|By Michael Kopp||
|May 25, 2011 11:15 AM EDT||
Most articles about Garbage Collection ignore the fact that the Sun Hotspot JVM is not the only game in town. In fact whenever you have to work with either IBM WebSphere or Oracle WebLogic you will run on a different runtime. While the concept of Garbage Collection is the same, the implementation is not and neither are the default settings or how to tune it. This often leads to unexpected problems when running the first load tests or in the worst case when going live. So let’s look at the different JVMs, what makes them unique and how to ensure that Garbage Collection is running smooth.
The Garbage Collection ergonomics of the Sun Hotspot JVM
Everybody believes to know how Garbage Collection works in the Sun Hotspot JVM, but lets take a closer look for the purpose of reference.
The Generational Heap
The Hotspot JVM is always using a Generational Heap. Objects are first allocated in the young generation, specifically in the Eden area. Whenever the Eden space is full a young generation garbage collection is triggered. This will copy the few remaining live objects into the empty survivor space. In addition objects that have been copied to Survivor in the previous garbage collection will be checked and the live ones will be copied as well. The result is that objects only exist in one survivor, while eden and the other survivor is empty. This form of Garbage Collection is called copy collection. It is fast as long as nearly all objects have died. In addition allocation is always fast because no fragmentation occurs. Objects that survive a couple of garbage collections are considered old and are promoted into the Tenured/Old space.
Tenured Generation GCs
The Mark and Sweep algorithms used in the Tenured space are different because they do not copy objects. As we have seen in one of my previous posts garbage collection takes longer the more objects are alive. Consequently GC runs in tenured are nearly always expensive which is why we want to avoid them. In order to avoid GCs we need to ensure that objects are only copied from Young to Old when they are permanent and in addition ensure that the tenured does not run full. Therefore generation sizing is the single most important optimization for the GC in the Hotspot JVM. If we cannot prevent objects from being copied to Tenured space once in a while we can use the Concurrent Mark and Sweep algorithm which collects objects concurrent to the application.
While that shortens the suspensions it does not prevent them and they will occur more frequently. The Tenured space also suffers from another problem, fragmentation. Fragmentation leads to slower allocation, longer sweep phases and eventually out of memory errors when the holes get too small for big objects.
This is remedied by a compacting phase. The serial and parallel compacting GC perform compaction for every GC run in the Tenured space. Important to note is that, while the parallel GC performs compacting every time, it does not compact the whole Tenured heap but just the area that is worth the effort. Worth the effort means when the heap has reached a certain level of fragmentation. In contrast, the Concurrent Mark and Sweep does not compact at all. Once objects cannot be allocated anymore a serial major GC is triggered. When choosing the concurrent mark and sweep strategy we have to be aware of that side affect.
The second big tuning option is therefore the choice of the right GC strategy. It has big implications for the impact the GC has on the application performance. The last and least known tuning option is around fragmentation and compacting. The Hotspot JVM does not provide a lot of options to tune it, so the only way is to tune the code directly and reduce the number of allocations.
There is another space in the Hotspot JVM that we all came to love over the years, the Permanent Generation. It holds classes and string constants that are part of those classes. While Garbage Collection is executed in the permanent generation, it only happens during a major GC. You might want to read up what a Major GC actually is, as it does not mean a Old Generation GC. Because a major GC does not happen often and mostly nothing happens in the permanent generation, many people think that the Hotspot JVM does not do garbage collection there at all.
Over the years all of us run into many different forms of the OutOfMemory situations in PermGen and you will be happy to hear that Oracle intends to do away with it in the future versions of Hotspot.
Now that we had a look at Hotspot, let us look at the difference in the Oracle JRockit. JRockit is used by Oracle WebLogic Server and Oracle has announced that it will merge it with the Hotspot JVM in the future.
The biggest difference is the heap strategy itself. While Oracle JRockit does have a generational heap it also supports a so called continuous heap. In addition the generational heap looks different as well.
The Young space is called Nursery and it only has two areas. When objects are first allocated they are placed in a so called Keep Area. Objects in the Keep Area are not considered during garbage collection while all other objects still alive are immediately promoted to tenured. That has major implications for the sizing of the Nursery. While you can configure how often objects are copied between the two survivors in the Hotspot JVM, JRockit promotes objects in the second Young Generation GC.
In addition to this difference JRockit also supports a completely continuous Heap that does not distinguish between young and old objects. In certain situations, like throughput orientated batch jobs, this results in better overall performance. The problem is that this is the default setting on a server JVM and often not the right choice. A typical Web Application is not throughput but response time orientated and you will need to explicitly choose the low pause time garbage collection mode or a generational garbage collection strategy.
Mostly Concurrent Mark and Sweep
If you choose Concurrent Mark and Sweep strategy you should be aware about a couple of differences here as well. The mostly concurrent mark phase is divided into four parts:
- Initial marking, where the root set of live objects is identified. This is done while the Java threads are paused.
