Welcome!

Weblogic Authors: Yeshim Deniz, Elizabeth White, Michael Meiner, Michael Bushong, Avi Rosenthal

Related Topics: Weblogic

Weblogic: Article

Common WebLogic Server Deadlocks and How To Avoid Them

Common WebLogic Server Deadlocks and How To Avoid Them

Every developer has experienced it. The application that ran so well in testing hangs or performs miserably under load.

While there are many possible causes of performance degradation or hangs, this article can't possibly cover them all. Instead, we'll look at three common mistakes in WebLogic Server applications that can deadlock the server or bring your performance to a screeching halt.

Thread Dumps
The best Java tool for diagnosing deadlocks is a Java virtual machine thread dump. A thread dump is a snapshot of the virtual machine's current state, including stack traces for each Java thread. Many virtual machines also include information about the Java monitors held by each thread. Monitor information is especially useful for diagnosing deadlocks and performance problems in your application. On Windows platforms, you can generate a thread dump by pressing Ctrl-Break in the virtual machine's window. On Unix systems, a SIGQUIT signal must be sent to the Java virtual machine process. This can be done with a kill -3 <process id>.

Deadly Embrace
The classic deadlock problem is the deadly embrace: Thread 1 owns Lock A and waits on Lock B, Thread 2 owns Lock B and waits on Lock A. These threads are deadlocked and will remain blocked in this state. In many cases, the remaining threads will eventually enter the deadlock by attempting to acquire Lock A or Lock B and waiting. For instance, you might have a servlet that calls a synchronized method on the B object. If B's monitor is already held in a deadlock, any subsequent servlet request that attempts to acquire that monitor will enter the deadlock.

A thread dump is one of the best ways to discover a deadly embrace deadlock. Most virtual machines include a thread state for each Java thread in the dump. The most common thread states are: R - running; MW - monitor wait; CW - condition wait. Threads in the MW state are blocked, waiting to enter a synchronized block and acquire a Java monitor. Since the thread dump includes the Java thread's stack trace, it's also possible to determine which monitor is blocking the thread. If multiple threads are in the MW state on the same monitor, it's a good indication that there's either a lot of contention for this monitor, or the server is deadlocked. In a deadlock situation, you should be able to determine the other threads blocked in MW and their held monitors.

There are two classic techniques for solving deadly embrace deadlocks: deadlock avoidance and deadlock detection. Deadlock avoidance is merely changing or structuring your code so that it can't hit the deadlock case. A common solution is to implement lock ordering. If Lock A is always acquired before Lock B then you can't have a deadly embrace on these two locks. It's also a good idea to minimize the time that a monitor is held. Every extra line of code that runs under a monitor is another chance for someone to add a deadlock. Deadlock detection is commonly implemented by databases for their locks, but it doesn't usually find its way into programming languages. In deadlock detection, deadlocks are automatically discovered and one or more deadlock participants (known as the victims) are killed and release their locks to break the deadlock. Java virtual machines do not break deadlocks on Java monitors, so deadlock avoidance is necessary.

Out-of-Threads Deadlock
Another common way to deadlock an application is what I call the "out of threads" deadlock. Unfortunately, this deadlock often doesn't show up until a load test or, in the worst case, when your production application receives a lot of traffic. In this scenario, your WebLogic Server is running with a fixed number of threads. The application includes logic where a given request or action performs work in one thread and then blocks on work that must be done in another thread.

For instance, an application might open an HTTP socket connection to its own server instance, post a request, and wait for a response. If all threads post at the same time, there are no available threads to generate a response. Another common scenario is when Server A makes an RMI call to Server B and blocks waiting for a response. Server B then calls back into A before it generates the response to A's initial call. If all threads in A are exhausted then A is waiting for B's response, and B is waiting for A's response.

The first clue in an "out of threads" dump is that there are no idle WebLogic Server Execute Threads. The Execute Threads are the worker threads in the server that run the application code. An idle ExecuteThread will be in condition wait (CW) in a method named ExecuteThread.waitForRequest. This thread is available and waiting for incoming work. In an "out of threads" deadlock, you won't see any idle threads, and all other worker threads will be blocked, usually waiting for a response.

Unfortunately, determining that there are no idle threads is slightly more complicated than just searching for ExecuteThread.waitForRequest. WebLogic Server uses thread pools internally to avoid having to continually create new threads. It also offers multiple thread pools and uses several different thread pools within the server. Thread pools would be a good topic for an entire article so we won't cover them in detail here, but it's important to understand for this topic that most user work is performed on the "default" queue. In an "out of threads" deadlock, it's important to note whether there are no idle threads on the "default" queue.

