Why is the maximum size of the Java heap fixed?_问答_开发者

It is not possible to increase the maximum size of Java's heap after the VM has started. What are the technical reasons for this? Do the garbage collection algorithms depend on having a fixed amount of memory to work with? Or is it for security reasons, to prevent a Java application from DOS'ing other applications on the system by consuming all ava开发者_运维问答ilable memory?

In Sun's JVM, last I knew, the entire heap must be allocated in a contiguous address space. I imagine that for large heap values, it's pretty hard to add to your address space after startup while ensuring it stays contiguous. You probably need to get it at startup, or not at all. Thus, it is fixed.

Even if it isn't all used immediately, the address space for the entire heap is reserved at startup. If it cannot reserve a large enough contiguous block of address space for the value of -Xmx that you pass it, it will fail to start. This is why it's tough to allocate >1.4GB heaps on 32-bit Windows - because it's hard to find contiguous address space in that size or larger, since some DLLs like to load in certain places, fragmenting the address space. This isn't really an issue when you go 64-bit, since there is so much more address space.

This is almost certainly for performance reasons. I could not find a terrific link detailing this further, but here is a pretty good quote from Peter Kessler (full link - be sure to read the comments) that I found when searching. I believe he works on the JVM at Sun.

The reason we need a contiguous memory region for the heap is that we have a bunch of side data structures that are indexed by (scaled) offsets from the start of the heap. For example, we track object reference updates with a "card mark array" that has one byte for each 512 bytes of heap. When we store a reference in the heap we have to mark the corresponding byte in the card mark array. We right shift the destination address of the store and use that to index the card mark array. Fun addressing arithmetic games you can't do in Java that you get to (have to :-) play in C++.

This was in 2004 - I'm not sure what's changed since then, but I am pretty sure it still holds. If you use a tool like Process Explorer, you can see that the virtual size (add the virtual size and private size memory columns) of the Java application includes the total heap size (plus other required space, no doubt) from the point of startup, even though the memory 'used' by the process will be no where near that until the heap starts to fill up...

Historically there has been a reason for this limitiation, which was not to allow Applets in the browser to eat up all of the users memory. The Microsoft VM which never had such a limitiation actually allowed to do this which could lead to some sort of Denial of Service attack against the users computer. It was only a year ago that Sun introduced in the 1.6.0 Update 10 VM a way to let applets specify how much memory they want (limited to a certain fixed share of the physical memory) instead of always limiting them to 64MB even on computers that have 8GB or more available.

Now since the JVM has evolved it should have been possible to get rid of this limitation when the VM is not running inside a browser, but Sun obviously never considered it such a high priority issue even though there have been numerous bug reports been filed to finally allow the heap to grow.

I think the short, snarky, answer is because Sun hasn't found it worth the time and cost to develop.

The most compelling use case for such a feature is on the desktop, IMO, and Java has always been a disaster on the desktop when it comes to the mechanics of launching the JVM. I suspect that those who think the most about those issues tend to focus on the server side and view any other details best left to native wrappers. It is an unfortunate decision, but it should just be one of the decision points when deciding on the right platform for an application.

My gut feel is that it has to do with memory management with respect to the other applications running on the operating system.

If you set the maximum heap size to, for example, the amount of RAM on the box you effectively let the VM decide how much memory it requires (up to this limit). The problem with this is that the VM could effectively cripple the machine it is running on because it will take over all the memory on the box before it decides that it needs to garbage collect.

When you specify max heap size, what you're saying to the VM is, you are allowed to use this amount of memory before you need to start garbage collecting. You cannot have more because if you take more then the other applications running on the box will slow down and you will start swapping to the disk if you use more than this.

Also be aware that they are two values with respect to memory, that is "current heap size" and "max heap size". The current heap size is how much memory the heap size is currently using and, if it requires more it can resize the heap but it cannot resize the heap above the value of maximum heap size.

From IBM's performance tuning tips (so may not be directly applicable to Sun's VMs)

The Java heap parameters influence the behavior of garbage collection. Increasing the heap size supports more object creation. Because a large heap takes longer to fill, the application runs longer before a garbage collection occurs. However, a larger heap also takes longer to compact and causes garbage collection to take longer.

The JVM has thresholds it uses to manage the JVM's storage. When the thresholds are reached, the garbage collector gets invoked to free up unused storage. Therefore, garbage collection can cause significant degradation of Java performance. Before changing the initial and maximum heap sizes, you should consider the following information: In the majority of cases you should set the maximum JVM heap size to value higher than the initial JVM heap size. This allows for the JVM to operate efficiently during normal, steady state periods within the confines of the initial heap but also to operate effectively during periods of high transaction volume by expanding the heap up to the maximum JVM heap size. In some rare cases where absolute optimal performance is required you might want to specify the same value for both the initial and maximum heap size. This will eliminate some overhead that occurs when the JVM needs to expand or contract the size of the JVM heap. Make sure the region is large enough to hold the specified JVM heap. Beware of making the Initial Heap Size too large. While a large heap size initially improves performance by delaying garbage collection, a large heap size ultimately affects response time when garbage collection eventually kicks in because the collection process takes more time.

So, I guess the reason that you can't change the value at runtime is because it may not help: either you have enough space in your heap or you don't. Once you run out, a GC cycle will be triggered. If that doesn't free up the space, you're stuffed anyway. You'd need to catch the OutOfMemoryException, increase the heap size, and then retry you calculation, hoping that this time you have enough memory.

In general the VM won't use the maximum heap size unless you need it, so if you think you might need to expand the memory at runtime, you could just specify a large maximum heap size.

I admit that's all a bit unsatisfying, and seems a bit lazy, since I can imagine a reasonable garbage collection strategy which would increase the heap size when GC fails to free enough space. Whether my imagination translates to a high performance GC implementation is another matter though ;)