# Memory Limiter Processor

<!-- status autogenerated section -->
| Status        |           |
| ------------- |-----------|
| Stability     | [beta]: traces, metrics, logs   |
| Distributions | [core], [contrib], [k8s] |
| Issues        | [![Open issues](https://img.shields.io/github/issues-search/open-telemetry/opentelemetry-collector?query=is%3Aissue%20is%3Aopen%20label%3Aprocessor%2Fmemorylimiter%20&label=open&color=orange&logo=opentelemetry)](https://github.com/open-telemetry/opentelemetry-collector/issues?q=is%3Aopen+is%3Aissue+label%3Aprocessor%2Fmemorylimiter) [![Closed issues](https://img.shields.io/github/issues-search/open-telemetry/opentelemetry-collector?query=is%3Aissue%20is%3Aclosed%20label%3Aprocessor%2Fmemorylimiter%20&label=closed&color=blue&logo=opentelemetry)](https://github.com/open-telemetry/opentelemetry-collector/issues?q=is%3Aclosed+is%3Aissue+label%3Aprocessor%2Fmemorylimiter) |

[beta]: https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/component-stability.md#beta
[core]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol
[contrib]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-contrib
[k8s]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-k8s
<!-- end autogenerated section -->

## Overview

The memory limiter processor is used to prevent out of memory situations on
the collector. Given that the amount and type of data the collector processes is
environment-specific and resource utilization of the collector is also dependent
on the configured processors, it is important to put checks in place regarding
memory usage.

## Functionality

The memory limiter processor performs periodic checks of memory
usage and will begin refusing data and forcing GC to reduce
memory consumption when defined limits have been exceeded.

The processor uses soft and hard memory limits. The hard limit is defined via the
`limit_mib` configuration option, and is always above or equal
to the soft limit. The difference between the soft limit and hard limit is defined via
the `spike_limit_mib` configuration option.

The processor will enter memory limited mode and will start refusing the data when
memory usage exceeds the soft limit. This is done by returning errors to the preceding component
in the pipeline that made the ConsumeLogs/Trace/Metrics function call.

In memory limited mode the error returned by ConsumeLogs/Trace/Metrics function is a
non-permanent error. When receivers see this error they are expected to retry sending
the same data. The receivers may also apply backpressure to their own data sources
in order to slow the inflow of data into the Collector, and to allow memory usage
to go below the set limits.

> Warning: Data will be permanently lost if the component preceding the memory limiter
> in the telemetry pipeline does not correctly retry sending data after it has
> been refused by the memory limiter.
> We consider such components to be incorrectly implemented.

When the memory usage is above the hard limit the processor will additionally
force garbage collection to be performed.

Normal operation is resumed when memory usage drops below the soft limit, meaning data
will no longer be refused and the processor won't force garbage collection to
be performed.

## Best Practices

Note that while the processor can help mitigate out of memory situations,
it is not a replacement for properly sizing and configuring the
collector. Keep in mind that if the soft limit is crossed, the collector will
return errors to all receive operations until enough memory is freed. This may
eventually result in dropped data since the receivers may not be able to
retry the data indefinitely.

It is highly recommended to configure the `GOMEMLIMIT`
[environment variable](https://pkg.go.dev/runtime#hdr-Environment_Variables) as well
as the `memory_limiter` processor on every collector. `GOMEMLIMIT` should be set to
80% of the hard memory limit of your collector. For the `memory_limiter` processor, the
best practice is to add it as the first processor in a pipeline. This is to ensure that backpressure
can be sent to applicable receivers and minimize the likelihood of dropped data when the
`memory_limiter` gets triggered.

The value of the `spike_limit_mib` configuration option should be selected in a way
that ensures that memory usage cannot increase by more than this value within a single
memory check interval. Otherwise, memory usage may exceed the hard limit, even if temporarily.
A good starting point for `spike_limit_mib` is 20% of the hard limit. Bigger
`spike_limit_mib` values may be necessary for spiky traffic or for longer check intervals.


## Configuration

Please refer to [memorylimiter.go](../../internal/memorylimiter/memorylimiter.go) for the config spec.

The following configuration options **must be changed**:
- `check_interval` (default = 0s): Time between measurements of memory
usage. The recommended value is 1 second.
If the expected traffic to the Collector is very spiky then decrease the `check_interval`
or increase `spike_limit_mib` to avoid memory usage going over the hard limit.
- `limit_mib` (default = 0): Maximum amount of memory, in MiB, targeted to be
allocated by the process heap. Note that typically the total memory usage of
process will be about 50MiB higher than this value.  This defines the hard limit.
- `spike_limit_mib` (default = 20% of `limit_mib`): Maximum spike expected between the
measurements of memory usage. The value must be less than `limit_mib`. The soft limit
value will be equal to (limit_mib - spike_limit_mib).
The recommended value for `spike_limit_mib` is about 20% `limit_mib`.
- `limit_percentage` (default = 0): Maximum amount of total memory targeted to be
allocated by the process heap. This configuration is supported on Linux systems with cgroups
and it's intended to be used in dynamic platforms like docker.
This option is used to calculate `memory_limit` from the total available memory.
For instance setting of 75% with the total memory of 1GiB will result in the limit of 750 MiB.
The fixed memory setting (`limit_mib`) takes precedence
over the percentage configuration.
- `spike_limit_percentage` (default = 20% of `limit_percentage`): Maximum spike expected between the
measurements of memory usage. The value must be less than `limit_percentage`.
This option is used to calculate `spike_limit_mib` from the total available memory.
For instance setting of 25% with the total memory of 1GiB will result in the spike limit of 250MiB.
This option is intended to be used only with `limit_percentage`.

Examples:

```yaml
processors:
  memory_limiter:
    check_interval: 1s
    limit_mib: 4000
    spike_limit_mib: 800
```

- Hard limit will be set to **4000 MiB**.
- Soft limit will be set to 4000 - 800 = **3200 MiB**.

```yaml
processors:
  memory_limiter:
    check_interval: 1s
    limit_percentage: 80
    spike_limit_percentage: 15
```

On a machine with 1000 MiB total memory available:

- Hard limit will be set to 1000 * 0.80 = **800 MiB**.
- Soft limit will be set to 1000 * 0.80 - 1000 * 0.15 = 1000 * 0.65 = **650 MiB**.

Refer to [config.yaml](../../internal/memorylimiter/testdata/config.yaml) for detailed
examples on using the processor.
