Optimize allocations in ActorPath.Join()

[petrikero]

Optimize ActorPath.Join() to perform fewer allocations. Accessing Elements causes quite a bit of garbage to get generated.

In .NET Core or .NET Standard 2.1, the char[] buffer allocation could be avoided, but I'm sure this isn't good enough reason to require either.

The calls are mainly triggered by serialization of ActorRefs when running with Akka.Remote. We have thousands of actors running on each node and sometimes sending ActorRefs over the wire. We're mostly sending unique ActorRefs (each gets sent only once), so the caching doesn't help.

[petrikero]

I tried looking at some of the failing logs, but as far as I can tell, all the errors seem unrelated. There's a lot of logs, though, so I may have easily missed something real.

[Aaronontheweb]

Before:

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.19041.388 (2004/?/20H1)
AMD Ryzen 7 1700, 1 CPU, 16 logical and 8 physical cores
.NET Core SDK=3.1.300
[Host] : .NET Core 3.1.4 (CoreCLR 4.700.20.20201, CoreFX 4.700.20.22101), X64 RyuJIT
DefaultJob : .NET Core 3.1.4 (CoreCLR 4.700.20.20201, CoreFX 4.700.20.22101), X64 RyuJIT

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
ActorPath_Parse	1,926.04 ns	22.125 ns	20.695 ns	0.2689	-	-	1128 B
ActorPath_Concat	57.26 ns	0.351 ns	0.329 ns	0.0421	-	-	176 B
ActorPath_Equals	25.21 ns	0.270 ns	0.252 ns	-	-	-	-
ActorPath_ToString	161.39 ns	2.374 ns	2.221 ns	0.0210	-	-	88 B

[Aaronontheweb]

After:

// * Summary *

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.19041.388 (2004/?/20H1)
AMD Ryzen 7 1700, 1 CPU, 16 logical and 8 physical cores
.NET Core SDK=3.1.300
[Host] : .NET Core 3.1.4 (CoreCLR 4.700.20.20201, CoreFX 4.700.20.22101), X64 RyuJIT
DefaultJob : .NET Core 3.1.4 (CoreCLR 4.700.20.20201, CoreFX 4.700.20.22101), X64 RyuJIT

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
ActorPath_Parse	1,908.42 ns	7.849 ns	7.342 ns	0.2689	-	-	1128 B
ActorPath_Concat	57.57 ns	0.552 ns	0.516 ns	0.0421	-	-	176 B
ActorPath_Equals	25.12 ns	0.137 ns	0.122 ns	-	-	-	-
ActorPath_ToString	126.39 ns	0.770 ns	0.720 ns	0.0210	-	-	88 B

[Aaronontheweb]

I believe Join gets called in ToString, but let me double check - just in case we need a better benchmark for this case.

[Aaronontheweb]

Yep

akka.net/src/core/Akka/Actor/ActorPath.cs

Lines 537 to 543 in 602013c

	public string ToStringWithAddress(Address address)
	{
	if (Address.Host != null && Address.Port.HasValue)
	return $"{Address}{Join()}";
	return $"{address}{Join()}";
	}

Looks like this PR improves the speed of that method by up ~22% - nicely done.

[Aaronontheweb]

@petrikero yeah, those errors were unrelated - we have a large test suite and some of the tests are timing-sensitive (assert that work gets done within a bounded window of time) and those are notoriously brittle. We've been going through and hardening some, disabling others. An ongoing battle for the maintenance team here.

Thanks for this PR though - this is terrific.