The best way to avoid "out of threads" deadlocks is to analyze your architecture and remove the common mistakes that produce these deadlocks. In particular, never open a socket connection to your own server instance. Also, avoid synchronous request/response APIs that include callbacks. In general, asynchronous communication works well for server-to-server or application-to-application calls. Both messaging (JMS) and Web services include asynchronous support. One of the advantages of asynchronous communication is that the calling thread is not blocked waiting for the response.

Monitors and External Systems
In addition to diagnosing deadlocks, thread dumps can be useful for analyzing performance problems. One architectural mistake that is often evident from thread dumps is holding Java monitors while making external requests. For instance, an application might enter a synchronized block on its business data object and then log some information on an external system. In the common case, this might work fine if the logging to the external system is running relatively quickly. However, it really runs into trouble under load. In a production system, the external system is probably slower, and there are many concurrent requests trying to get access to the monitor. Usually, the entire system slows down dramatically and essentially becomes single-threaded.

The "single-threaded" problem shows up in thread dumps when there are many threads that are blocked in MW for the same monitor. There is then a single thread that already owns the monitor and is often blocked waiting for some external system to return. For instance, it's often blocked waiting for a network socket read to return. In thread dumps that display the current monitor owners, it's obvious which thread holds the contested monitor. In less verbose thread dumps, you'll have to analyze the stack trace for each thread and determine which monitors it's holding. Since a monitor is only ever owned by one thread, this is not as bad as it initially sounds.

The "single-threaded" problem can be avoided by never holding Java monitors while making requests to external systems. Fundamentally, holding monitors and running someone else's code is inviting deadlock or extremely poor performance. The external system may be down, performing slowly, or blocked on some other resource. If your application is holding a Java monitor, these external events can drastically reduce your performance to a crawl.

Conclusion
This article demonstrates some common WebLogic Server application mistakes and how they contribute to deadlocks or poor performance. Thread dumps are invaluable in debugging these types of problems. Hopefully, the techniques in this article will help you find your way out the next time your application becomes unresponsive.

Comments (0)

