If I have 16 GB of RAM on my machine, how much can I allocate to a java command line program I'm executing? I assume java -Xmx 16g... will crash my system?
EDIT:
In light of the comments, I tried java -Xmx16g..., and it did not crash my machine. The program still ran out of memory. I tried java -Xmx32g..., which did crash my machine.
From the comments below (which have been really enlightening), I guess I just need to keep playing around with the allocations.
The size the heap memory of the virtual machine is controlled by the options - Xms
and - Xmx .
The first specifies the initial heap size and the second maximum size.
The allocation is done within the JVM itself ,and not on the OS. As more memory is needed, the JVM allocates a block of memory until the Xmx is reached.
If you need more than that, an OutOfMemoryError is thrown.
It is common to use the same value for Xms and Xmx,
causing the VM to allocate memory in the operating system only at the beginning of an execution,
not depending on the OS specific behavior.
In your case, since you set 32g as your Xmx and your system crashed, it seems you don't have this memory (swap + memory ram)
If you manually change the virtual memory size of your system, the configuration will work.
It is not possible for your OS to allocate more than 16GB and therefore your system crash. Apart from XMX and XMS that is stated in another answer there is also an -XX:+AggressiveHeap option which 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
Related
If I have a smaller-ram machine and a larger-ram machine. I run the same java code on them.
Will jvm do garbage collection more lazily on the machine with larger ram?
The problem I am trying to solve is an out of memory issue. People reported that they have Out of memory issue on small ram machine. I want to test that but the only machine I have now has a much larger ram than theirs. I am wondering if I do the test on this larger-ram machine and keep track of the memory usage, will the memory usage be the same on a smaller-ram machine or it will use even less memory?
Thanks!
Erben
You need to take a look at the JVM memory parameters. actually you can set the as much memory as you want to your JVM :
-Xmx2048m -> this param to set the max memory that the JVM can allocate
-Xms1024m -> the init memory that JVM will allocate on the start up
-XX:MaxPermSize=512M -> this for the max Permanent Generation memory
so in your case you can set the much memory as in the another machine. so you machine will not take more RAM than the Xmx value
and you may want to check this parameters also.
-XX:MaxNewSize= -> this need to be 40% from your Xmx value
-XX:NewSize=614m -> this need to be 40% from your Xmx value
also you may tell you JVM what type of GC to use like :
-XX:+UseConcMarkSweepGC
SO if you set this parameters in the both machines, you will get the same results and the same GC activity most likely.
Yes it will. This depends on the default maximum heap size. You can check your current maximum heap size using this command:
java -XshowSettings:vm
On my wife's laptop (Windows 8.1, 4 GB RAM, 32-Bit-Java-Runtime) it is 247.5 MB, while on my laptop (Windows 7, 8 GB RAM, 64-Bit-Java-Runtime) it is 903.12 MB.
This is determined by Java (see https://stackoverflow.com/a/4667635/3236102, though the values shown there are for server-class-machines, they might be different from normal machines).
If you want your vm to simulate a low-RAM-machine just use the -Xmx flag to limit your machine to less RAM (e.g. -Xmx128m for 128 MB RAM allocation).
The best thing might be to ask the users that encounter the Out Of Memory-issues to check their maximum heap size (using the command above) and set your machine to the same maximum heap size, so you have the same conditions as they have.
The issue can be reproduced with larger RAM.
First you need to get the heap size configuration from the people who reported the issue.
Use the same heap size to reproduce the issue.
Use below jvm params for heap settings.
-Xmx512m Max heap memory that is used to store objects
-XX:MaxPermSize=64m Max perm gen size. This space is used to store meta info like loaded classes etc
I am working on java application and I have set the following configurations in the VM option
-Xms and -Xmx options are set to 1024m
-XX:MaxPermSize=128m Hardware :32 bit Windows 7 system with 2GB ram.
I often encounter with java out of swap space error. What could be the reason? Please help me.
The reason is that your operating system is not configured with enough swap space for the job mix that you are running. Swap space is an area on disc where the operating system puts copies memory pages when there are more virtual memory pages than physical memory pages.
So what has happened is that your JVM has asked for more virtual memory than the operating system can give it.
(Updated to include Peter's comments)
Some possible fixes:
Add more physical memory assuming that the hardware and OS allow this. (In this case, the answer the OS should allow it ...)
Configure the system with more swap space.
Kill some of the other non-essential applications and services running on the machine.
Change the Java application's JVM options to reduce the heap size.
The Java release notes under "Hotspot VM" say about this exact error;
If you see this symptom, consider increasing the available swap space
by allocating more of your disk for virtual memory and/or by limiting
the number of applications you run simultaneously. You can also
decrease your usage of memory by reducing the value of the -Xmx flag,
which limits the size of the Java object heap.
In other words, your machine is doing too many other things using up memory to be able to supply the 1GB you're telling the Java VM that it should be able to use.
java -Xms is apparently not having an affect on the amount of memory the java process consumes during a run.
