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 message 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. .. _ProfileChunkedBuffer: https://searchfox.org/mozilla-central/search?q=ProfileChunkedBuffer&path=&case=true®exp=false .. _ProfileChunkManager: https://searchfox.org/mozilla-central/search?q=ProfileBufferChunkManager.h&path=&case=true®exp=false .. _ProfileBufferChunk: https://searchfox.org/mozilla-central/search?q=ProfileBufferChunk&path=&case=true®exp=false .. _ProfileBufferChunkManagerWithLocalLimit: https://searchfox.org/mozilla-central/search?q=ProfileBufferChunkManagerWithLocalLimit&case=true&path= .. _ProfilerParent.h: https://searchfox.org/mozilla-central/source/tools/profiler/public/ProfilerParent.h .. _ProfilerChild.h: https://searchfox.org/mozilla-central/source/tools/profiler/public/ProfilerChild.h .. _ProfileBuffer.h: https://searchfox.org/mozilla-central/source/tools/profiler/core/ProfileBuffer.h .. _ProfileBuffer: https://searchfox.org/mozilla-central/search?q=ProfileBuffer&path=&case=true®exp=false .. _ProfilerParent: https://searchfox.org/mozilla-central/search?q=ProfilerParent&path=&case=true®exp=false .. _ProfilerChild: https://searchfox.org/mozilla-central/search?q=ProfilerChild&path=&case=true®exp=false .. _DestroyReleasedChunksAtOrBefore: https://searchfox.org/mozilla-central/search?q=DestroyReleasedChunksAtOrBefore&path=&case=true®exp=false