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Getting a Handle on Rogue Transactions and Execute Threads

Getting a Handle on Rogue Transactions and Execute Threads

Q. What can I do about "rogue" transactions that are affecting the overall performance of my application?
A.
Many production J2EE applications suffer from rogue transactions. A rogue transaction is a particular use case or click-through in the application that results in enormous resource consumption or unusually high response times when compared with its peers. In practice, an application suffering from such a problem will display erratic and unpredictable resource consumption patterns or response times.

If we take an application with some single point of dispatch (e.g., controller servlet), it might display two very distinct response times and throughput patterns if there are one or many threads involved in a rogue transaction. Before the rogue transaction hits the server, the CPU and memory usage patterns may be calm. Like a large rock hitting the surface of a pond, from the moment a rogue transaction has entered the application and begun to be dispatched, its voracious CPU and memory consumption may ripple through the rest of the JVM and hardware, perturbing the other, better-behaved transactions in mid-dispatch.

If the rogue transactions are rare, they can show up in performance metrics as sustained anomalies in response time and throughput. If there are a few consistently running at any given time, however, response time and throughput may consistently show poor numbers. In this case, you may not have rogue transaction so much as a couple of consistently expensive use cases. In the latter case, you can generally identify the problem with a profiler or any other tool that will be able to breakdown the responsiveness of the application by application logic or use case.

But what of the situation when you truly have a rare rogue transaction that traverses your system, wreaking havoc in its wake and disappearing as suddenly as it appeared? Your only real option before resorting to tools is to thread dump the JVM while the rogue transaction is being dispatched. Much has been written about collecting and reading thread dumps. You should try to perform at least three thread dumps while your server is in its exhausted state, reading them for any thread that appears to be busy in the same program logic across all three.

Now you should focus your response time analysis on this piece of program logic. If you are sure you can reproduce the same datasets being used by this logic in a staging area, try profiling the logic and measuring its CPU and clock time outside production. If not, consider adding logging or other tooling to monitor the responsiveness of the routines that appear to be costly and slow. One common source of rogue transactions that burn CPU is XML parsing and transformation - some implementations will generate enormous numbers of temporary objects in the course of doing their work, which in turn forces more object instantiation and garbage collection. A cousin of the rogue transaction is the infinite loop: the use case or click-through causes a thread to enter a loop without a break condition. Infinite loops tend to be easier to diagnose. An application server or JVM with a thread in an infinite loop will cause one CPU on the server per thread stuck in a loop, to sit saturated at 100% utilization. The loops should jump out in a series of thread dumps - a thread will literally not have moved from a particular piece of application code in any of them.

Q. What are execute threads and how do I configure them appropriately?
A.
If BEA WebLogic Server were said to have any single "utility" it required in order to handle incoming transactions, you might pick execute threads.

One of the main ideas behind J2EE for Web application development was the notion of a servlet container living in a live JVM. With the JVM as a process always resident in memory, incoming requests could be handled by invoking lightweight threads. Even better, you could pool these threads and reuse them when you were finished dispatching a request.

Naturally, like most pooled resources, thread pools have some settings for their initial size and maximum size. They may also have settings for how quickly to grow the pool, how often to check that resources have been returned to it, and so on.

In the BEA WebLogic Server, the Execute Queue functions as the thread pool for incoming requests. If you see your response times slow down dramatically as you increase load on your production application - even though you haven't maxed out the CPU(s) of the hardware on which your application is hosted - then you may be bottlenecking your WebLogic Server with its Execute Queue.

A good setting strikes a suitable balance between the available CPU resources and the volume of incoming requests. If your CPU is already saturated, increasing the thread pool size is not going to help the situation and may even make it worse. Larger thread pools mean more work for the thread scheduler in the JVM and for the WebLogic Server itself. The best way to set the Execute Queue's size is to keep an eye on the following four variables:

  • CPU utilization
  • Application responsiveness
  • Application throughput
  • WebLogic Server Execute Queue threads busy

    BEA WebLogic Server publishes a metric, via JMX, showing how many of the execute threads are busy. If you have the luxury of a staging environment with a load similar to your production environment, try setting the Execute Queue size all the way down to five available application threads - then monitor the business of these threads from the WebLogic Server JMX metrics or some other tool. Slowly increase the number of threads in increments of five, watching how this affects the application responsiveness, throughput, and CPU utilization.

    Once you have a thread pool that is staying more than 80% available without the CPU being saturated, you should stop increasing the pool size. Any further increases and you can end up slowing everything down.

    As always, I invite you to send an e-mail to [email protected] if you have any performance-related questions about JVMs, Java applications, WebLogic Server, or connections to back-end systems.

  • More Stories By Lewis Cirne

    Lew Cirne is the founder of New Relic, the first provider of on-demand (SaaS) application management tools for cloud or datacenter applications. A seasoned entrepreneur, technologist, and enterprise software pioneer, he has been focused on application performance management for more than ten years. Cirne holds seven patents related to application performance technology. Most recently he was an Entrepreneur in Residence at Benchmark Capital. He founded and was first CEO of Wily Technology and earlier held senior engineering positions at Apple and Hummingbird Communications.

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