Akka.Remote: DotNetty never disposes HeapArena following ActorSystem shutdown

[Aaronontheweb]

Version: v1.3.16 and all prior versions.

Reproduction: https://github.com/Aaronontheweb/DotNettyPoolAreaLeak - run this sample, using .NET Core 3.0 or .NET 4.6.1.

Expected behavior: all system-specific resources are released once the ActorSystem is terminated.

Actual behavior: in this sample we spawn 31 ActorSystems and successively churn connections to the original target ActorSystem. Even after all ActorSystem instances are terminated, there's still ~500mb of memory allocated - and as it turns out, the majority of this are leaked resources from previous DotNetty connections, specifically the byte buffer pooling:

In most applications this won't be a problem, but it's a big issue for our test suite and for applications that make temporary use of ActorSystems (i.e. thick client software.) We should change this behavior such that when DotNetty is terminated, so are all of its resources for that ActorSystem.

[Aaronontheweb]

If I run the reproduction sample with the following HOCON configuration:

akka{
    actor.provider = remote
    remote{
        dot-netty.tcp{
            enable-pooling = false
            hostname = 127.0.0.1
            port = 0
        }
    }
}

This disable the buffer pooling from running. If I use this setting, the memory leak doesn't occur:

Only 100mb allocated to the process - 99% of it doesn't linger by the time all of the ActorSystems are terminated.

[Aaronontheweb]

So I scaled up my reproduction sample from 30 ActorSystems to 70. The amount of memory allocated was pretty consistent with my earlier results - about 545mb allocated.

Based on the default rules used in the PooledByteBufferAllocator

https://github.com/Azure/DotNetty/blob/47f5ec7303037f1360615d182939e04d8619a2b3/src/DotNetty.Buffers/PooledByteBufferAllocator.cs#L71-L73

On my 12 core development laptop (12 logical CPU) I should get back 24 16mb buffers, roughly ~400mb. That's what I see.

So the allocation size appears to be scaled to the hardware, but what's annoying is there's no way to de-allocate any of these buffers even when DotNetty is no longer running.

[Aaronontheweb]

https://github.com/Azure/DotNetty/blob/9e3a84189f149fe3aa149851a1f083f2333ace7c/src/DotNetty.Common/FastThreadLocal.cs#L50 - I might be able to call this when the ActorSystem gets destroyed, but unfortunately.... If there are multiple ActorSystems running inside the same process, this will have consequences.

[Aaronontheweb]

So this doesn't appear to be a critical issue, after all. The memory is capped to the size of your hardware. If this is an issue, you can disable memory pooling via setting `akka.remote.dot-netty.tcp.enable-pooling = false'.