload and performance of the platform resources are represented symbolically without needing detailed executable descriptions.
Application workload is a quantity which informs how much capacity is required from the underlying platform components to execute certain functionality. In model-based performance evaluation the workloads can be estimated based on, for example, standard specifications, prior experience from the application domain, or available processing capacity. Legacy application components, on the other hand, can be profiled and performance models of these components can be evaluated together with the models of components yet to be developed.
In addition to computational demands, communication demands between application parts must be considered. In practice, the communication is realized as data messages transmitted between real-time operating system (RTOS) threads or between processing elements over an on-chip communication network. Shared buses and Network-on-Chip (NoC) links and routers perform scheduling for transmitted data packets in an analogous way as PEs execute and schedule computational tasks. Moreover, inter-PE communication can be alternatively performed using a shared memory. The performance characteristics of memories as well as their utilization play a major role in the overall system performance. The impact of computation, communication, and storage activities should all be considered in system-level analysis to enable successful performance evaluation of a modern SoC.
2.2. Model-Based RTES Performance Evaluation Process
RTES performance evaluation process must follow disciplined steps to be effective. From SoC designer's perspective, a generic performance evaluation process consists of the following steps. Some of the concepts of this and the next subsection have been reused and modified from the work in :
(1)selection of the evaluation techniques and tools,
(2)measuring, profiling, and estimating workload characteristics of application and determining platform performance characteristics by benchmarking, estimation, and so forth,
(3)constructing system performance model,
(4)measuring, executing, or simulating system performance models,
(5)interpreting, validating, monitoring, and back-annotating data received from previous step.
The selection of the evaluation techniques and tools is the first and foremost step in the performance evaluation process. This phase includes considering the requirements of the performance analysis and availability of tools. It determines the modeling methods used and the effort required to perform the evaluation. It also determines the abstraction level and accuracy used. All further steps in the process are dependent on this step.
The second step is performed if the system performance model requires initial data about application task workloads or platform performance. This is based on profiling, specifications, or estimation. The application as well as platform may be alternatively described using executable behavioral models. In that case, such additional information may not be needed as all performance data can be determined during system model execution.
The actual system model is constructed in the third step by a system architect according to defined metamodel and model representation methods. Gathered initial performance data is annotated to the system model. The annotation of the profiling results can also be accelerated by combining the profiling and back-annotation with automation tools such as .
After system modeling, the actual analysis of the model is carried out. This may involve several model transformations, for example, from UML to SystemC. The analysis methods can be classified into dynamic and static methods . Dynamic methods are based on executing the system model with simulations. Simulations can be categorized into cycle-accurate and system-level simulations. Cycle-accurate simulation means that the timing of system behavior is defined by the precision of a single clock cycle. Cycle-accuracy guarantees that at any given clock cycle, the state of the simulated system model is identical with the state of the real system. System-level simulation uses higher abstraction level. The system is represented at IP-block level consisting coarse grained models of processing, memory, and communication elements. Moreover, the application functionality is presented by coarse-grained models such as interacting tasks.
Static (or analytic) methods are typically used in early design-space exploration to find different corner cases. Analytical models cannot take into consideration sporadic effects in the system behavior, such as aperiodic interrupts or other aperiodic external events. Static models are suited for performance evaluation when deterministic behavior of the system is accurate enough for the analysis.
Static methods are faster and provide significantly larger coverage of the design-space than dynamic methods. However, static methods are less accurate as they cannot take into account dynamic performance aspects of a multiprocessor system. Furthermore, dynamic methods are better suited for spotting delayed task response times due to blocking of shared resources.
Analysing, measuring, and executing the system performance models produces usually a massive amount of data from the modeled system. The final step in the flow is to select, interpret, and exploit the relevant data. The selection and interpretation of the relevant data depends on the purpose of the analysis. The purpose can be early design-space exploration, for example. In that case, the flow is usually iterative so that the results are used to optimize the system models after which the analysis is performed again for the modified models. In dynamic methods, an effective way of analysing the system behavior is to visualize the results of simulation in form of graphs. This helps the designer to efficiently spot changes in system behavior over time.
2.3. Modeling Language and Tool Development
SoC designers typically utilize predefined modeling languages and tools to carry out the performance evaluation process. On the other hand, language and tool developers have their own steps to provide suitable evaluation techniques and tools for SoC designers. In general they are as follows:
(1)formulation of metamodel,
(2)developing methods for model representation and capturing,
(3)developing analysis tools according to selected modeling methods.
