Characterizing Java Virtual Machine for More Efficient Processor Power Management. Abstract

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1 Characterizing Java Virtual Machine for More Efficient Processor Power Management Marcelo S. Quijano, Lide Duan Department of Electrical and Computer Engineering University of Texas at San Antonio Abstract With CPUs dominating the power consumption of a datacenter, improving energy efficiency is a primary design goal for current processors, servers, and even datacenters. Therefore, power management schemes such as clock gating and power gating have been proposed to lower the processor power consumption. These schemes utilize different sleep states implemented in current commercial CPUs, dynamically transitioning the processor into a suitable sleep state based on its workload. In particular, power gating is a vital solution to eliminate leakage power nowadays representing a significant amount of the processors power dissipation. Although operating at various sleep states reduces CPU idle power, transitioning to and from these states incurs overhead in power consumption and wake-up latencies. The existing power management solutions for a computer system usually focus on the architecture layer with no notion of higher layers such as operating system and virtualization. As a result, their power saving may be suboptimal if cross-layer interaction is taken into account. In this paper, we propose investigating the Java Virtual Machine (JVM) to generate power saving hints at virtualization layer and provide useful information to the hardware system for selecting adequate power limits for the underlying processor. Different components of the JVM are capable of characterizing the software being compiled for optimization purposes. For instance, the garbage collector keeps track of the lifetime of different objects in the JVM; the Just In Time (JIT) compiler determines the frequency of the different methods. Useful information gathered by different features within the JVM can be used to construct a framework that provides guidance for effective low-level power management. Furthermore, benchmarking software can be used to characterize a certain JVM implementation and isolate the least or the most power hungry components and program phases. Introduction Popular demand for websites, cloud storage, and databases, among other components in the preceding years, has caused data centers to grow and expand. Increasing the number of servers running in a datacenter increases one important factor; power consumption. From 25 to 21 the power consumed by datacenters in the U.S. increased approximately 56% [reference]. This increase in power consumption has driven an interest in optimizing power efficiency of a datacenter. In modern datacenters, CPU makes up 42% of the total peak power, making it the top power consumer [reference]. Therefore lowering the power consumption of the CPU in servers will reduce the overall datacenter power. The motive of this paper is to provide a new approach to make clock and power gating more efficient, power friendly, and cost effective. Clock gating is a technique used to put the processor in different

2 sleep states depending on its demand. Power gating is the technique of completely shutting down the processor whenever it is advantageous towards reducing power consumption. The goal is to use data gathered by different components of the JVM to provide clues to the power management hardware to determine the correct power state for the underlying processor. This paper explores the use of Powertop, a tool that provides real time information about a program s effect toward power driving factors of a processor, to gather information about the processor s states while running different benchmarks in different JVMs. First, some useful background information about the JVM s architecture, high level functionality of the JVM and power consumption of JVMs are provided to help the reader understand certain data and conclusions we will provide throughout the paper. Suggestions on how JVMs can be used to reduce processor s power are also provided in the Background section. Then all the experimental components for the experiment in this paper will listed, defined, and their functionalities towards the experiment will be explained including: Powertop, the DaCapo Benchmarks, and Icedtea and Jre8 JVMs. The paper will then be followed by the results and analyses section which will be composed of the results provided by Powertop when running a single benchmark in Icedtea, each benchmark individually in Icedtea, and each benchmark individually in Icedtea and Jre8. The results will then be analyzed and finally conclusions will be made based on the results. Background The Java Virtual Machine A JVM is composed of three main sections: the Virtual Machine Runtime Area, the Garbage Collector, and the Just In Time (JIT) compiler. The VM Runtime Area is responsible for the JVM life cycle, exception handling, error handling, class loading, interpreter, Java native interface and thread management and Synchronization. The Garbage Collector is used for Java object memory allocation and reclamation. The JIT Compiler interprets Java byte codes into native code for the underlying platform [3]. Java programs are compiled by a Java compiler to generate class files consisting of bytecode, i.e. the instruction set of the JVM. During execution, the class loader loads the class files into to the RAM. The bytecode in the class files is then converted into native machine code by the execution engine using a JIT compiler. Figure 1 provides a visual representation of a JVM [3], and one can see how the runtime area communicates with the class loader and the execution engine.

