Sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics as a percentage of the maximum amount determined as described in the -XX:MaxRAM option. The default value is 25 percent. Specifying this option disables automatic use of compressed oops if the combined result of this and other options influencing the maximum amount of memory is larger than the range of memory addressable by compressed oops.
The following example shows how to set the percentage of the maximum amount of memory used for the Java heap:. Sets the maximum size in bytes of the heap for the young generation nursery. The default value is set ergonomically. Sets the maximum tenuring threshold for use in adaptive GC sizing.
The largest value is The default value is 15 for the parallel throughput collector, and 6 for the CMS collector. Sets the size of the allocated class metadata space that will trigger a garbage collection the first time it is exceeded.
This threshold for a garbage collection is increased or decreased depending on the amount of metadata used. The default size depends on the platform. Sets the minimum allowed percentage of free heap space 0 to after a GC event. If free heap space falls below this value, then the heap will be expanded.
Sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics as a percentage of the maximum amount determined as described in the -XX:MaxRAM option for small heaps.
A small heap is a heap of approximately MB. The default value is 50 percent. The following example shows how to set the percentage of the maximum amount of memory used for the Java heap for small heaps:. Sets the ratio between young and old generation sizes. By default, this option is set to 2. Sets the initial size in bytes of the heap for the young generation nursery. If the size for the young generation is too low, then a large number of minor GCs will be performed. If the size is too high, then only full GCs will be performed, which can take a long time to complete.
The following examples show how to set the initial size of young generation to MB using various units:. Sets the number of threads used for parallel garbage collection in the young and old generations. For example, to set the number of threads for parallel GC to 2, specify the following option:. Enables printing of information about adaptive generation sizing. Enables printing of messages at every GC. Enables printing of how much time elapsed since the last pause for example, a GC pause.
Enables printing of how much time the pause for example, a GC pause lasted. Enables printing of a date stamp at every GC. Enables printing of detailed messages at every GC. Enables printing of time stamps for every individual GC worker thread task. Enables printing of time stamps at every GC. Prints detailed deduplication statistics. Enables printing of tenuring age information. The following is an example of the output:.
Age 1 objects are the youngest survivors they were created after the previous scavenge, survived the latest scavenge, and moved from eden to survivor space. Age 2 objects have survived two scavenges during the second scavenge they were copied from one survivor space to the next. And so on. In the preceding example, 28 bytes survived one scavenge and were copied from eden to survivor space, 1 bytes are occupied by age 2 objects, etc. The third value in each row is the cumulative size of objects of age n or less.
Enables GC of the young generation before each full GC. Oracle recommends that you do not disable it, because scavenging the young generation before a full GC can reduce the number of objects reachable from the old generation space into the young generation space. Sets the amount of time in milliseconds a softly reachable object is kept active on the heap after the last time it was referenced.
The default value is one second of lifetime per free megabyte in the heap. This difference means that the Client VM tends to flush soft references rather than grow the heap, whereas the Server VM tends to grow the heap rather than flush soft references.
In the latter case, the value of the -Xmx option has a significant effect on how quickly soft references are garbage collected. String objects reaching the specified age are considered candidates for deduplication. An object's age is a measure of how many times it has survived garbage collection. Note that String objects that are promoted to an old heap region before this age has been reached are always considered candidates for deduplication. The default value for this option is 3.
Sets the ratio between eden space size and survivor space size. By default, this option is set to 8. Sets the desired percentage of survivor space 0 to used after young garbage collection.
Sets the initial size in bytes of a thread-local allocation buffer TLAB. If this option is set to 0, then the JVM chooses the initial size automatically. Enables the use of adaptive generation sizing. Enables the use of the occupancy value as the only criterion for initiating the CMS collector.
By default, this option is disabled and other criteria may be used. Enables the use of the CMS garbage collector for the old generation. By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM. Enables the use of the garbage-first G1 garbage collector.
It is a server-style garbage collector, targeted for multiprocessor machines with a large amount of RAM. It meets GC pause time goals with high probability, while maintaining good throughput.
The G1 collector is recommended for applications requiring large heaps sizes of around 6 GB or larger with limited GC latency requirements stable and predictable pause time below 0. When the heap is small, this feature can be used to prevent applications from running for long periods of time with little or no progress.