The formulation of the metamodel requires very similar kind of consideration on the objectives of the performance analysis as the selection of the techniques and tools by SoC designers. The created metamodel determines the effort required to perform the evaluation as well as the abstraction level and accuracy used. In particular, it defines whether the system performance model can be executed, simulated, or statically analysed.
The second step is to define how the model is captured by a designer. This phase includes the selection or definition of the modeling language (such as UML, SystemC or a custom domain-specific language). The selection of notations also requires transformation rules defined between the elements of the metamodel and the elements of the selected description language. In case of UML2, the metamodel concepts are mapped to UML2 metaclasses, stereotyped model elements, and diagrams.
We want to emphasize the importance of performing these first two steps exactly in this order. The definition of the metamodel should be performed independently from the utilized modeling language and with full concentration on the primary objectives of the analysis. The selection of the modeling language should not alter the metamodel nor bias the definition of it. Instead, the modeling language and notations should be tailored for the selected metamodel, for instance, by utilizing extension mechanisms of the UML2 or defining completely new domain-specific language. The reason for this is that model notations contribute only to presentational features. Model semantics truly determine whether the model is usable for the analysis. Nevertheless, presentational features determine the feasibility of the model for a human designer.
The final step is the development of the tools. To provide efficient evaluation techniques, the implementation of the tools should follow the created metamodel and its original objectives. This means that the original metamodel becomes the foundation of the internal metamodel of the tools. The system modeling language and tools are linked together with model transformations. These transformations are used to convert the notations of the system modeling language to the format understood by the tools, while the semantics of the model is maintained.
2.4. RTES Timing Analysis Concepts
A typical SoC contains heterogeneous processing elements executing complex application tasks in parallel. The timing analysis of such a system requires abstraction and parameterization of the key concerns related to resulting performance.
Hansson et al. define concepts for RTES timing analysis . In the following, a short introduction to these concepts is given.
Task execution time is the time in which (in clock cycles or absolute time) a set of sequential operations are executed undisturbed on a processing element. It should be noted that the term task is here considered more generally as a sequence of operations or actions related to single-threaded execution, communication, or data storing. The term thread is used to denote typical schedulable object in an RTOS. profiling the execution time does not consider background activities in the system, such as RTOS thread pre-emptions, interrupts, or delays for waiting a blocked shared resource. The purpose of execution time is to determine how much computing resources is required to execute the task. Task response time , on the other hand, is the actual time it takes from beginning to the end of the task in the system. It accounts all interference from other system parts and background activities.
Execution time and response time can be further classified into worst case (wc), best case (bc), and average case (ac) times. Worst case execution time is the worst possible time the task can take when not interfered by other system activities. On the other hand, worst case response time is the worst possible time the task may take when considering the worst case scenario in which other system parts and activities interfere its execution. In multimedia applications that require streaming data processing, the worst case and average case response times are usually the ones needed to be analysed. However, in some hard real-time systems, such as a car air bag controller, also the best case response time ( ) may be as important as the . Average case response time is usually not so significant. Jitter is a measure for time variability. For a single task, jitter in execution time can be calculated as . Respectively, jitter in response time can be calculated as .
It is assumed that the execution time is constant for a given task-PE pair. It should be noted that in practice the execution time of a function may vary depending on the processed data, for example. For these kinds of functions the constant task execution time assumption is not valid. Instead, different execution times of such functions should be modeled by selecting a suitable value to characterize it (e.g., worst or average case) or by defining separate tasks for different execution scenarios. As opposed to execution time, response time varies dynamically depending on the task's surrounding system it is executed on. The response time analysis must be repeated if
(1)mapping of application tasks is changed,
(2)new functionalities (tasks) are added to the application,
(3)underlying execution platform is modified,
(4)environment (stimuli from outside) changes.
In contrast, a single task execution time does not have to be profiled again if the implementation of the task is not changed (e.g., due to optimization) assuming that the PE on which the profiling was carried out is not changed. If the PE executing is changed and the profiling uses absolute time units, then a reprofiling is needed. However, this can be avoided by utilizing PE-neutral parameters, such as number of operation, to characterize the execution load of the task. Other possibility is to represent processing element performances using a relative speed factor as in .
