Buffers and Memory Management

In a post-Fission world, precise memory management across many threads and processes is especially important. In order for the profiler to achieve this, it uses a chunked buffer strategy.

The ProfileBuffer is the overall buffer class that controls the memory and storage for the profile, it allows allocating objects into it. This can be used freely by things like markers and samples to store data as entries, without needing to know about the general strategy for how the memory is managed.

The ProfileBuffer is then backed by the ProfileChunkedBuffer. This specialized buffer grows incrementally, by allocating additional ProfileBufferChunk objects. More and more chunks will be allocated until a memory limit is reached, where they will be released. After releasing, the chunk will either be recycled or freed.

The limiting of memory usage is coordinated by the ProfilerParent in the parent process. The ProfilerParent and ProfilerChild exchange IPC messages with information about how much memory is being used. When the maximum byte threshold is passed, the ProfileChunkManager in the parent process removes the oldest chunk, and then the ProfilerParent sends a DestroyReleasedChunksAtOrBefore message to all of child processes so that the oldest chunks in the profile are released. This helps long profiles to keep having data in a similar time frame.

Profile Buffer Terminology

ProfilerParent

The main profiler machinery is installed in the parent process. It uses IPC to communicate to the child processes. The PProfiler is the actor which is used to communicate across processes to coordinate things. See ProfilerParent.h. The ProfilerParent uses the DestroyReleasedChunksAtOrBefore meessage to control the overall chunk limit.

ProfilerChild

ProfilerChild is installed in every child process, it will receive requests from DestroyReleasedChunksAtOrBefore.

Entry

This is an individual entry in the ProfileBuffer.h,. These entry sizes are not related to the chunks sizes. An individual entry can straddle two different chunks. An entry can contain various pieces of data, like markers, samples, and stacks.

Chunk

An arbitrary sized chunk of memory, managed by the ProfileChunkedBuffer, and IPC calls from the ProfilerParent.

Unreleased Chunk

This chunk is currently being used to write entries into.

Released chunk

This chunk is full of data. When memory limits happen, it can either be recycled or freed.

Recycled chunk

This is a chunk that was previously written into, and full. When memory limits occur, rather than freeing the memory, it is re-used as the next chunk.