Add load biaser reference (#2136)

Author: travisbeckham

linkerd.io/content/2-edge/reference/load-balancing.md

@@ -0,0 +1,38 @@
+---
+title: Load Balancing
+description: How Linkerd implements load balancing.
+---
+
+Linkerd uses a sophisticated
+[EWMA load balancing algorithm](../features/load-balancing.md) which typically
+requires no configuration or tuning. However, it also supports special handling
+for backends which support rate limiting. This special handling can be enabled
+with Service annotations.
+
+## Load Biaser
+
+When backends implement rate limiting and return
+[HTTP 429](https://www.rfc-editor.org/rfc/rfc6585.html#page-3) or
+[gRPC RESOURCE_EXHAUSTED](https://grpc.github.io/grpc/core/md_doc_statuscodes.html)
+by default, the proxy treats these as successful responses from a load balancing
+perspective. Since these types of responses are typically very fast, Linkerd’s
+[EWMA load balancing](https://linkerd.io/docs/features/load-balancing/) may
+actually send more traffic to these rate-limited endpoints. This can create a
+feedback loop where clients experience high 429 or RESOURCE_EXHAUSTED rates.
+
+Linkerd can be configured to use a more sophisticated version of the EWMA load
+balancing algorithm which takes rate-limit responses (HTTP 429 or gRPC
+RESOURCE_EXHAUSTED) into account.
+
+To enable Linkerd to use the Load Biaser for a Service, set the following
+annotation on the Service resource:
+
+{{< keyval >}}
+
+| annotation                                              | value                                                                                  |
+| ------------------------------------------------------- | -------------------------------------------------------------------------------------- |
+| `balancer.alpha.linkerd.io/penalize-failures`           | Enables the Load Biaser for this Service. Defaults to `false`.                         |
+| `balancer.alpha.linkerd.io/load-biaser-penalty`         | The latency value to inject for rate-limited responses and failures. Defaults to `5s`. |
+| `balancer.alpha.linkerd.io/load-biaser-max-retry-after` | The maximum allowed value of a Retry-After header. Defaults to `300s`.                 |
+
+{{< /keyval >}}

linkerd.io/content/docs/reference/load-balancing.md

@@ -0,0 +1,38 @@
+---
+title: Load Balancing
+description: How Linkerd implements load balancing.
+---
+
+Linkerd uses a sophisticated
+[EWMA load balancing algorithm](../features/load-balancing.md) which typically
+requires no configuration or tuning. However, it also supports special handling
+for backends which support rate limiting. This special handling can be enabled
+with Service annotations.
+
+## Load Biaser
+
+When backends implement rate limiting and return
+[HTTP 429](https://www.rfc-editor.org/rfc/rfc6585.html#page-3) or
+[gRPC RESOURCE_EXHAUSTED](https://grpc.github.io/grpc/core/md_doc_statuscodes.html)
+by default, the proxy treats these as successful responses from a load balancing
+perspective. Since these types of responses are typically very fast, Linkerd’s
+[EWMA load balancing](https://linkerd.io/docs/features/load-balancing/) may
+actually send more traffic to these rate-limited endpoints. This can create a
+feedback loop where clients experience high 429 or RESOURCE_EXHAUSTED rates.
+
+Linkerd can be configured to use a more sophisticated version of the EWMA load
+balancing algorithm which takes rate-limit responses (HTTP 429 or gRPC
+RESOURCE_EXHAUSTED) into account.
+
+To enable Linkerd to use the Load Biaser for a Service, set the following
+annotation on the Service resource:
+
+{{< keyval >}}
+
+| annotation                                              | value                                                                                  |
+| ------------------------------------------------------- | -------------------------------------------------------------------------------------- |
+| `balancer.alpha.linkerd.io/penalize-failures`           | Enables the Load Biaser for this Service. Defaults to `false`.                         |
+| `balancer.alpha.linkerd.io/load-biaser-penalty`         | The latency value to inject for rate-limited responses and failures. Defaults to `5s`. |
+| `balancer.alpha.linkerd.io/load-biaser-max-retry-after` | The maximum allowed value of a Retry-After header. Defaults to `300s`.                 |
+
+{{< /keyval >}}