In multiprocessor SoC performance evaluation, simulating the profiled or estimated execution times (or number of operations) of tasks on abstract HW resource models is an effective way of observing combined effects of task execution times, mapping, scheduling, and HW platform parameters on resulting task response times, response time jitters, and processing element utilizations.
Timing requirements of SoC functions are compared against estimated, simulated, or measured response times. It is typical that timing requirements are given as combined response times of several individual tasks. This is naturally completely dependent on the granularity used in identifying individual tasks. For instance, a single WLAN data transmission task could be decomposed into data processing, scheduling, and medium access tasks. Then examining if the timing requirement of a single data transmission is met requires examining the response times of the composite tasks in an additive manner.
2.5. On UML in Simulation-Based RTES Performance Evaluation
Related work has several static and dynamic methods for performance evaluation of parallel computer systems. A comprehensive survey on methods and tools used for design-space exploration is presented in . Our focus is on dynamic methods and some of the closest related research to our work are examined in the following.
Erbas et al.  present a system-level modeling and simulation environment called Sesame, which aims at efficient design space exploration of embedded multimedia system architectures. For application, it uses KPN for modeling the application performance with a high-level programming language. The code of each Kahn process is instrumented with annotations describing the application's computational actions, which allows to capture the computational behavior of an application. The communication behavior of a process is represented by reading from and writing to FIFO channels. The architecture model simulates the performance consequences of the computation and communication events generated by an application model. The timing of application events are simulated by parameterizing each architecture model component with a table of operation latencies. The simulation provides performance estimates of the system under study together with statistical information such as utilization of architecture model components. Their performance metamodel and approach has several similarities with ours. The biggest differences are in the abstraction level of HW communication modeling and visualization of the system models and performance results.
Balsamo and Marzolla  present how UML use case, activity and deployment diagrams can be used to derive performance models based on multichain and multiclass Queuing Networks. The UML models are annotated according to the UML Profile for Schedulability, Performance and Time Specification . This approach has been developed for SW architectures rather than for embedded systems. No specific tool framework is presented.
Kreku et al.  propose a method for simulation-based RTES performance evaluation. The method is based on capturing application workloads using UML2 state-machine descriptions. The platform model is constructed from SystemC component models that are instantiated from a library. Simulation is enabled with automatic C++ code generation from UML2 description, which makes the application and platform models executable in a SystemC simulator. Platform description provides dedicated abstract services for application to project its computational and communicational loads on HW resources. These functions are invoked from actions of the state-machines. The utilization of UML2 state-machine enables efficiently capturing the control structures of the application. This is a clear benefit in comparison to plain data flow graphs. The platform services can be used to represent data processing and memory accesses. Their method is well suited for control-intensive applications as UML state-machines are used as the basis of modeling. Our method targets at modeling embedded streaming data applications with less effort required in modeling using UML activity diagrams.
Madl et al.  present how distributed real-time embedded systems can be represented as discrete event systems and propose an automated method for verification of dense time properties of such systems. The model of computation (MoC) is based on tasks connected with channels. Tasks are mapped onto machines that represent computational resources of embedded HW.
Our performance evaluation method is based on executable streaming data application workload model specified as UML activity diagrams and abstract platform performance model specified in composite structure diagrams. In comparison to related work, this is the first proposal that defines transformation between UML activity diagrams and streaming data application workload models and successfully adopts it for embedded RTES performance evaluation.
3. Performance Metamodel for Streaming Data Embedded Systems
The foundations of the performance metamodel defined in this work is based on the earlier work on Model of Computation (MoC) for architecture exploration described in . We introduce storage tasks, storage elements, and timing constraints as new features. The metamodel definition is given using mathematical equations and set theory. Another alternative would be to utilize Meta Object Facility (MOF) . MOF is often used to define the metamodels from which UML profiles are derived as the model elements and notations of MOF are a subset of UML model elements. Next, detailed formulation of the performance metamodel is carried out.
3.1. Application Performance Metamodel
where is task that emits tokens to the channel, task that consumes tokens, and is the set of buffered tokens in the channel. Tokens in channels represent the flow of control as well as flow of data in the application. A token carries certain amount of data from task to another. This has two impacts. First, the load on the communication medium for the time of the transfer. Second, the execution load when the next task is triggered after reception. Latter enables data amount-dependent dynamic variations in execution of application tasks. Similar to traditional KPN model, channels between tasks (or processes) are uni-directional, unbounded FIFO buffers and tasks use a blocking read as a synchronization mechanism.
