Docker containers' memory consumption never decreases (or does it?)

Question

we're running several containers on a single docker host, mainly to run R and Python apps for data analysis. So when I load a big table into one of the containers, its memory footprint on the docker host increases.

However, when I close the Jupyter Notebook or R session, the container's memory footprint appears to remain unchanged on the host. It seems that the memory consumption of a docker container can only go up, and not down.

So I know that Linux in general occupies memory which is not needed by other applications (stuff is cached). However, how is this dealt with in the case of Docker containers? From the individual containers' perspectives there is a lot of memory (we don't want to limit the memory available to containers), and even if it is not needed inside this particular container, it would remain "occupied" in the container, and therefore inaccessible by other containers. And the host doesn't know if this memory is really needed or simply used for caching.

So how is this dealt with? I can imagine a situation where several people have started containers in which they have loaded or generated big data sets, but this was only temporary, and now the host's memory is all occupied because the memory is not freed.

I'm pretty sure that this is not how it works, so can someone explain this to me, please?

Many thanks,

Enno

Answer 1

In the Docker documentation, under resource constraints, there is an explanation about limiting memory to containers. When running a container, the memory is not freed based on the processes running in the container. The docs explain how the host system manages memory:

It is important not to allow a running container to consume too much of the host machine’s memory. On Linux hosts, if the kernel detects that there is not enough memory to perform important system functions, it throws an OOME, or Out Of Memory Exception, and starts killing processes to free up memory. Any process is subject to killing, including Docker and other important applications. This can effectively bring the entire system down if the wrong process is killed.

Docker attempts to mitigate these risks by adjusting the OOM priority on the Docker daemon so that it is less likely to be killed than other processes on the system...

The Docker containers can use memory but is prevented by the Docker daemon from crashing the host system. The memory allotted to Docker containers can also be limited:

Docker can enforce hard memory limits, which allow the container to use no more than a given amount of user or system memory, or soft limits, which allow the container to use as much memory as it needs unless certain conditions are met, such as when the kernel detects low memory or contention on the host machine.

We do not want to limit memory to the containers, but there are options to do so, like --memory=<value> , --memory-swap , and --memory-reservation. So no, the host cannot free up memory of a container that is running, but it does prevent the risk of all memory being occupied and making the kernel potentially kill a crucial system process.

Please excuse the formatting. Hope this helps; I also linked the related documentation. Also, not completely related, but maybe you can check this out about using a Java application in a container:
Why the docker container memory usage doesn't decrease?