Is there a way to measure direct memory usage in Java?

Question

If I create a buffer via ByteBuffer.allocateDirect(), the memory exists outside of the Java heap. Is there a way to measure this kind of memory usage from my application in a cross-platform way similar to how I can measure Java heap usage with Runtime.totalMemory() and Runtime.freeMemory()?

Answer 1

You can use reflections to get Bits.reservedMemory on OpenJDK/HotSpot Java 7. There is no platform independent way and this only shows you the usage via ByteBuffer.allocateDirect() not any other way of allocating native memory.

An alternative is to parse /proc/{pid}/maps and exclude file mappings. This is an approximation of the virtual memory your process is using.

Answer 2

You can use MXBeans to get the memory used by the direct byte buffers and mapped byte buffers:

List<BufferPoolMXBean> pools = ManagementFactory.getPlatformMXBeans(BufferPoolMXBean.class);
for (BufferPoolMXBean pool : pools) {
    System.out.println(pool.getName());
    System.out.println(pool.getCount());
    System.out.println("memory used " + toMB(pool.getMemoryUsed()));
    System.out.println("total capacity" + toMB(pool.getTotalCapacity()));
    System.out.println();
}

prints something like:

direct
122
memory used 456.1562509536743 MB
total capacity456.15625 MB

mapped
0
memory used 0.0 MB
total capacity0.0 MB

where toMB function is this:

private static String toMB(long init) {
    return (Long.valueOf(init).doubleValue() / (1024 * 1024)) + " MB";
}

However, I am not 100% sure if direct byte buffers are the only things that can live in the direct memory. Perhaps there are other things...