3.2. External Events and Constraints
where determines whether the event is fired once or periodically. is the absolute time or period when the event is triggered, and is the channel where events are fed.
in which is a consecutive path of tasks and channels and and are the required worst-case response time and best case response time for the to be completed after the first element of has been triggered.
3.3. Platform Performance Metamodel
Links connect processing and storage elements to the communication element . The carries out the required data exchange between PEs and SEs.
in which and are performance indices for reading and writing from and to storage element. The time which it takes to read or write to the storage is calculated in the same manner as in (17).
3.4. Metamodel for Functionality Mapping
Each task is mapped only onto one platform element and several tasks can be mapped onto a single platform element. Events are not mapped to any platform element. The mapping of channels onto communication element is not explicitly modeled. Instead, they are implicitly mapped onto the single communication element that interconnects processing and storage elements.
3.5. Example Model
4. UML2 and the MARTE Profile
UML has been traditionally used for specifying software-intensive systems but currently it is seen as a promising language for developing embedded systems as well. Natively UML2 lacks some of the key concepts that are crucial for embedded systems such as quantifiable notion of time, nonfunctional properties, embedded execution platform, and mapping of functionality. However, the language has extension mechanisms that can be used for tailoring the language for desired domains. One of such mechanisms is to use profiles that add custom semantics to be used with the set of model elements offered by the language itself. Profiles are defined with stereotype extensions, tag definitions, and constraints. Stereotypes give new semantics to existing UML2 metaclasses. Tagged values are attributes of a stereotype that are used to further specify the stereotyped model element. Constraints limit the meta -model by defining how model elements can be used.
One model element can have multiple stereotypes. Consequently it gets all the properties, tagged values, and constraints of those stereotypes. For example, a PE may have different stereotypes for defining its performance characteristics and its power consumption characteristics. The separation of concerns (one stereotype for one purpose) when defining profiles is recommended to keep the set of model elements concise for a designer.
4.1. Utilized MARTE Architecture
In this work, a subset of the MARTE profile is used as the foundation for creating our domain-specific modeling language for performance modeling. The concepts of the created performance evaluation metamodel are mapped to the stereotypes defined by MARTE. Thereafter, custom stereotypes with associated tag definitions for the rest of the metamodel concepts are defined.
From MARTE design model, the HW resource modeling (HRM) profile is adopted to identify and give semantics to different types of HW elements. It should be noted that HRM profile has dependencies in other profiles in the foundations, such as general resource modeling (GRM) profile, but it is not included to the figure, since the stereotypes from there are not directly adopted.
The MARTE analysis model contains pre-defined packages that are dedicated for generic quantitative analysis modeling (GQAM), schedulability analysis modeling (SAM), and performance analysis modeling (PAM). MARTE profile specification defines that this analysis model can be extended for other domains as well, such as for power consumption. We do not utilize the pre-defined analysis concepts but define own extensions that implement the metamodel defined in Section 3. This is because the MARTE analysis packages have been defined according to their own metamodel that differs from ours. Although there are some similarities in the modeling concepts, we define dedicated stereotype extensions to allow as straightforward way of capturing the performance models as possible.
5. Performance Model Specification in UML2
The extension of modeling capabilities for our performance metamodel is specified by refining the elements of UML and MARTE with additional stereotypes. These stereotypes specify the performance characteristics of particular elements to which they are applied to. The additional stereotypes are designed so that they can be used with other profiles similar to MARTE. The requirements for such profile is that it supports embedded HW modeling and a functionality mapping mechanism. As mentioned, the additional stereotypes have been successfully used also with the TUT-Profile. The defined stereotypes are, however, dependent on the nonfunctional property data types and measurement units defined by MARTE nonfunctional property and model library packages. These data types are used in tag definitions.
5.1. Application Workload Model Presentation
UML2 activity diagrams have been selected as the view for application workload models. The reasons for this are
(i)activity diagrams are a natural view for presenting control and data flow between functional elements of the application,
(ii)activity diagrams have enough expression power to present the application task network of the workload model,
(iii)reuse of activity diagrams created for describing task-level behaviour becomes possible.