3 Figure 1. The Java Virtual Machine Architecture In the runtime area in Figure 1, there are five components, for the purpose of this paper, I will only explain the heap, since it is the section of memory that is segregated for JVM use and it is the area were the garbage collector operates. The heap is separated into two sections, young and old generation. In the young generation section, short lived data is stored, and in the old generation section, data that has survived several garbage collections is stored. The heap is also designed to keep track of how many garbage collections each surviving object has survived. This data can helpful in determining which objects are used the most [3]. Power Consumption of JVMs Research shows that the peak power consumption of JVMs was usually caused by the application they were running and not by its components [4]. For this reason, research focused on reducing the power consumption of JVMs by implementing optimizations on the different components has faded, and the focus has shifted more on to modulating application peak power. Using the JVM to Reduce Power After having a general idea of how a JVM and its components function, it seems feasible to use certain information, gathered by different components of the JVM, about the program being executed to assist the power management hardware decide the adequate processor power state. For instance, the garbage collector records the lifetime of different objects in the JVM s heap. The Just In Time (JIT) compiler uses different methods to determine the frequency of different methods called in the application executed by the JVM and collecting this data may be helpful towards assisting in determining the power state the processor should be running in.

4 Experimental Setup This section provides a summary of what Powertop is capable of providing and how it was used in the experiment. It also summarizes the purpose of the DaCapo benchmarks and how they are used in the experiments. Power and Energy Measurements Using Powertop Powertop is a Linux based tool that provides real time information about programs influence to a processor s state and activity while they are executed. Powertop has seven different modes, and two of them were used in our experiments: html mode and workload mode. Html mode and workload mode were used for Powertop to run a specified workload and store its results in an html file. As depicted in Figure 2, the html file consists of 8 tabs: Summary tab, CPU Idle tab, CPU Frequency Tab, Software Info Tab, Device info Tab, Tuning Tab, Advanced Host Controller Interface (AHCI) tab, and All tab. The summary tab lists the top power consuming processes which kept waking the processor up while running the benchmark. The CPU Idle tab denotes the different Idle states (i.e. the C-states) broken up by CPU, core, and package. The server s processor used in this experiment has 4 different C-states, C, C1,C3, and C6, with C being the active state, C6 the power gating deep sleep state, and C1 and C3 being the clock gating idle states in between active and deep sleep. The frequency tab presents the different scalable active state frequencies also broken up by CPU, core, and package. There are 16 scalable frequencies in the processor used and they vary from 12 MHz to more than 2.6 GHz. The highest being turbo mode varies from 2.7 GHz to 3.2 GHz, and the lowest being 12 MHz. The software info tab depicts the same data as the Summary tab, which shows the top power consuming items which kept waking up the processor, but shows more details like the GPU s operations per second, disk input output per second, and graphics card wakeups per second caused by each program. The device info tab lists the hardware devices that consume the most power. The tuning tab denotes the list of devices that are not tuned for power management and shows commands on how to tune them individually. The AHCI tab is not supported by the server used. Finally, the All tab shows all the tabs data in one place [5].

5 Figure 2. Html File Created by Powertop The DaCapo Benchmark Suite The DaCapo Benchmarks are a set of realistic, open source Java applications. They were created as an effort to produce Java benchmarks that followed two main criteria: diverse real applications and ease of use. These criteria were met by gathering diverse Java programs to increase the coverage of application behavior, using benchmarks that are easy to measure and have minimal dependencies, and provide a range of various input sizes [6]. The benchmarks used for this experiment are: batik, fop, h2, luindex, lusearch, sunflow, and xalan. Batik benchmark creates a number of Scalable Vector Graphics (SVG) files. Fop benchmark converts ax XLS-FO file to a PDF file. H2 benchmark is an in-memory database benchmark executing transactions against a baking like application. Luindex indexes a set of documents comprised of the works of Shakespeare and King James Bible. Lusearch benchmark does a text search of keywords over a data composed of works of Shakespeare and King James Bible. Sunflow benchmark generates a set of images using ray tracing technique. Xalan benchmark transforms XML documents into HTML files [6]. The Java Virtual Machines The two JVMs used in this experiment are Icedtea and Jre8. Icedtead JVM is an open source that was invented to provide a solution to build OpenJDK using free non-proprietary tools. OpenJDK is

