Menu
We ship a Java application whose memory demand can vary quite a lot depending on the size of the data it is processing. If you don't set the max VM (virtual memory) size, quite often the JVM quits with an GC failure on big data.
What we'd like to see, is the JVM requesting more memory, as GC fails to provide enough, until the total available VM is exhausted. e.g., start with 128Mb, and increase geometrically (or some other step) whenever the GC failed.
The JVM ("Java") command line allows explicit setting of max VM sizes (various -Xm* commands), and you'd think that would be designed to be adequate. We try to do this in a .cmd file that we ship with the application. But if you pick any specific number, you get one of two bad behaviors: 1) if your number is small enough to work on most target systems (e.g., 1Gb), it isn't big enough for big data, or 2) if you make it very large, the JVM refuses to run on those systems whose actual VM is smaller than specified.
How does one set up Java to use the available VM when needed, without knowing that number in advance, and without grabbing it all on startup?
608
-xmx and -xms are the parameters used to adjust the heap size. -Xms: It is used for setting the initial and minimum heap size. It is recommended to set the minimum heap size equivalent to the maximum heap size in order to minimize the garbage collection. -Xmx: It is used for setting the maximum heap size.
The theoretical limit is 2^64 bytes, which is 16 exabytes (1 exabyte = 1024 petabytes, 1 petabyte = 1024 terabytes). However, most OS's can't handle that. For instance, Linux can only support 64 terabytes of data. Note: We don't recommend you exceed 2 GB of in use JVM heap.
This resource memory used by the JVM is often called overhead. The recommended minimum starting memory point for 64-bit Maximo 7.5 JVMs systems is 3584 MB. Therefore we recommended that physical memory availability for each JVM be 4096 MB;0.5 GB is for JVM allocation and 512 MB is for overhead.
You can also use the option: -XX:+AggressiveHeap
This according to the [documentation][1]:
The -XX:+AggressiveHeap option inspects the machine resources (size of memory and number of processors) and attempts to set various parameters to be optimal for long-running, memory allocation-intensive jobs. It was originally intended for machines with large amounts of memory and a large number of CPUs, but in the J2SE platform, version 1.4.1 and later it has shown itself to be useful even on four processor machines. With this option the throughput collector (-XX:+UseParallelGC) is used along with adaptive sizing (-XX:+UseAdaptiveSizePolicy). The physical memory on the machines must be at least 256MB before AggressiveHeap can be used. The size of the initial heap is calculated based on the size of the physical memory and attempts to make maximal use of the physical memory for the heap (i.e., the algorithms attempt to use heaps nearly as large as the total physical memory).
[1]: http://java.sun.com/docs/hotspot/gc1.4.2/#4.2.2. AggressiveHeap|outline
139
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