In the workload model, the basic activities are used as the level of detail in activity diagrams. UML2 basic activity is presented as a graph of actions and edges connecting them. Here, actions correspond to tasks and edges to channels . Basic activities allow modeling of control and data flow, but explicit forks and joins of control, as well as decisions and merges, are not supported . Still, the expression power is adequate for our workload model.
Stereotypes ExecutionWorkload and StorageWorkload are applied to actions that represent execution tasks and storage tasks . The tag definitions for these stereotypes define other properties of the represented tasks, including trigger conditions, computational workload indices, and sent data tokens. The index of tagged value lists represent an individual trigger condition and its related actions (operations to be calculated, data to be sent to the next tasks) when the trigger condition is satisfied.
Action nodes are connected together using activity edges. This notation is used in our model presentation to represent a channel between two tasks. The direction of the data flow in the channel is the same as the direction of the activity edge. The names of the channels are directly referenced as strings in trigger condition as well as in tagged values indicating outgoing channels.
An external event is presented as an action node stereotyped as WorkloadEvent. Such action has always a single outgoing channel that carries tokens to the task network. The top-level activity which defines a single complete workload model of the system is stereotyped as WorkloadModel.
Timing constraints are defined by applying the stereotype ResponseTiming for a single action or a complete activity and defining the response timing requirements in terms of worst and best case response times. The timing requirement for an activity is defined as the time it takes to execute the activity from its initial state to its exit state.
5.2. Platform Performance Model Presentation
The platform is modeled with stereotyped UML2 classes and class instances. Other alternative would be to use stereotyped UML nodes and node instances. Nodes and devices in deployment diagrams are the native way in UML to model coarse grained HW architecture that serves as the target to SW artifacts. Memory and communication resource modeling are not natively supported by UML2. Therefore, MARTE hardware resource modeling (HRM) package is utilized to classify different types of HW elements.
MARTE hardware resource modeling package offers several stereotypes for modeling embedded HW platform. The complete hardware resource model is divided into logical and physical views. Logical view defines HW resources according to their functional properties whereas physical view defines their physical properties, such as area and power. The performance modeling does not require considering physical properties, and thus, only stereotypes related to the logical view are enough for our needs. Next, the stereotypes utilized from MARTE HRM to categorize different HW elements are discussed in detail.
HW_ComputingResource is a generic MARTE stereotype that is used to represent elements in the HW platform which can execute application functionality. It can be specialized to, for example, HW_Processor to indicate its properties as a programmable computing resource. This stereotype or any of its inherited stereotypes is used to represent processing element .
HW_Memory is a generic MARTE stereotype for resources that are capable of storing data. This stereotype and its inherited stereotypes, such as HW_RAM, are used to represent storage element .
Finally, generic MARTE stereotype HW_CommunicationResource and its inherited stereotypes, such as HW_Bus, are used to represent communication element .
5.3. Mapping Model Presentation
MARTE allocation package is used to model the mapping of application tasks onto platform resources. MARTE allocation mechanism allows hybrid allocation in which application behavioral elements are associated with structural platform resources. The hybrid allocation is performed with two stereotypes ApplicationAllocationEnd and ExecutionPlatformAllocationEnd. In UML diagrams they are written as app_allocated and ep_allocated for conciseness. Application allocation end has a tagged value that describes the platform resources to which the particular application element is mapped. Execution platform allocation end identifies the platform resources onto which application elements can be mapped. A dependency stereotyped Allocated is used to bind application behaviour elements onto platform elements.
5.4. Parameterizing Models with MARTE VSL Expressions
The MARTE value specification language (VSL) has been developed to specify the values of constraints, properties and stereotype attributes particularly for nonfunctional properties. It is an extension to the Value specification and DataType concepts provided by UML. It can be used in any UML-based specification for extending the base expression infrastructure provided by UML. The VSL addresses how to specify variables, constants, and expressions in textual form. It also deals with time values and assertions as well as how to specify composite values such as collection, interval, and tuples in UML models.
