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Java Virtual Memory Recovery Tips


Over the past few days, some readers have told us that they have experienced Java virtual memory.


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    When it comes to decimation, Java virtual memory is called JVM. The JVM has an interpreter to help you interpret your Java program. class file in byte code that can be defined by your computer or computer. The JVM is mainly used for memory allocation, garbage collection, and for throwing runtime exceptions.



    java virtual memory

    This has been a Java problem for a long time, but it’s actually pretty much pointless and mostly based on a willingness to provide false information. Ordinary words are something like “Hello World in Java, megabytes are needed! 10 Why does he really need it? ” Well, here’s a way to help Hello World with a trial version of a 64-bit JVM take over 4 gigabytes … which can be found with at least some form of measurement.

    java -Xms1024m -Xmx4096m com.example.Hello

    On Linux, the main clients of the command assign several different memory numbers to memory. Here’s what it says about the Hello World example:

    PID USER PR NI VIRT RES S shr% CPU% MEM TIME + COMMAND 2120 kg regory 20 6 4373m 15m 7152 S nothing 0,2 0: 00.10 java

    • VIRT is undoubtedly virtual storage: a layer of everything in a virtual memory map (see below). This is pretty much pointless unless it is (see below).
    • RES is just the size of the resident seta: The number of the World Wide Web currently in RAM. In almost all cases, this is the only number that can be used when specifying “too large”. But this is still not the best indicator, especially when it comes to Java.
    • SHR is the amount of resident memory shared with other processes. For a Java process, this is usually reasonably limited to shared libraries and memory mapped JAR files. In this example, I only had one Java process running, so my guess is that 7k is usually the result of libraries used by the operating system.
    • SWAP is not enabled by default and is not displayed here. How much virtual memory can currently be on disk, whether or not it is in swap space. The operating system supports writes to RAM very efficiently, and the only solutions to replace them are (1) buy more storage devices, or (2) reduce the number of tactics, so it is best to ignore this type of number.

    java virtual memory

    The situation with Windows Task Manager is somewhat more complicated. Windows Vista has a columnThere are s “Memory Usage” and “Virtual Memory Size”, but the official documentation says nothing about what they mean. Windows Vista and even Windows 7 add more columns and these types are actually documented. Of these, the most useful measure is “working set”; this is roughly the sum of RES and SHR in Linux. Virtual

    Memory consumed during a process is the sum of the total memory allocated to a process. This includes data (for example, the Java heap), that is, all the shared libraries and therefore the memory-mapped files used by the program. On Linux, you can use the pmap purchase to see everything that is scheduled in the process area here (internally and externally, I’ll just refer to Linux as I’m using it; I’m sure there are equivalent tools for Windows). Here is an excerpt from a Google memory stick of the “Hello World” program; the existing memory allocation is 100 lines long, and there is often a new list of thousands of lines.

    000000004000000 36K r-x – /usr/local/java/jdk-1.6-x64/bin/java0000000040108000 8K rwx – /usr/local/java/jdk-1.6-x64/bin/java0000000040eba000 676K rwx– [anonymous]00000006fae00000 21248K rwx – [announce]00000006fc2c0000 62720K rwx– [anonymous]0000000700000000 699072K rwx – [announce]000000072aab0000 2097152K rwx– [announce]00000007aaab0000 349504K rwx– [announce]00000007c0000000 1048576K rwx– [announce]…00007fa1ed00d000 1652K r-xs- /usr/local/java/jdk-1.6-x64/jre/lib/rt.jar…00007fa1ed1d3000 1024K rwx – [announce]00007fa1ed2d3000 4K —– [Announced]00007fa1ed2d4000 1024K rwx – [announce]00007fa1ed3d4000 4K —– [Announced]…00007fa1f20d3000 164 KB r-x – /usr/local/java/jdk-1.6-x64/jre/lib/amd64/libjava.so00007fa1f20fc000 1020 KB —– /usr/local/java/jdk-1.6-x64/jre/lib/amd64/libjava.so00007fa1f21fb000 28K rwx – /usr/local/java/jdk-1.6-x64/jre/lib/amd64/…00007fa1f34aa000 1576K r-x– /lib/x86_64-linux-gnu/libc-2.13.so00007fa1f3634000 2044 KB —– /lib/x86_64-linux-gnu/libc-2.13.so00007fa1f3833000 16K r-x– /lib/x86_64-linux-gnu/libc-2.13.so00007fa1f3837000 4K rwx – /lib/x86_64-linux-gnu/…

    Brief explanation of the format: the line almost starts with a virtual memory address that refers to the segment. This is followed by the specific segment size, resolutions, and source that the segment affects. This last element is usually a file or “anon” that indicates the correct block of memory allocated via mmap.

