Home > Articles > Software Development & Management > Architecture and Design

Design Patterns for Real-Time Systems: Resource Patterns

Bruce Powel Douglass provides a number of patterns to help organize, manage, use, and share finite resources when building real-time and embedded systems.
This chapter is from the book

The following patterns are presented in this chapter.

  • Critical Section Pattern: Uses resources run-to-completion

  • Priority Inheritance Pattern: Limits priority inversion

  • Highest Locker Pattern: Limits priority inversion

  • Priority Ceiling Pattern: Limits priority inversion and prevents deadlock

  • Simultaneous Locking Pattern: Prevents deadlock

  • Ordered Locking Pattern: Prevents deadlock

7.1 Introduction

One of the distinguishing characteristics of real-time and embedded systems is the concern over management of finite resources. This chapter provides a number of patterns to help organize, manage, use, and share such resources. There is some overlap of concerns with the patterns in this and other chapters. For example, the Smart Pointer Pattern provides a robust access to resources of a certain type: those that are dynamically allocated. However, that pattern has already been discussed in Chapter 6. Similarly, the synchronization of concurrent threads may be thought of as resource management, but it is dealt with using the Rendezvous Pattern from Chapter 5. This chapter focuses on patterns that deal with the sharing and management of resources themselves and not the memory they use. To this end, we'll examine a number of ways to manage resources among different, possibly concurrent, clients.

A resource, as used here, is a thing (an object) that provides services to clients that have finite properties or characteristics. This definition is consistent with the so-called Real-Time UML Profile1, where a resource is defined as follows.


An element that has resource services whose effectiveness is represented by one or more /blockquote

Quality of Service (QoS) characteristics.

The QoS properties are the quantitative properties of the resource, such as its capacity, execution speed, reliability, and so on. In real-time and embedded systems, it is this quantifiable finiteness that must be managed. For instance, it is common for a resource to provide services in an atomic fashion; this means that the client somehow "locks" the resource while it needs it, preventing other clients from accessing that resource until the original client is done. This accomplishes the more general purpose of serialization of resource usage, crucial to the correct operation in systems with concurrent threads. This is often accomplished with a mutex semaphore (see the Guarded Call Pattern in Chapter 5) or may be done by serializing the requests themselves (see the Message Queuing Pattern, also in Chapter 5).

The management of resources with potentially many clients is one of the more thorny aspects of system design, and a number of patterns have evolved or been designed over time to deal specifically with just that.

The first few patterns (Priority Inheritance, Highest Locker, Priority Ceiling) address the schedulability of resources in a priority-based preemptive multitasking environment, which can be a major concern for real-time systems design. In static priority scheduling approaches (see, for example, the Static Priority Pattern in Chapter 5), the priorities of the tasks are known at design time. The priority of the task determines which tasks will run preferentially when multiple tasks are ready to run—the highest-priority task that is ready. This makes the timing analysis of such systems very easy to compute, as long as certain assumptions are not violated too badly. These assumptions are the following.

  • Tasks are periodic with the deadlines coming at the end of the periods.

  • Infinite preemptibility—a lower-priority task can be preempted immediately when a higher-priority task becomes ready to run.

  • Independence—tasks are independent from each other.

When these conditions are true, then the following standard rate monotonic analysis formula may be applied.

Note that it is "2 raised to the power of (1/n)", where Cj is the worst-case amount of time required for task j to execute, Tj is its period, and n is the number of tasks.2, 3 If the inequality is true, then the system is schedulable—that is, the system will always meet its deadlines. Aperiodic tasks are generally handled by assuming they are periodic and using the minimum arrival time between task invocations as the period, often resulting in an overly strong but sufficient condition. The assumption of infinite preemptibility is usually not a problem if the task has very short critical sections during which it cannot be preempted—short with respect to the execution and period times. The problem of independence is, however, much stickier.