In our approach the syntax of VSL is utilized to define expressions on application workload models and platform performance models. It is an efficient way for parameterizing the workload models according to application-related values. Top-right corner of Figure 6 shows an example of using VSL syntax to parameterize application workload models according to video quality metrics that are dependent on the application. In the example, frame rate (fr) is set to frames per second and this constant variable is utilized to determine the time period for the VideoInput workload event when a single image is fed to the process network. Further, the macroblock size in pixels (MBPixelSize) and image size (Xres and Yres) are used to determine the data amounts transferred between tasks.
6. Tool Framework for Model-Driven SoC Performance Evaluation and Exploration
6.1. Performance Model Capture and System-Level Simulation
The flow begins from capturing the system performance modeling in UML2 using the presented model elements and profiles. This is followed by the model parsing phase in which the models are transformed into XML system model (XSM) [24, 25]. This is the corresponding XML presentation of the UML2 performance models. The XSM is a common format between tools to exchange information on the designed system. The XSM can be modified by tools after its creation during the design-space exploration iterations.
After model creation the XSM file is fed to the simulator. The simulator is divided into two parts: computation and communication. The computation part is in practice realized with a configurable transaction generator (TG) . The computation part simulates the execution and scheduling of tasks on processing and memory elements. It also feeds the underlying communication part with data tokens transmitted between tasks which are mapped onto different platform elements. The abstraction level of the computation part is the same with the metamodel defined in Section 3.
Due to high abstraction level of the computation part, the executed tasks do not contain any specific functionality, but they only reserve the processing or memory element and block it from other tasks for certain amount of time. For example, for execution tasks this time is derived with (17).
The computation part (TG) is configured automatically based on the abstract task, processing and storage resource models defined in UML. The configuration is based on generating corresponding SystemC code containing the same tasks, processing and memory elements. This is done by instantiating generic task and HW element SystemC components with parameters (operation counts, performance indices, etc.) defined in UML the models.
The computation and communication parts are interfaced with Open Core Protocol (OCP)  TL2 compatible interfaces. This means that the communication part can be changed to any SystemC-based network model that implements OCP TL2 compatible interfaces for interconnected elements. This allows simulation of low abstraction level models of communication (such as NoCs) with high abstraction level models of computation. Currently, the earlier presented simple performance model for communication element is not used in our framework. Instead, a more accurate SystemC defined TLM model for the communication part is used in simulations.
6.2. Execution Monitoring
Summary of collected and monitored performance statistics.
For example, frame rate, radio throughput
Signals in/out, avg./tot. communication cycles, communication % of execution time, intra/inter-PE communication bytes and cycles, communication cycles/byte
Execution count, avg./tot. execution cycles, execution % of thread/service total, signal queue, execution latency, response time
Task to thread/PE
Utilization, inter-PE communication bytes, avg./tot. execution cycles
Performance bottlenecks can be detected by observing the amount of tokens in signal queues and the utilization of PEs. If the number of tokens in the incoming channel of a task is increasing it is usually an indication of that task being the bottleneck in a chain of several tasks. On the other hand, a bottleneck can be located when a single processor has a considerably higher utilization than other collaborating processors.
In practice, the modeled response time requirements are validated by observing the maximum response time of a task in different execution scenarios. Meeting throughput requirements can be also observed in a similar manner.
6.3. Design-Space Exploration
After simulation and performance monitoring, the performance simulation results and XSM are fed to the design-space exploration tool which tries to optimize the platform parameters and task mapping so that user-defined cost function is minimized. The cost function can contain several nonfunctional properties such as power, frequency, area, or response time of an individual task. The design space exploration tool has several mapping heuristics supported: simulated annealing, group migration, hybrid of the previous two , optimal subset mapping , genetic algorithm, and random. The design-space exploration cycle continues by performing the simulation after each remapping or modification in the execution platform.
After the design-space exploration cycle ends, the optimized system description is again written to the XSM file. The back-annotator tool is used to change the UML2 models according to the results of the design-space exploration (updated platform and mapping).
6.4. Governing the Tool Flow Execution
The execution of the design flow is governed by a customizable Java-based tool for configuring and executing SoC design flows. This tool is called Koski Graphical User Interface. The idea of this tool is that a user selects tools to the flow to be executed from a library of tools. New tools can be imported to the library in a plug-and-play fashion. Each tool includes a section of XML which specifies the input and output tokens (files and parameters) of that particular tool. Parameters of individual tools can be set via the GUI. For example, the platform constraints such as maximum and minimum number of PEs and the cost function of the design-space exploration tool are these kind of parameters. Due to its flexibility, this tool has shown to be very effective in researching and evaluating different methodologies and tool flow configurations.