    • JVM loader (i.e. the program starts if you enter java ). Is that too little? it is only loaded during shared libraries where the real JVM code is stored.
    • A group of anonymous streets containing a bunch of Java and internal content. This is the Sun JVM, the heap of which is divided into several generations, each of which corresponds to a different retention period. Note that the virtual JVM allocates storage space based on the -Xmx value; this superior quality results in consistent styling. The -Xms value is used internally to indicate how much of the heap is constantly “used” at program startup to run the garbage collector if that limit is reached. >
    • JAR file, in this skin is a file that contains “JDK classes”. When you map a JAR to memory, you can easily extract the files it contains (instead of having to read it from the beginning every time). This way, the JVM will map all JAR classpaths into memory; If your code requires access to the JAR file, someone might map it in memory as well.
    • Stream data associated with streams. Block 1M is a stack of wires. I didn’t have a good breakdown for the 4k block, but I defined @ericsoe as a “guard block”: it’s advancing, doesn’t have read permissionsno / write, results in an access error segment and some JVM traps that eventually translate to StackOverFlowError . For a real software package, you will see that dozens, if not hundreds, handle these records being repeated across the storage card.
    • One of the shared libraries that contains the actual JVM code for each one. Several of them have been created.
    • A generic library for the C storage library. This is just one of many things loaded by the JVM that are not necessarily part of Java.

    Shared collections are especially interesting: every shared library has at least two segments: a read-only part that contains the library code, and a read-write segment that contains global data related to the process in the library (I don’t know which segment forbidden; I’ve only seen it on Linux x64). The read-only portion of a library can get stuck between all shared processes using that particular library; For example, libc has 1.5 MB of electronic memory that can be shared.

    Internet CardIt contains a lot of Boxercise. A portion is read-only, a portion is shared, and a portion is allocated but never touched (for example, in this example, essentially the entire heap is 4 GB). But the operating system is really smart enough to only load what is needed, so the size of the virtual memory is still largely irrelevant.

    The size of virtual memory is often large when using a mission-critical 32-bit operating system, where you can only define 2 GB (or in some cases 3 GB) including free space. In case you are still dealing with a limited resource and you might have to compromise for example.
    java virtual memory

    But since 64-bit machines are ubiquitous, I think it usually doesn’t take long before the amount of virtual memory becomes a completely irrelevant statistic.

    The resident set size is the virtual memory space that actually resides in RAM. If your RSS feed makes up a significant portion of your physical storagemaybe it’s time to worry. Basically, when your RSS grows to contain all of your physical, secure digital information, and your system starts communicating, worrying is essentially an exaggeration.


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    But RSS is also misleading, especially on lightly loaded computers. The operating system does its best to retrieve the content pages used by the process. There is little benefit to high-value secondary debt if the process makes it onto the page of the future. Consequently, RSS statistics can contain many pages that are not actively used.

    If you’re not trading, you’re too worried about what various storage statistics tell you. A warning that an ever-growing RSS will indicate some kind of memory leak.

    For a particular Java program, it is much more important to pay attention to exactly what is happening on the heap. The total amount of space to eat is large and certain actions can be taken to reduce it. Free time you spend collecting wastera, more importantly, not to mention those parts of the heap that are collected.

    Access to the hard disk (ie, the database) is generally expensive and storage is cheap. If one can exchange for another, write it down.



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    Does Java use virtual memory?

    The server will potentially use a very large amount of virtual memory and if it is configured to use Java version 1.7. Indeed, the default behavior of JVM 1.7 (64-bit JVM on Windows) is to reserve 1/4 of all physical memory.

    How does JVM use memory?

    Please note that the JVM requires more memory than one heap. For example, Java methods, thread stacks, and organic in-memory descriptors are assigned separately on the heap, while the JVM is assigned in data structures. The stack is sometimes divided into two parts (or generations), which are called child (or youth) and elderly.

    What is JVM and its use?

    A Java virtual machine (JVM) is a virtual machine that allows a computer to run Java programs as well as programs written in other languages ​​that are also compiled to Java bytecode. The JVM reference application is developed by the OpenJDK project and is open source and includes the HotSpot JIT compiler.




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