6 Event/s an open source JVM implemented by Sun Microsystems in order to be able to use it in free GNU/Linux distributions. Jre8 JVM is a proprietary JVM owned by Sun Microsystems. Since Jre8 is a proprietary JVM, it is expected to be a better JVM when comparing both power and effectiveness. Results and Analyses This section presents Powertop s results when running different experiments with different JVMs and different Benchmarks. The first section focuses in running one benchmark and depicts useful data that Powertop provides. The second section shows results and conclusions regarding running each benchmark once using Icedtea JVM. Finally, the third section compares Powertop s results when running each benchmark in each JVM, Icedtea and Jre8. Similarities, differences, and conclusions are stated based on the results. Sunflow Benchmark Analyses Using Powertop To demonstrate what Powertop is capable of providing, Powertop was used to run sunflow benchmark once in a single thread. It was given a large input, and an html file was generated with the data gathered while the benchmark was executed. Icedtea JVM was used to run the benchmark. Figures 3, 4, and 5 depict the data gathered from the html file created by Powertop after running sunflow with the criteria above. 25 Wakeups/s by Sunflow Benchmark sunflow Benchmark Figure 3. Wakeups/s while Executing Sunflow Benchmark Using Icedtea The number of wakeups per second by a processor is an important aspect to consider when looking at power efficiency, and Powertop can gather this information when executing a benchmark. Figure 3 illustrates the number of wakeups per second (around 225 wakeups/s) that occurred while the sunflow benchmark was executed. Data presented in later sections will conclude that sunflow wakes up the processor quite often compared to other benchmarks executed in Icedtea JVM.

7 Percentage of Time Spend in Each State Percent of Time in an Idle State Sunflow C % C1 % C3 % C6 % Idle States Figure 4. Powertop Results Regarding Idle States while Executing Sunflow Benchmark Using Icedtea Figure 4, denotes the percentage of time that the processor spends in each of the idle states. Since the sunflow benchmark uses ray tracing to generate a set of images, it is expected to be computation intensive, and Figure 4 conveys that. When running the sunflow benchmark, the processor spends over 8 percent of the time in the C or active state, and a low 15 percent of the time, the processor goes into deep sleep while it is in the idle states. Percentage of Time in Turbo Mode vs Idle Mode sunflow Benchmark Turbo Mode Idle Figure 5. Powertop Results Regarding Turbo Mode vs Idle Mode while Executing Sunflow Benchmark Using Icedtea To compare the time spent in turbo mode vs idle mode, Figure 5 was introduced. Turbo mode

8 Percentage of Time Spend in a State Percentage of Time in an Idle State represents the processor running at its maximum frequency. These results were gathered from the frequency tab in the html file. The graph shows that the processor was in turbo mode close to 8 percent of the time and only 2 percent in idle mode. Figures 6 and 7 demonstrate the power effects of varying the input size of the benchmark. Based on the two figures, it can be determined that the greater the input size, the more power the processor will consume. This is because the smaller input spends more time in idle states rather than active states compared to the results given with the bigger input. Small vs Large Input Idle States Comparison C % C1 % C3 % C6 % Idle States sunflow large input sunflow small input Figure 6. Powertop Results Regarding Turbo Mode vs Idle Mode while Executing Sunflow Benchmark Using Icedtea Small vs Large Input Active vs Idle State Comparison sunflow large input Input Size sunflow small input Turbo Mode Idle Figure 7. Powertop Results Regarding Turbo Mode vs Idle Mode while Executing Sunflow Benchmark Using Icedtea

