Overload manager

The overload manager is configured in the Bootstrap overload_manager field.

An example configuration of the overload manager is shown below. It shows a configuration to disable HTTP/1.x keepalive when heap memory usage reaches 95% and to stop accepting requests when heap memory usage reaches 99%.

refresh_interval:
  seconds: 0
  nanos: 250000000
resource_monitors:
  - name: "envoy.resource_monitors.fixed_heap"
    typed_config:
      "@type": type.googleapis.com/envoy.config.resource_monitor.fixed_heap.v2alpha.FixedHeapConfig
      max_heap_size_bytes: 2147483648
actions:
  - name: "envoy.overload_actions.disable_http_keepalive"
    triggers:
      - name: "envoy.resource_monitors.fixed_heap"
        threshold:
          value: 0.95
  - name: "envoy.overload_actions.stop_accepting_requests"
    triggers:
      - name: "envoy.resource_monitors.fixed_heap"
        threshold:
          value: 0.99

Resource monitors

The overload manager uses Envoy’s extension framework for defining resource monitors. Envoy’s builtin resource monitors are listed here.

Triggers

Triggers connect resource monitors to actions. There are two types of triggers supported:

Type

Description

threshold

Sets the action state to 1 (= saturated) when the resource pressure is above a threshold, and to 0 otherwise.

scaled

Sets the action state to 0 when the resource pressure is below the scaling_threshold, (pressure - scaling_threshold)/(saturation_threshold - scaling_threshold) when scaling_threshold < pressure < saturation_threshold, and to 1 (saturated) when the pressure is above the saturation_threshold.”

Overload actions

The following overload actions are supported:

Name

Description

envoy.overload_actions.stop_accepting_requests

Envoy will immediately respond with a 503 response code to new requests

envoy.overload_actions.disable_http_keepalive

Envoy will stop accepting streams on incoming HTTP connections

envoy.overload_actions.stop_accepting_connections

Envoy will stop accepting new network connections on its configured listeners

envoy.overload_actions.reject_incoming_connections

Envoy will reject incoming connections on its configured listeners without processing any data

envoy.overload_actions.shrink_heap

Envoy will periodically try to shrink the heap by releasing free memory to the system

envoy.overload_actions.reduce_timeouts

Envoy will reduce the waiting period for a configured set of timeouts. See below for details on configuration.

Reducing timeouts

The envoy.overload_actions.reduce_timeouts overload action will reduce the amount of time Envoy will spend waiting for some interactions to finish in response to resource pressure. The amount of reduction can be configured per timeout type by specifying the minimum timer value to use when the triggering resource monitor detects saturation. The minimum value for each timeout can be specified either by providing a scale factor to apply to the configured maximum, or as a concrete duration value.

As an example, here is a single overload action entry that enables timeout reduction:

name: "envoy.overload_actions.reduce_timeouts"
triggers:
  - name: "envoy.resource_monitors.fixed_heap"
    scaled:
      scaling_threshold: 0.85
      saturation_threshold: 0.95
typed_config:
  "@type": type.googleapis.com/envoy.config.overload.v3.ScaleTimersOverloadActionConfig
  timer_scale_factors:
    - timer: HTTP_DOWNSTREAM_CONNECTION_IDLE
      min_timeout: 2s

It configures the overload manager to change the amount of time that HTTP connections are allowed to remain idle before being closed in response to heap size. When the heap usage is less than 85%, idle connections will time out at their usual time, which is configured through RouteAction.idle_timeout. When the heap usage is at or above 95%, idle connections will be closed after the specified min_timeout, here 2 seconds. If the heap usage is between 85% and 95%, the idle connection timeout will vary between those two based on the formula for the scaled trigger So if RouteAction.idle_timeout = 600 seconds and heap usage is at 92%, idle connections will time out after \(2s + (600s - 2s) \cdot (95\% - 92\%) / (95\% - 85\%) = 181.4s\).

Note in the example that the minimum idle time is specified as an absolute duration. If, instead, min_timeout: 2s were to be replaced with min_scale: { value: 10 }, the minimum timer value would be computed based on the maximum (specified elsewhere). So if RouteAction.idle_timeout is again 600 seconds, then the minimum timer value would be \(10\% \cdot 600s = 60s\).

Limiting Active Connections

Currently, the only supported way to limit the total number of active connections allowed across all listeners is via specifying an integer through the runtime key overload.global_downstream_max_connections. The connection limit is recommended to be less than half of the system’s file descriptor limit, to account for upstream connections, files, and other usage of file descriptors. If the value is unspecified, there is no global limit on the number of active downstream connections and Envoy will emit a warning indicating this at startup. To disable the warning without setting a limit on the number of active downstream connections, the runtime value may be set to a very large limit (~2e9).

If it is desired to only limit the number of downstream connections for a particular listener, per-listener limits can be set via the listener configuration.

One may simultaneously specify both per-listener and global downstream connection limits and the conditions will be enforced independently. For instance, if it is known that a particular listener should have a smaller number of open connections than others, one may specify a smaller connection limit for that specific listener and allow the global limit to enforce resource utilization among all listeners.

An example configuration can be found in the edge best practices document.

Statistics

Each configured resource monitor has a statistics tree rooted at overload.<name>. with the following statistics:

Name

Type

Description

pressure

Gauge

Resource pressure as a percent

failed_updates

Counter

Total failed attempts to update the resource pressure

skipped_updates

Counter

Total skipped attempts to update the resource pressure due to a pending update

Each configured overload action has a statistics tree rooted at overload.<name>. with the following statistics:

Name

Type

Description

active

Gauge

Active state of the action (0=scaling, 1=saturated)

scale_percent

Gauge

Scaled value of the action as a percent (0-99=scaling, 100=saturated)