What are the reasons why Docker does not release memory?

There are several reasons why memory is not released in Docker.

1. Memory leak: The application may have a memory leak issue, where allocated memory is not properly released. This can be caused by code errors, resource management problems, or infinite loops, among other reasons. This results in the continuous increase of memory usage in Docker containers and the inability to release it.
2. Image caching: Docker stores cached images locally, which helps to improve the speed of container startup. However, these caches take up memory space and if not cleared properly, can lead to memory not being released.
3. Mounting a volume: If a storage volume is mounted in a Docker container and the files in the volume are constantly changing, the container may not be able to release the memory occupied by the volume. This is because the changes in the files of the volume will cause an increase in the file system cache, and this part of the cache cannot be released.
4. Garbage collection delay: Docker uses a garbage collection mechanism to release unused memory and resources, but the triggering time of garbage collection may be limited by certain conditions, such as only triggering when memory usage reaches a certain threshold. If these conditions are not met, it can result in memory not being released in a timely manner.
5. Resource constraint: Docker containers may have memory limits set, and if the application within the container exceeds these limits, it can lead to memory not being released. This is because the excess memory cannot be released by the container, but instead must be reclaimed by the host operating system.

It should be noted that the issues mentioned above could be caused by problems with the application, Docker configuration, or operating system configuration, and need to be investigated and resolved based on the specific situation.