Enables performance optimization of an application on a machine with nonuniform memory architecture NUMA by increasing the application's use of lower latency memory. By default, this option is disabled and no optimization for NUMA is made. Enables the use of the parallel scavenge garbage collector also known as the throughput collector to improve the performance of your application by leveraging multiple processors.
Enables the use of the parallel garbage collector for full GCs. Enables the use of parallel threads for collection in the young generation. Enables the use of the serial garbage collector. This is generally the best choice for small and simple applications that do not require any special functionality from garbage collection. Enables string deduplication.
To use this option, you must enable the garbage-first G1 garbage collector. String deduplication reduces the memory footprint of String objects on the Java heap by taking advantage of the fact that many String objects are identical. Instead of each String object pointing to its own character array, identical String objects can point to and share the same character array.
Enables the use of thread-local allocation blocks TLABs in the young generation space. These options were included in the previous release, but have since been considered unnecessary. Enables incremental garbage collection.
This option was deprecated in JDK 8 with no replacement. This option was superseded by the -agentlib option. Sets the percentage of time 0 to between minor collections that the concurrent collector is allowed to run. Enables the incremental mode for the CMS collector. Sets the percentage of time 0 to by which the incremental mode duty cycle is shifted to the right within the period between minor collections. Enables automatic adjustment of the incremental mode duty cycle based on statistics collected while the JVM is running.
Sets the percentage of time 0 to used to add conservatism when computing the duty cycle. Sets the percentage of the permanent generation occupancy 0 to at which to start a GC.
Sets the maximum permanent generation space size in bytes. Sets the space in bytes allocated to the permanent generation that triggers a garbage collection if it is exceeded. Enables splitting of the verification process. By default, this option was enabled in the previous releases, and verification was split into two phases: type referencing performed by the compiler and type checking performed by the JVM runtime.
This option was deprecated in JDK 8, and verification is now split by default without a way to disable it. Enables caching of commonly allocated strings. This option was removed from JDK 8 with no replacement. The following examples show how to use experimental tuning flags to either optimize throughput or to provide lower response time.
Also known as huge pages, large pages are memory pages that are significantly larger than the standard memory page size which varies depending on the processor and operating system. Large pages optimize processor Translation-Lookaside Buffers. A Translation-Lookaside Buffer TLB is a page translation cache that holds the most-recently used virtual-to-physical address translations.
TLB is a scarce system resource. A TLB miss can be costly as the processor must then read from the hierarchical page table, which may require multiple memory accesses. By using a larger memory page size, a single TLB entry can represent a larger memory range. There will be less pressure on TLB, and memory-intensive applications may have better performance. However, large pages page memory can negatively affect system performance. For example, when a large mount of memory is pinned by an application, it may create a shortage of regular memory and cause excessive paging in other applications and slow down the entire system.
Also, a system that has been up for a long time could produce excessive fragmentation, which could make it impossible to reserve enough large page memory. Only Windows Server supports large pages. To use this feature, the administrator must first assign additional privilege to the user who will be running the application:. Note that these steps are required even if it is the administrator who will be running the application, as administrators by default do not have the privilege to lock pages in memory.
AppCDS reduces the footprint and decreases start-up time of your applications provided that a substantial number of classes are loaded from the application class path. The following steps create a shared archive file that contains all the classes used by the test. Hello application. The last step runs the application with the shared archive file.
Create a list of all classes used by the test. The following command creates a file named hello. Create a shared archive, named hello. Note that the -cp parameter used at archive creation time must be the same as or a prefix of the -cp used at run time. Run the application test.
Hello with the shared archive hello. Ensure that you have specified the option -Xshare:on or - Xshare:auto. Verify that the test. Hello application is using the class contained in the hello. Loaded test.
You can share the same archive file across multiple applications processes that have the exact same class path or share a common class path prefix. This reduces memory usage as the archive is memory-mapped into the address space of the processes. The operating system automatically shares the read-only pages across these processes. The following steps create a shared archive that both applications Hello and Hi can use.
Create a list of all classes used by the Hello application and another list for the Hi application:. Note that because the Hello and Hi applications share a common class path prefix both of their class paths start with common. Create a single list of classes used by all the applications that will share the shared archive file. The following commands combine the files hello. Create a shared archive, named common. The value of the -cp parameter is the common class path prefix shared by the Hello and Hi applications.
Run the Hello and Hi applications with the same shared archive:. The following exit values are typically returned by the launcher when the launcher is called with the wrong arguments, serious errors, or exceptions thrown by the JVM.