If resources are sharable (in the sense that they can permit simultaneous access by multiple clients), then no problem exists. However many, if not most, resources cannot be shared. The common solution to this problem was addressed in the Guarded Call Pattern of Chapter 5 using a mutual-exclusion semaphore to serialize access to the resource. This means that if a low-priority task locks a resource and then a higher-priority task that needs the resource becomes ready to run, it must block and allow the low-priority task to run until it can release the resource so that the higher-priority task can run. A simple example of this is shown in the timing diagram in Figure 7-1.

Figure 7-1Figure 7-1: Task Blocking4

In the figure, Task 1 is the higher-priority task. Since Task 2 runs first and locks the resource, when Task 1 is ready to run, it cannot because the needed resource is unavailable. It therefore must block and allow Task 2 to complete its use of the resource. During the period of time between marks C and D, Task 1 is said to be blocked. A task is blocked when it is prevented from running by a lower-priority task. This can only occur when resources are shared via mutual exclusion.

The problem with blocking is that the analysis of the timeliness becomes more difficult. When Task 1 is blocked, the system is said to be in a state of priority inversion because a lower-priority task has the thread focus even though a higher-priority task is ready to run. One can imagine third and fourth tasks of intermediate priority that don't share the resource (and are therefore able to preempt Task 2) running and preempting Task 2, thereby lengthening the amount of time before Task 2 releases the resource and allowing Task 1 to run. Because an arbitrary number of tasks can be fit in the priority scheme between Task 1 and Task 2, this problem is called unbounded priority inversion and is a serious problem for the schedulability of tasks. Figure 7-2 illustrates this problem by adding intermediate-priority Tasks X and Y to the system. Note that for some period of time, Task 1, the highest-priority task in the system, is blocked by all three remaining tasks.

Figure 7-2Figure 7-2: Unbounded Task Blocking

To compute the schedulability for task sets with blocking, the modified RMA inequality is used.

where Bj is the blocking time for task j—that is, the worst-case time that the task can be prevented from execution by a lower-priority task owning a needed resource. The problem is clear from the inequality—unbounded blocking means unbounded blocking time, and nothing useful can be said about the ability of such a system to meet its deadlines.

Unbounded priority inversion is a problem that is addressed by the first three patterns in this chapter. Note that priority inversion is a necessary consequence of resource sharing with mutual exclusion locking, but it can be bounded using these patterns.

These first three patterns solve, or at least address, the problem of resource sharing for schedulability purposes, but for the most part they don't deal with the issue of deadlock. A deadlock is a condition in which clients of resources are waiting for conditions to arise that cannot in principle ever occur. An example of deadlock is shown in Figure 7-3.

Figure 7-3Figure 7-3: Deadlock

In Figure 7-3, there are two tasks, Task 1 and Task 2, that share two resources, R1 and R2. Task 1 plans to lock R2 and then lock R1 and release them in the opposite order. Task 2 plans to lock R1 and then R2 and release them in the reverse order. The problem arises when Task 1 preempts Task 2 when it has a single resource (R1) locked. Task 1 is a higher priority, so it can preempt Task 1, and it doesn't need a currently locked resource, so things are fine. It goes ahead and locks R2. Now it decides that it needs the other resource, R1, which, unfortunately is locked by the blocked task, Task 2. So Task 1 cannot move forward and must block in order to allow Task 2 to run until it can release the now needed resource (R1). So Task 2 runs but finds that it now needs the other resource (R2) owned by the blocked Task 1. At this point, each task is waiting for a condition that can never be satisfied, and the system stops.

In principle, a deadlock needs the following four conditions to occur.

  1. Mutual exclusion (locking) of resources

  2. Resources are held (locked) while others are waited for

  3. Preemption while holding resources is permitted

  4. A circular wait condition exists (for example, P1 waits on P2, which waits on P3, which waits on P1)

The patterns for addressing deadlock try to ensure that at least one of the four necessary conditions for deadlock cannot occur. The Simultaneous Locking Pattern breaks condition 2, while the Ordered Locking Pattern breaks condition 4. The Priority Ceiling Pattern is a pattern that solves both the scheduling problem and the deadlock problem.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020