7. Case Study: Performance Evaluation and Exploration of a Video Codec on Multiprocessor SoC
This section presents a case study that illustrates the applicability of the modeling methods and tool framework in practice. The application is a video codec on a multiprocessor platform. We used an approach in which new functionality representing web client was modeled and added to an existing video codec system in Figure 6 and the system was simulated and optimized based on the monitored information.
7.1. Profiling and Modeling
All the functions were modeled by their workload and simulated in SystemC using TG. The workload model of the video codec was originally profiled from real FPGA execution trace whereas the model of the web client was only a single task which had an early estimate of its behavior.
The performance requirement of the video codec was set to frames per second (FPS). Thus, an external event representing the camera triggered at 35 Hz frequency. The HW platform consisted of three processors connected through a shared bus. The operating frequencies of the processors were set to MHz, MHz, and MHz. The frequency of the bus was set to MHz.
7.2. Simulating and Monitoring
When the original system was simulated, it was observed that it met the FPS requirement. Next, functionality for the web client was added to run in parallel with the video codec. The web client was mapped to cpu1(see Figure 11) because it was observed that the utilization of cpu1 was the lowest in the original system. Simulations indicated that the performance of the video codec was decreased to FPS. In addition, cpu1 became fully utilized at all times whereas the utilizations of the other two processors decreased. This indicated a clear bottleneck on cpu1 as it was not able to forward processed data fast enough to other processors. This could also be observed from the signal queues of the tasks mapped onto cpu1. The environment produced raw frames so fast that they started accumulating at the cpu1.
Thereafter, a remapping of the application tasks was performed since the workload of the processors was clearly imbalanced. The mapping was done manually so that all the encoder tasks were mapped to cpu1, the decoder tasks to cpu2, and the web client functionality was isolated to cpu3. During the simulation it was observed that this improved the FPS to 22.
Because the manual mapping did not result in the required performance, the next phase was automatic exploration of the task mapping. The result mapping was nonobvious because the tasks of the encoder and decoder were distributed among all the processors. Hence, it is unlikely that we had ended to it with manual mapping.
Finally, automated exploration was performed for the operating frequencies of the processors. The result of the exploration was that the frequency of cpu1 was increased MHz to MHz, and the frequencies of the other two processors were increased MHz to MHz. The simulation on this system model showed that the FPS requirement should be met, and the tasks could process all the signals which they received.
In early performance evaluation, the key issue is the tradeoff between accuracy and development time of the model. The best accuracy is achieved from cycle-accurate simulations or from actual implementation. However, constructing the cycle-accurate model or integrating the system is very time consuming in comparison to using system-level models and simulations. Thus, utilization of abstract system-level models allow the designer to explore the design space more efficiently. The actual simulation time is also faster in system-level simulations in comparison to cycle-accurate simulations.
In this work we concentrate on reducing the effort in specifying and managing the performance models for system-level simulations. This has been done by utilizing graphical UML2 models. As a result, the degree of readability of the models is improved in comparison to textual presentation. The case study showed that the system model is easy to construct, interpret, and modify with the presented UML model elements. The case study models were constructed in few hours. Profiling and estimating operation counts for workload tasks can be considered time-consuming and hard. In our case, it was done by profiling similar application executing on FPGA.
MARTE VSL was found useful for defining expressions. It significantly simplified modifying the models with different application-specific parameters in comparison to using constant values.
In earlier study  the average error in frame-rate was . This article uses the same metamodel. Hence, it can be concluded that our method offers designer-friendly, rapid yet rather accurate performance evaluation for RTES.
9. Conclusions and Future Work
This article presented an efficient method to model and evaluate streaming data embedded system performance with UML2 and system-level simulations. The modeling methods were successfully utilized in a tool framework for early performance evaluation and design-space exploration. The case study showed that UML2, the presented modeling methods, and the utilized performance evaluation tools form a designer-friendly, rapid yet rather accurate way of modeling and evaluating RTES performance before actual implementation. Future work consists of taking account the impact of SW platform in the RTES performance metamodel. This includes the workload of SW platform services (such as file access and memory allocation) as well as scheduling of tasks with different policies.
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