I have an app that consumes about 1Gb from the system point of view. I tried setting -Xms2048m (and -Xmx4096m) and I see absolutely no change in memory consumption.
The hotspot docs claim the heap size is bounded below by the Xms value or the default.
The only thing I can think of is maybe the process cannot grab a contiguous block of memory, so it grabbed all it could and then will allocate more later, or maybe windows is not letting it have that much memory to start with. (64-bit windows 7)
(I don't need this for anything, it is just something curious I noticed)
The default memory usage windows task manager shows you is not what's allocated in the processes virtual memory space. It's how much that process has actually written into the virtual space that has had to be mapped onto real memory. If you enable the column for 'Commit Size' in your task manager that will show what is actually considered "used" from the perspective of your processes's virtual address space. (roughly Xms + permsize + size of VM and system stuff itself.)
For Java 1 try with -ms and -mx
Since Java2 you can use -Xms and -Xmx
My experience is, that -ms and `-mx works also in Java2. See http://www.devx.com/tips/Tip/5578
The JVM need a continuous region of memory for the heap. This means it allocates the maximum size as virtual memory on startup. This is not as bad as it sounds as the OS only allocates main memory to the application as it uses it, (not when it allocates virtual memory)
If you look at the amount of memory used in a tool like VisualVM, you can find that even with overhead of 150 - 500 MB, the size is less than the minimum size. This is because Java doesn't just use the minimum size if it doesn't have a use for it.
Instead the minimum size is the point below which it makes only minor attempts to clean up memory. (You may see it perform minor GCs) In most cases this means the application will use the minimum size very quickly. However, a "hello world" program will not use the minimum size.
maybe windows is not letting it have that much memory to start with
The JVM will fail to start if it cannot allocate the maximum size as a continuous block. (This was a common problem on 32-bit Window, such that the limit could be 1.5 GB or as low as 1.2 GB)
just my complete Linux box crashed with OOM (OOM Killer Process killed the wrong processes), due to a java application consumed too much memory and there was no memroy left.
My question is, if I use the JVM Paramter -XmX, does this limit Java to no more use Memory as specified by the -XmX option? Or said differently, If I do NOT specify the -XmX than java might allocate more and more memory with the result my linux box is crahsing itself with OOM?
Thank you very much!
Jens
The default maximum for Java 6 is 1/4 of the main memory. This can mean the total virtual memory of your applications can exceed the main memory and swap space.
Given the cost of memory (8 GB costs less than £40) you should buy more memory. However, an alternative is to use less memory or increase the swap space, so you are less likely to run out.
There's a default maximum heap size (used to be 64M, I think it's 128M now.) The -Xmx parameter changes that maximum size. Oracle's JVMs will never allocate a larger heap than specified in that parameter.
That's not to say that -Xmx gives the total amount of RAM used by the JVM; it'll actually use more than that. Some is for the executable code of the JVM implementation itself; there's also memory used for the "permgen" area, and possibly memory-mapped buffers for other purposes. But Oracle's JVMs, in any event, will not grow their RAM usage without bound; there's always an upper limit.
Now, why doesn't your Linux box have more swap space? It's cheap, and it would prevent this sort of thing from happening in the first place.
I have defined -Xmsx 1.3GB in the java VM parameters and my Eclipse does not allow more than this, when running the application I got the below exception:
Exception in thread "Thread-3" java.lang.OutOfMemoryError: Java heap space
What can I do?
You can set the maximum memory eclipse uses with -mx1300m or the like. This limitation will be because you are running 32-bit java on Windows. On a 64-bit OS, you won't have this problem.
However, its the maximum memory size you set for each application in eclipse which matters. What have you set in your run options in eclipse?
Your question is very unclear:
Are you running the application in a new JVM?
Did you set the -Xmx / -Xms parameters in the launcher for the child JVM?
If the answer to either of those questions is "no", then try doing ... both. (In particular, if you don't set at least -Xmx for the child JVM, you'll get the default heap size which is relatively small.)
If the answer to both of those questions is "yes", then the problem is that you are running into the limits of your hardware and/or operating system configuration:
On a typical 32bit Windows, a user process can only address a total 2**31 bytes of virtual memory, and some of that will be used by the JVM binaries, native libraries and various non-heap memory allocations. (On a 32 bit Linux, I believe you can have up to 2**31 + 2**30). The "fix" for this is to use a 64bit OS and a 64bit JVM.
In addition, a JVM is limited on the amount of memory that it can request by the resources of the OS'es virtual memory subsystem. This is typically bounded by the sum of the available RAM and the size of the disc files / partitions used for paging. The "fix" for this is to increase the size of the paging file / partition. Adding more RAM would probably be a good idea too.
You may want to look at the aggressive Heap option http://java.sun.com/docs/hotspot/gc1.4.2/#4.2.2.%20AggressiveHeap|outline
It solved a similiar issue for me.