- Concurrent marking, where the references from the root set are followed in order to find and mark the rest of the live objects in the heap. This is done while the Java threads are running.
- Precleaning, where changes in the heap during the concurrent mark phase are identified and any additional live objects are found and marked. This is done while the Java threads are running.
- Final marking, where changes during the precleaning phase are identified and any additional live objects are found and marked. This is done while the Java threads are paused.
The sweeping is also done concurrent to your application, but in contrast to Hotspot in two separate steps. It is first sweeping the first half of the heap. During this phase threads are allowed to allocate objects in the second half. After a short synchronization pause the second half is sweeped. This is followed by another short final synchronization pause. The JRockit algorithm therefore stops more often than the Sun Hotspot JVM, but the remark phase should be shorter. Unlike the Hotspot JVM you can tune the CMS by defining the percentage of free memory that triggers a GC run.
The JRockit does compacting for all Tenured Generation GCs, including the Concurrent Mark and Sweep. It does so in an incremental mode for portions of the heap. You can tune this with various options like percentage of heap that should be compacted each time or how many objects are compacted at max. In addition you can turn off compacting completely or force a full one for every GC. This means that compacting is a lot more tunable in the JRockit than in the Hotspot JVM and the optimum depends very much on the application itself and needs to be carefully tested.
Thread Local Allocation
Hotspot does use thread local allocation, but it is hard to find anything in the documentation about it or how to tune it. The JRockit uses this on default. This allows threads to allocate objects without any need for synchronization, which is beneficial for allocation speed. The size of a TLA can be configured and a large TLA can be beneficial for applications where multiple threads allocate a lot of objects. On the other hand a too large TLA can lead to more fragmentation. As a TLA is used exclusively by one thread, the size is naturally limited by the number of threads. Thus both decreasing and increasing the default can be good or bad depending on your applications architecture.
Large and small objects
The JRockit differentiates between large and small objects during allocation. The limit for when an object is considered large depends on the JVM version, the heap size, the garbage collection strategy and the platform used. It is usually somewhere between 2 and 128 KB. Large objects are allocated outside thread local area in in case of a generational heap directly in the old generation. This makes a lot of sense when you start thinking about it. The young generation uses a copy ccollection. At some point copying an object becomes more expensive than traversing it in ever garbage collection.
No permanent Generation
And finally it needs to be noted that the JRockit does not have a permanent generation. All classes and string constants are allocated within the normal heap area. While that makes life easier on the configuration front it means that classes can be garbage collected immediately if not used anymore. In one of my future posts I will illustrate how this can lead to some hard to find performance problems.
The IBM JVM
The IBM JVM shares a lot of characteristics with JRockit: The default heap is a continuous one. Especially in WebSphere installation this is often the initial cause for bad performance. It differentiates between large and small objects with the same implications and uses thread local allocation on default. It also does not have a permanent generation, but while the IBM JVM also supports a generational Heap model it looks more like Sun’s rather than JRockit.
Allocate and Survivor act like Eden and Survivor of the Sun JVM. New objects are allocated in one area and copied to the other on garbage collection. In contrast to JRockit the two areas are switched upon gc. This means that an object is copied multiple times between the two areas before it gets promoted to Tenured. Like JRockit the IBM JVM has more options to tune the compaction phase. You can turn it off or force it to happen for every GC. In contrast to JRockit the default triggers it due to a series of triggers but will then lead to a full compaction. This can be changed to an incremental one via a configuration flag.
We see that while the three JVMs are essentially trying to achieve the same goal, they do so via different strategies. This leads to different behaviour that needs tuning. With Java 7 Oracle will finally declare the G1 (Garbage First) production ready and the G1 is a different beast altogether, so stay tuned.
If you’re interested in hearing me discuss more about WebSphere in a production environment, then check out our upcoming webinar with The Bon-Ton Stores. I’ll be joined by Dan Gerard, VP of Technical & Web Services at Bon-Ton, to discuss the challenges they’ve overcome in operating a complex Websphere production eCommerce site to deliver great web application performance and user experience. Reserve your seat today to hear me go into more detail about Websphere and production eCommerce environments.
- The impact of Garbage Collection on Java performance // In my last post I explained what a major...
- Major GCs – Separating Myth from Reality In a recent post we have shown how the Java...
- JDK6 Update 23 changes CMS Collection counters Stefan Frandl, Test Automation Team Lead at dynaTrace, recently tested...
- The Top Java Memory Problems – Part 1 // Memory and Garbage Collection problems are still the most...
- Troubleshooting response time problems – why you cannot trust your system metrics // Production Monitoring is about ensuring the stability and health...