9 Events/s Comparing Benchmarks Using Powertop Using Powertop, seven benchmarks were used for this experiment. This section explores the results provided by Powertop when each of these benchmarks was executed individually. Each benchmark was executed once, given the biggest input possible, and in a single thread. Powertop then stored the results in an html file after each execution where the data for the graphs was gathered from Events/s for Each Benchmark Using Icedtea batik fop h2 luindex lusearch sunflow xalan Benchmark Figure 8. Wakeups/s while Executing Each Benchmark Independently Using Icedtea Figure 8 denotes the processor s wakeups per second during the execution of each of the stated benchmarks. It is obvious that lusearch, sunflow, and xalan benchmarks wake the processor up with the highest frequencies. This is due to the fact that those three benchmarks are the most computational intensive. Looking at Figure 8, one can also determine that the luindex benchmark is the benchmark that needed the least processor assistance. This makes sense since it is only indexing documents, which does not require much effort by the processor.

10 Percent of Time in an Idle State Time Spend in Different Idle States based on Pecentage Running Icedtea C % C1 % C3 % C6 % Idle States batik fop h2 luindex lusearch sunflow xalan Figure 9. Powertop Results Regarding Idle States while Executing Each Benchmark Independently Using Icedtea After analyzing Figure 9, one can conclude that sunflow and xalan spend the most percentage of time in C active state, lusearch is the only benchmark that spend some time in the C1 state, and luindex is the benchmark with the most percentage of time in C6 deep sleep state. Sunflow and xalan benchmarks spending most of their time in the C active state is in accordance with Figure 8, since they were also the benchmarks that caused the most wakeups per second along with lusearch. Generating a set of images using ray tracing technique and transforming XML documents into HTML files, which are done in these two benchmarks, are both computation intensive. The fact that lusearch is only in active state 5 plus percent of the time being one of the benchmarks that woke up the processor the most is due to the chuck of time spend in the C1 state. Since a text search of keywords over a corpus of data may require the processor constantly, but it is not as computational intensive as sunflow and xalan benchmarks. Finally, it is in agreement with Figure 8 that luindex should spend the most time in deep sleep since it is the least computational intensive.

11 Execution Time in ms Percentage of Time Spend in State Percentage of Time in Turbo Mode vs Idle State batik fop h2 luindex lusearch sunflow xalan Benchmarks Turbo Mode Idle Figure 1. Powertop Results Regarding Turbo Mode vs Idle Mode while Each Benchmark Independently Using Icedtea Figure 1 presents the percentage of time spent in turbo mode vs idle mode. The conclusions made from this graph are the same as in Figure 8 and 9 for the same reasons. The most computational intensive benchmarks, sunflow and xalan, spend the most time in turbo mode and the least intensive program consumed the greatest percentage of time in idle mode Execution by Each Benchmark batik fop h2 luindex lusearch sunflow xalan Benchmarks Figure 11. Execution Time by Each Benchmark Using Icedtea If execution time per benchmark is taken into account, the results do not vary much. Figure 11 demonstrates that again both sunflow and xalan benchmarks take the longest time to execute and luindex benchmark the least time.

12 Percent of Time in an Idle State Percent of Time in an Idle State Comparing JVMs Using Powertop This section compares two java virtual machines (JVM), Icedtea and Jre8, and their results when running the benchmarks. First, comparisons between the time spent in different idle states for different benchmarks between both JVMs will be made, followed by comparisons between time spent in turbo vs idle mode, and finally, contrasts between execution times of each benchmark between both JVMs will be presented. Time Spend in Different Idle States based on Pecentage Icedtea C % C1 % C3 % C6 % Idle States batik fop h2 luindex lusearch sunflow xalan Time Spend in Different Idle States based on Pecentage Running Jre C % C1 % C3 % C6 % Idle States batik fop h2 luindex lusearch sunflow xalan Figure 12. Powertop Results Regarding Idle States while Executing Each Benchmark Independently By observing Figure 12, one can infer that Jre8 is more power efficient since it spends more percentage of time in deep sleep state, C6. It demonstrates this conclusion more drastically when looking at batik and fop benchmarks. All other benchmarks gave roughly the same results.

