Optimize IPC RAM usage

Introduction

On smaller systems, or systems with complex IPC APIs, the IPC memory usage can be a significant contributor to the overall system RAM footprint. This can extend up to 20-30% of the total RAM usage of the system.

When your application uses an API, the Legato framework uses RAM for this API in four ways:

  • Message buffers for sending or receiving messages.
  • For APIs with events or callbacks, callback contexts to track each callback or event which has been registered.
  • For async IPC servers, async contexts tracking each outstanding async call.
  • Various messaging state and synchronization variables.

The callback and async contexts are both typically 16-32 bytes, so by far the largest memory user is usually the message buffers.

This page describes how to measure the memory required for API messages, and techniques which can be used to reduce this memory usage, such as:

Measuring IPC RAM Usage

The ifgen tool has a report mode which can be used to measure and report on

  • the memory used for each message buffer, and
  • the data size required for each API call and callback message.

To generate this report, run

# ifgen --lang Report --gen-mem-report <apiFile>

This will generate the memory report in the current directory in <api>_mem_report.html. If the API includes any other APIs via USETYPES you will need to add --import-dir flags to point to the directories containing those APIs.

This report contains a column for RTOS message size; this can be ignored unless you are using the experimental RTOS support.

Adjusting Buffer Sizes and Message Counts

The easiest way to reduce memory usage is decrease the size of strings and arrays in messages, or decrease the number of messages the server will process at a time. The message buffer is sized to accommodate the largest strings and arrays allowed by the API. Decreasing these, where possible, will result in a smaller message buffer. In a similar vein, consider if a single API which requires multiple string or array parameters can be split into different APIs with a smaller number of parameters.

Similarly, the Legato framework will estimate the number of message buffers which will be needed. This can be fine-tuned by overriding the API's Messages pool size. See the description of the cdef pools section for details.

Using References and Accessors

There are two common patterns to access bulk data in Legato APIs. Either a copy of the array data structure can be returned all at once to the client, or the client can return a reference to the data using a REF return value, and the client can make further API calls to access the individual elements. Returning a copy of the entire array or data structure can be convenient and quick if the caller will want every element. But it will also require larger IPC buffers, using more memory if the server is also keeping a copy of the data anyway.

This is especially important for handlers on RTOS systems. If a string or array is passed to a handler, the entire data is copied into the message buffer. If however a string or array is returned as an OUT parameter on a regular function, only a pointer to the buffer needs to be passed over IPC. This does not apply on Linux, where the string or array must be passed over IPC in both cases.

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