WebRTC defines no default signaling protocol, causing fragmentation between WebRTC silos. SIP and XMPP provide possibilities, but come with considerable complexity and are not designed for use in a web environment. In his session at Internet of @ThingsExpo, Matthew Hodgson, technical co-founder of the Matrix.org, will discuss how Matrix is a new non-profit Open Source Project that defines both a new HTTP-based standard for VoIP & IM signaling and provides reference implementations.
Oct. 22, 2014 05:15 AM EDT Reads: 997
SUNNYVALE, Calif., Oct. 20, 2014 /PRNewswire/ -- Spansion Inc. (NYSE: CODE), a global leader in embedded systems, today added 96 new products to the Spansion® FM4 Family of flexible microcontrollers (MCUs). Based on the ARM® Cortex®-M4F core, the new MCUs boast a 200 MHz operating frequency and support a diverse set of on-chip peripherals for enhanced human machine interfaces (HMIs) and machine-to-machine (M2M) communications. The rich set of periphera...
Oct. 21, 2014 08:30 PM EDT Reads: 1,360
SYS-CON Events announced today that Aria Systems, the recurring revenue expert, has been named "Bronze Sponsor" of SYS-CON's 15th International Cloud Expo®, which will take place on November 4-6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Aria Systems helps leading businesses connect their customers with the products and services they love. Industry leaders like Pitney Bowes, Experian, AAA NCNU, VMware, HootSuite and many others choose Aria to power their recurring revenue business and deliver exceptional experiences to their customers.
Oct. 21, 2014 06:00 PM EDT Reads: 1,178
The Internet of Things (IoT) is going to require a new way of thinking and of developing software for speed, security and innovation. This requires IT leaders to balance business as usual while anticipating for the next market and technology trends. Cloud provides the right IT asset portfolio to help today’s IT leaders manage the old and prepare for the new. Today the cloud conversation is evolving from private and public to hybrid. This session will provide use cases and insights to reinforce the value of the network in helping organizations to maximize their company’s cloud experience.
Oct. 21, 2014 05:15 PM EDT Reads: 1,171
The Internet of Things (IoT) is making everything it touches smarter – smart devices, smart cars and smart cities. And lucky us, we’re just beginning to reap the benefits as we work toward a networked society. However, this technology-driven innovation is impacting more than just individuals. The IoT has an environmental impact as well, which brings us to the theme of this month’s #IoTuesday Twitter chat. The ability to remove inefficiencies through connected objects is driving change throughout every sector, including waste management. BigBelly Solar, located just outside of Boston, is trans...
Oct. 21, 2014 09:00 AM EDT Reads: 1,494
SYS-CON Events announced today that Matrix.org has been named “Silver Sponsor” of Internet of @ThingsExpo, which will take place on November 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Matrix is an ambitious new open standard for open, distributed, real-time communication over IP. It defines a new approach for interoperable Instant Messaging and VoIP based on pragmatic HTTP APIs and WebRTC, and provides open source reference implementations to showcase and bootstrap the new standard. Our focus is on simplicity, security, and supporting the fullest feature set.
Oct. 20, 2014 11:45 PM EDT Reads: 1,242
Predicted by Gartner to add $1.9 trillion to the global economy by 2020, the Internet of Everything (IoE) is based on the idea that devices, systems and services will connect in simple, transparent ways, enabling seamless interactions among devices across brands and sectors. As this vision unfolds, it is clear that no single company can accomplish the level of interoperability required to support the horizontal aspects of the IoE. The AllSeen Alliance, announced in December 2013, was formed with the goal to advance IoE adoption and innovation in the connected home, healthcare, education, aut...
Oct. 20, 2014 11:15 PM EDT Reads: 1,681
SYS-CON Events announced today that Red Hat, the world's leading provider of open source solutions, will exhibit at Internet of @ThingsExpo, which will take place on November 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Red Hat is the world's leading provider of open source software solutions, using a community-powered approach to reliable and high-performing cloud, Linux, middleware, storage and virtualization technologies. Red Hat also offers award-winning support, training, and consulting services. As the connective hub in a global network of enterprises, partners, a...
Oct. 20, 2014 09:45 PM EDT Reads: 1,247
The only place to be June 9-11 is Cloud Expo & @ThingsExpo 2015 East at the Javits Center in New York City. Join us there as delegates from all over the world come to listen to and engage with speakers & sponsors from the leading Cloud Computing, IoT & Big Data companies. Cloud Expo & @ThingsExpo are the leading events covering the booming market of Cloud Computing, IoT & Big Data for the enterprise. Speakers from all over the world will be hand-picked for their ability to explore the economic strategies that utility/cloud computing provides. Whether public, private, or in a hybrid form, clo...