13 Percentage of Time Spend in State Percentage of Time Spend in State Percentage of Time in Turbo Mode vs Idle State Running in Icedtea Turbo Mode Idle Benchmarks Percentage of Time in Turbo Mode vs Idle State Running in Jre Turbo Mode Idle Benchmarks Figure 13. Powertop Results Regarding Turbo Mode vs Idle Mode while Each Benchmark Independently When observing the time spend in Turbo vs Idle mode, the same holds true, Jre8 is more efficient. This statement can be made by observing Figure 13. Similar to when comparing idle states, the batik and fop benchmarks spend a greater percent of time notoriously when running in Jre8 compared to running them in Icedtea. When comparing Turbo vs Idle mode xalan benchmark also spends a significantly greater percentage of time in Idle mode when running in Jre8 compared than running in Icedtea. Besides those benchmarks, all other benchmarks showed similar results when running in each JVM when comparing Turbo and Idle mode. When looking at the execution time for each benchmark using each of the JVMs in Figure 13, one can conclude that Jre8 is quicker at executing most of the benchmarks. Fop, luindex, sunflow, and xalan were execute slightly faster by Jre8, and Icedtea was quicker at executing batik, h2, and lusearch. Something to take into account is that only substantial difference between the execution times between the JVMs occurs when executing the batik benchmark, in which Jre8 takes nearly twice as long as Icedtea.

14 Conclusions and Future Work The motivation of this work is to generate power saving hints at virtualization layer and provide useful information to the hardware system for selecting adequate power limits for the underlying processor. Powertop was presented as a possible solution for a JVM power measurement tool. It was successful at providing data that lead to conclusions on which type of applications are power hungry and how Jre8 is better than Icedtea JVM. First, it can be concluded that graphic related programs such as generating images are power hungry when running in the JVM. Also, any complex transformation from one file type to another can also consume a fair amount of power, e.g. in the case of the xalan benchmark converting XML files to HTML files. On the contrary, a simpler file conversion, which was implemented by flop benchmark, from XLS-FO file to a PDF file does not consume as much power. Also, a simple program like indexing a set of documents is also power friendly when implemented. The next step to take based on these conclusions is looking into what is happening inside the JVM that is causing a program to consume or not consume that much power? What is the state of the JVM when an application is causing it to consume power? What type of functions are the ones that make the processor be active the longest? Second, Jre8 was slightly more efficient then Icedtea when taking execution time into account, but the differences were minimal for most benchmarks. Jre8 was also more efficient when it came down to power management, and the differences there were more notable. The next step will be to analyze why Jre8 is more power efficient than Icedtea? What are the difference in the internal states when running a power hungry vs a low power program? Additional future studies include analyzing the garbage collector when running a power hungry application. Useful data on objects lifetime may be acquired and analyzed and conclusions can be made on why a particular object has a short or long lifetime within the JVM. The JIT compiler is another component that may store useful data that may be used to draw conclusions, e.g. it uses different techniques to determine the frequency of the different methods implemented by the program running in the JVM. References [1] Koomey, Jonathan, Growth in Data Center Electricity Use 25 to 21, A report by Analytics Press, The New York Times (211). [2] L. Barroso, J. Clidaras, and U. Holzle, The Datacenter as a Computer: An Introduction to the Design of Warehouse-Scale Machines, Second Edition, Morgan & Claypool Publishers (213). [3] Venners, Bill, Inside the Java Virtual Machine, McGraw-Hill, Inc. Press, New York, NY, 1996, Chap. 5. [4] Contreras, Gilberto and Martonosi, Margaret Techniques for Real-System Characterization of Java Virtual Machine Energy and Power Behavior, IEEE (26). [5] Accardi, Kristen C.and Yates, Alexandra, Powertop User s Guide, Intel Corporation (214). [6] Blackburn, Garner, Hoffmann, Khan, McKinley, Bentzur, Diwan, Feinberg, Guyer, Hirzel, Hosking, Jump, Lee, Moss, Phansalkar, Stefanovi, VanDrunen, von Dincklage, Wiedermann The DaCapo

15 Benchmarks: Java Benchmarking Development and Analysis, OOPSLA (26).