Allocated Heap v.s. Max Heap and OutOfMemoryError

Question

In my application, I specify heap size like following:
-Xms384m -Xmx384m
I got few OOME error recently. From our monitoring tool, I can see the 'allocated heap' was only about 280 - 340MB range when OOME happened. That means that the used heap size were close or reach the 'allocated' heap size but not the max heap that defined in '-Xmx'. My question is:
Why does JVM act like this? What is the problem that prevents JVM from obtaining promised memory from OS?
We are using JDK 1.5
Thanks,
J

Answer 1

Its easy to check this, create as many threads as possible and configure a small heap size and a big stack size, count the number of created threads, then increase the heap and run the program again. Then you can see the number of threads increased too.
I have created a [java program to do exactly this, check it out|http://weblogs.java.net/blog/claudio/archive/2007/05/how_many_thread_1.html] .
See below, how thread stack size are allocated into the heap.
[http://java.sun.com/docs/hotspot/threads/threads.html|http://java.sun.com/docs/hotspot/threads/threads.html]
excerpt "TLEs (in 1.3) or TLABs (in 1.4) are thread local portions of the heap used in the young generation"
[Java Memory White Paper|http://java.sun.com/javase/technologies/hotspot/gc/memorymanagement_whitepaper.pdf]
For multithreaded applications, allocation operations need to be multithread-safe. If global locks were used to
ensure this, then allocation into a generation would become a bottleneck and degrade performance. Instead,
the HotSpot JVM has adopted a technique called Thread-Local Allocation Buffers (TLABs). This improves
multithreaded allocation throughput by giving each thread its own buffer (i.e., a small portion of the
generation) from which to allocate. Since only one thread can be allocating into each TLAB, allocation can take
place quickly by utilizing the bump-the-pointer technique, without requiring any locking. Only infrequently,
when a thread fills up its TLAB and needs to get a new one, must synchronization be utilized. Several techniques
to minimize space wastage due to the use of TLABs are employed. For example, TLABs are sized by the allocator
to waste less than 1% of Eden, on average. The combination of the use of TLABs and linear allocations using the
bump-the-pointer technique enables each allocation to be efficient, only requiring around 10 native instructions.