Stash LoadBalancingPolicy on ClusterConfig

[Kahbazi]

Currently the ILoadBalancePolicy is get from a dictionary on each request in LoadBalancingMiddleware.

reverse-proxy/src/ReverseProxy/Middleware/LoadBalancingMiddleware.cs

Line 54 in 5cfbc32

var currentPolicy = _loadBalancingPolicies.GetRequiredServiceById(proxyFeature.ClusterConfig.Options.LoadBalancingPolicy, LoadBalancingPolicies.PowerOfTwoChoices);

Can we stash a ILoadBalancePolicy on ClusterConfig when building a cluster to improve the performance?

[Tratcher]

That's an interesting idea. @MihaZupan @alnikola has the load balancing policy lookup shown up in the CPU profiles at all?

[Kahbazi]

The same thing also could be done for IPassiveHealthCheckPolicy.

reverse-proxy/src/ReverseProxy/Middleware/PassiveHealthCheckMiddleware.cs

Lines 40 to 42 in 5cfbc32

	var policy = _policies.GetRequiredServiceById(options.Policy);
	var cluster = context.GetRequiredRouteConfig().Cluster;
	policy.RequestProxied(cluster, proxyFeature.SelectedDestination, context);

[karelz]

Triage: We should first prove it shows up on profiles to justify the complexity.
Per @MihaZupan dictionary lookups are cheaper in .NET 5 and even more in .NET 6 as well. It didn't show up in @alnikola profiles so far ...

[MihaZupan]

A few Dictionary improvements I had in mind (looks like they all made it in 5.0):
dotnet/coreclr#27195
dotnet/coreclr#27299
dotnet/runtime#36252

[karelz]

Triage: Closing - the work in #734 did not bring the expected gains. Not worth it.