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


@ThingsExpo Stories
When shopping for a new data processing platform for IoT solutions, many development teams want to be able to test-drive options before making a choice. Yet when evaluating an IoT solution, it’s simply not feasible to do so at scale with physical devices. Building a sensor simulator is the next best choice; however, generating a realistic simulation at very high TPS with ease of configurability is a formidable challenge. When dealing with multiple application or transport protocols, you would be...
SYS-CON Events announced today that Datera, that offers a radically new data management architecture, has been named "Exhibitor" of SYS-CON's 21st International Cloud Expo ®, which will take place on Oct 31 - Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Datera is transforming the traditional datacenter model through modern cloud simplicity. The technology industry is at another major inflection point. The rise of mobile, the Internet of Things, data storage and Big...
SYS-CON Events announced today that GrapeUp, the leading provider of rapid product development at the speed of business, will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place October 31-November 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Grape Up is a software company, specialized in cloud native application development and professional services related to Cloud Foundry PaaS. With five expert teams that operate in various sectors of the market acr...
In the enterprise today, connected IoT devices are everywhere – both inside and outside corporate environments. The need to identify, manage, control and secure a quickly growing web of connections and outside devices is making the already challenging task of security even more important, and onerous. In his session at @ThingsExpo, Rich Boyer, CISO and Chief Architect for Security at NTT i3, discussed new ways of thinking and the approaches needed to address the emerging challenges of security i...
SYS-CON Events announced today that CA Technologies has been named "Platinum Sponsor" of SYS-CON's 21st International Cloud Expo®, which will take place October 31-November 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. CA Technologies helps customers succeed in a future where every business - from apparel to energy - is being rewritten by software. From planning to development to management to security, CA creates software that fuels transformation for companies in the applic...
In his opening keynote at 20th Cloud Expo, Michael Maximilien, Research Scientist, Architect, and Engineer at IBM, discussed the full potential of the cloud and social data requires artificial intelligence. By mixing Cloud Foundry and the rich set of Watson services, IBM's Bluemix is the best cloud operating system for enterprises today, providing rapid development and deployment of applications that can take advantage of the rich catalog of Watson services to help drive insights from the vast t...
There is only one world-class Cloud event on earth, and that is Cloud Expo – which returns to Silicon Valley for the 21st Cloud Expo at the Santa Clara Convention Center, October 31 - November 2, 2017. Every Global 2000 enterprise in the world is now integrating cloud computing in some form into its IT development and operations. Midsize and small businesses are also migrating to the cloud in increasing numbers. Companies are each developing their unique mix of cloud technologies and service...
WebRTC is great technology to build your own communication tools. It will be even more exciting experience it with advanced devices, such as a 360 Camera, 360 microphone, and a depth sensor camera. In his session at @ThingsExpo, Masashi Ganeko, a manager at INFOCOM Corporation, will introduce two experimental projects from his team and what they learned from them. "Shotoku Tamago" uses the robot audition software HARK to track speakers in 360 video of a remote party. "Virtual Teleport" uses a...
Recently, IoT seems emerging as a solution vehicle for data analytics on real-world scenarios from setting a room temperature setting to predicting a component failure of an aircraft. Compared with developing an application or deploying a cloud service, is an IoT solution unique? If so, how? How does a typical IoT solution architecture consist? And what are the essential components and how are they relevant to each other? How does the security play out? What are the best practices in formulating...
Internet of @ThingsExpo, taking place October 31 - November 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with 21st Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world. The Internet of Things (IoT) is the most profound change in personal and enterprise IT since the creation of the Worldwide Web more than 20 years ago. All major researchers estimate there will be tens of billions devic...
An increasing number of companies are creating products that combine data with analytical capabilities. Running interactive queries on Big Data requires complex architectures to store and query data effectively, typically involving data streams, an choosing efficient file format/database and multiple independent systems that are tied together through custom-engineered pipelines. In his session at @BigDataExpo at @ThingsExpo, Tomer Levi, a senior software engineer at Intel’s Advanced Analytics ...
In his session at @ThingsExpo, Arvind Radhakrishnen discussed how IoT offers new business models in banking and financial services organizations with the capability to revolutionize products, payments, channels, business processes and asset management built on strong architectural foundation. The following topics were covered: How IoT stands to impact various business parameters including customer experience, cost and risk management within BFS organizations.
SYS-CON Events announced today that Elastifile will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 - Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Elastifile Cloud File System (ECFS) is software-defined data infrastructure designed for seamless and efficient management of dynamic workloads across heterogeneous environments. Elastifile provides the architecture needed to optimize your hybrid cloud environment, by facilitating efficient...
SYS-CON Events announced today that Golden Gate University will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Since 1901, non-profit Golden Gate University (GGU) has been helping adults achieve their professional goals by providing high quality, practice-based undergraduate and graduate educational programs in law, taxation, business and related professions. Many of its courses are taug...
"We provide IoT solutions. We provide the most compatible solutions for many applications. Our solutions are industry agnostic and also protocol agnostic," explained Richard Han, Head of Sales and Marketing and Engineering at Systena America, in this SYS-CON.tv interview at @ThingsExpo, held June 6-8, 2017, at the Javits Center in New York City, NY.
SYS-CON Events announced today that DXWorldExpo has been named “Global Sponsor” of SYS-CON's 21st International Cloud Expo, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Digital Transformation is the key issue driving the global enterprise IT business. Digital Transformation is most prominent among Global 2000 enterprises and government institutions.
21st International Cloud Expo, taking place October 31 - November 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA, will feature technical sessions from a rock star conference faculty and the leading industry players in the world. Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud strategy. Me...
Recently, WebRTC has a lot of eyes from market. The use cases of WebRTC are expanding - video chat, online education, online health care etc. Not only for human-to-human communication, but also IoT use cases such as machine to human use cases can be seen recently. One of the typical use-case is remote camera monitoring. With WebRTC, people can have interoperability and flexibility for deploying monitoring service. However, the benefit of WebRTC for IoT is not only its convenience and interopera...
SYS-CON Events announced today that Secure Channels, a cybersecurity firm, will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Secure Channels, Inc. offers several products and solutions to its many clients, helping them protect critical data from being compromised and access to computer networks from the unauthorized. The company develops comprehensive data encryption security strategie...
From 2013, NTT Communications has been providing cPaaS service, SkyWay. Its customer’s expectations for leveraging WebRTC technology are not only typical real-time communication use cases such as Web conference, remote education, but also IoT use cases such as remote camera monitoring, smart-glass, and robotic. Because of this, NTT Communications has numerous IoT business use-cases that its customers are developing on top of PaaS. WebRTC will lead IoT businesses to be more innovative and address...