Oct. 20, 2014 07:00 PM EDT Reads: 1,963
Software AG helps organizations transform into Digital Enterprises, so they can differentiate from competitors and better engage customers, partners and employees. Using the Software AG Suite, companies can close the gap between business and IT to create digital systems of differentiation that drive front-line agility. We offer four on-ramps to the Digital Enterprise: alignment through collaborative process analysis; transformation through portfolio management; agility through process automation and integration; and visibility through intelligent business operations and big data.
Oct. 20, 2014 03:45 PM EDT Reads: 1,684
The Transparent Cloud-computing Consortium (abbreviation: T-Cloud Consortium) will conduct research activities into changes in the computing model as a result of collaboration between "device" and "cloud" and the creation of new value and markets through organic data processing High speed and high quality networks, and dramatic improvements in computer processing capabilities, have greatly changed the nature of applications and made the storing and processing of data on the network commonplace.
Oct. 20, 2014 02:00 PM EDT Reads: 1,674
Be Among the First 100 to Attend & Receive a Smart Beacon. The Physical Web is an open web project within the Chrome team at Google. Scott Jenson leads a team that is working to leverage the scalability and openness of the web to talk to smart devices. The Physical Web uses bluetooth low energy beacons to broadcast an URL wirelessly using an open protocol. Nearby devices can find all URLs in the room, rank them and let the user pick one from a list. Each device is, in effect, a gateway to a web page. This unlocks entirely new use cases so devices can offer tiny bits of information or simple i...
Oct. 20, 2014 02:00 PM EDT Reads: 1,776
Things are being built upon cloud foundations to transform organizations. This CEO Power Panel at 15th Cloud Expo, moderated by Roger Strukhoff, Cloud Expo and @ThingsExpo conference chair, will address the big issues involving these technologies and, more important, the results they will achieve. How important are public, private, and hybrid cloud to the enterprise? How does one define Big Data? And how is the IoT tying all this together?
Oct. 20, 2014 12:00 PM EDT Reads: 1,660
Oct. 20, 2014 12:00 PM EDT Reads: 1,862
TechCrunch reported that "Berlin-based relayr, maker of the WunderBar, an Internet of Things (IoT) hardware dev kit which resembles a chunky chocolate bar, has closed a $2.3 million seed round, from unnamed U.S. and Switzerland-based investors. The startup had previously raised a €250,000 friend and family round, and had been on track to close a €500,000 seed earlier this year — but received a higher funding offer from a different set of investors, which is the $2.3M round it’s reporting."
Oct. 20, 2014 09:00 AM EDT Reads: 1,600
The Industrial Internet revolution is now underway, enabled by connected machines and billions of devices that communicate and collaborate. The massive amounts of Big Data requiring real-time analysis is flooding legacy IT systems and giving way to cloud environments that can handle the unpredictable workloads. Yet many barriers remain until we can fully realize the opportunities and benefits from the convergence of machines and devices with Big Data and the cloud, including interoperability, data security and privacy.
Oct. 19, 2014 10:00 PM EDT Reads: 1,553
All major researchers estimate there will be tens of billions devices - computers, smartphones, tablets, and sensors - connected to the Internet by 2020. This number will continue to grow at a rapid pace for the next several decades. Over the summer Gartner released its much anticipated annual Hype Cycle report and the big news is that Internet of Things has now replaced Big Data as the most hyped technology. Indeed, we're hearing more and more about this fascinating new technological paradigm. Every other IT news item seems to be about IoT and its implications on the future of digital busines...
Oct. 19, 2014 09:00 PM EDT Reads: 1,829
Cultural, regulatory, environmental, political and economic (CREPE) conditions over the past decade are creating cross-industry solution spaces that require processes and technologies from both the Internet of Things (IoT), and Data Management and Analytics (DMA). These solution spaces are evolving into Sensor Analytics Ecosystems (SAE) that represent significant new opportunities for organizations of all types. Public Utilities throughout the world, providing electricity, natural gas and water, are pursuing SmartGrid initiatives that represent one of the more mature examples of SAE. We have s...
Oct. 19, 2014 07:30 PM EDT Reads: 1,481
The Internet of Things needs an entirely new security model, or does it? Can we save some old and tested controls for the latest emerging and different technology environments? In his session at Internet of @ThingsExpo, Davi Ottenheimer, EMC Senior Director of Trust, will review hands-on lessons with IoT devices and reveal privacy options and a new risk balance you might not expect.
Oct. 19, 2014 11:00 AM EDT Reads: 1,919
IoT is still a vague buzzword for many people. In his session at Internet of @ThingsExpo, Mike Kavis, Vice President & Principal Cloud Architect at Cloud Technology Partners, will discuss the business value of IoT that goes far beyond the general public's perception that IoT is all about wearables and home consumer services. The presentation will also discuss how IoT is perceived by investors and how venture capitalist access this space. Other topics to discuss are barriers to success, what is new, what is old, and what the future may hold.
Oct. 19, 2014 11:00 AM EDT Reads: 1,700