How to get Direct Memory Snapshot of a Netty 4 Application

Question

I have a Netty Based Server which handles numerous HTTP Request asynchronously.

Objective - Expose Direct Memory Usage of the application.

Now I understand the Reference Counting is one way of exposing memoryusage. But for each request, few objects (like httpContent etc) are retained explicit and for others Netty internally updates the reference count.

1. Since the server is capable of handling numerous requests at a time, how can I monitor the Direct Memory Usage of my application and expose it ?

2. Is there any way to fetch the total reference count throughout the application ?

3. Apart from ReferenceCount, What are the other ways to monitor Direct Memory Usage ?

Answer 1

Netty by default uses ByteBufAllocator.DEFAULT (that is actually is ByteBufUtil.DEFAULT_ALLOCATOR that is either UnpooledByteBufAllocator.DEFAULT or PooledByteBufAllocator.DEFAULT) allocator for the allocations. If you haven't explicitly set another allocator within your code you can use it to track your memory consumption.

You can do this with the next code:

public class MemoryStat {

    public final long heapBytes;

    public final long directBytes;

    public MemoryStat(ByteBufAllocator byteBufAllocator) {
        long directMemory = 0;
        long heapMemory = 0;

        if (byteBufAllocator instanceof ByteBufAllocatorMetricProvider) {
            ByteBufAllocatorMetric metric = ((ByteBufAllocatorMetricProvider) byteBufAllocator).metric();
            directMemory = metric.usedDirectMemory();
            heapMemory = metric.usedHeapMemory();
        }

        this.directBytes = directMemory;
        this.heapBytes = heapMemory;
    }
}

Usage: new MemoryStat(ByteBufAllocator.DEFAULT);

Both default netty allocators UnpooledByteBufAllocator, PooledByteBufAllocator implements ByteBufAllocatorMetricProvider that provides 2 methods:

public interface ByteBufAllocatorMetric {
    /**
     * Returns the number of bytes of heap memory used by a {@link ByteBufAllocator} or {@code -1} if unknown.
     */
    long usedHeapMemory();

    /**
     * Returns the number of bytes of direct memory used by a {@link ByteBufAllocator} or {@code -1} if unknown.
     */
    long usedDirectMemory();
}

There is no direct API to get total reference count.