The values are:. Skip to Content. Synopsis java [ options ] classname [ args ] java [ options ] -jar filename [ args ] javaw [ options ] classname [ args ] javaw [ options ] -jar filename [ args ] options Command-line options separated by spaces. Description The java command starts a Java application. The method declaration has the following form: public static void main String[] args The java command can be used to launch a JavaFX application by loading a class that either has a main method or that extends javafx.
Standard Options These are the most commonly used options that are supported by all implementations of the JVM. Brickbat , use the following command: java -ea:com.
Brickbat MyClass -disablesystemassertions -dsa Disables assertions in all system classes. Brickbat MyClass -enablesystemassertions -esa Enables assertions in all system classes. This option is equivalent to -XX:-BackgroundCompilation. Provided for backward compatibility. Example: -Xloggc:garbage-collection.
The following examples show how to set the initial and maximum size of young generation to MB using various units: -Xmnm -Xmnk -Xmn Instead of the -Xmn option to set both the initial and maximum size of the heap for the young generation, you can use -XX:NewSize to set the initial size and -XX:MaxNewSize to set the maximum size. The following examples show how to set the size of allocated memory to 6 MB using various units: -Xms -Xmsk -Xms6m If you do not set this option, then the initial size will be set as the sum of the sizes allocated for the old generation and the young generation.
Possible mode arguments for this option include the following: auto Use CDS if possible. Possible category arguments for this option include the following: all Shows all categories of settings. This is the default value. Possible mode arguments for this option include the following: remote Verifies all bytecodes not loaded by the bootstrap class loader.
This option checks if an application is using one of these mechanisms by checking the following: The java. Possible mode arguments for this option include the following: off Do not track JVM native memory usage. For more information, see "Large Pages". The style argument is an integer from 0 to 3: 0 Do not generate prefetch instructions.
String::indexOf If the method is specified without the signature, the command will be applied to all methods with the specified name. The following commands are available: break Set a breakpoint when debugging the JVM to stop at the beginning of compilation of the specified method. Advanced Serviceability Options These options provide the ability to gather system information and perform extensive debugging.
The following is an example of the output: Desired survivor size bytes, new threshold 10 max 10 - age 1: bytes, total - age 2: bytes, total - age 3: bytes, total The following example shows how to set the value to 2.
Deprecated and Removed Options These options were included in the previous release, but have since been considered unnecessary.
Performance Tuning Examples The following examples show how to use experimental tuning flags to either optimize throughput or to provide lower response time. Large Pages Also known as huge pages, large pages are memory pages that are significantly larger than the standard memory page size which varies depending on the processor and operating system. Reboot your system. Creating a Shared Archive File, and Running an Application with It The following steps create a shared archive file that contains all the classes used by the test.
Hello Ensure that you have specified the option -Xshare:on or - Xshare:auto. Unfortunately, every time I try to run the program I get this Java heap size error message:. Could not reserve enough space for object heap. Could not create the Java virtual machine. You set the maximum Java heap size of your program using the -Xmx option to the Java interpreter. To specifically limit your heap size to 64 MB the option should be specified like this:.
Young generation represents all the objects which have a short life of time. Young generation objects are in a specific location into the heap, where the garbage collector will pass often. All new objects are created into the young generation region called "eden". When an object survive is still "alive" after more than gc cleaning, then it will be swap has an "old generation" : they are "survivor".
And a more "official" source from IBM :. Sets the initial and maximum size of the new nursery heap to the specified value when using -Xgcpolicy:gencon. Equivalent to setting both -Xmns and -Xmnx. If you set either -Xmns or -Xmnx, you cannot set -Xmn. By default, -Xmn is selected internally according to your system's capability. You can use the -verbose:sizes option to find out the values that the VM is currently using. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
Create a free Team What is Teams? The Java heap is the amount of memory allocated to applications running in the JVM. Objects in heap memory can be shared between threads. The practical limit for Java heap size is typically about GB in a conventional JVM due to garbage collection pauses.
What does xmxm mean? In layman terms this means that the application can use a maximum of MB of memory. What is the default JVM memory size?
Client JVM Default Initial and Maximum Heap Sizes: The default maximum heap size is half of the physical memory up to a physical memory size of megabytes MB and otherwise one fourth of the physical memory up to a physical memory size of 1 gigabyte GB. How do I increase heap size? What does Minecraft use the most?
0コメント