CODEWITHSUNDEEP
cloud-foundry
atanu mallik
atanu mallik

Reputation: 53

App with Docker Image and Memory allocation

I came across a questionnaire on Containers and Buildpacks. I could not find the answer to the following questions :

  1. What buildpack is used to stage docker apps?
    I would assume a non-docker app would require a buildpack which would produce the container image during the staging process. For a docker based application a buildpack is not relevant since it is already a container image . Is this correct understanding ?

  2. When you allocate 1G to an app, how much memory does the application receive? What component determines this?
    I would assume, the actual application (For example: my_sample_spring_boot_app)would receive less memory than 1G , since some part of teh memory would be utilised for Infra + Stack + runtime etc. Is this correct understanding ?

Could you please help me with some guidance

Upvotes: 0

Views: 265

Answers (1)

Daniel Mikusa
Daniel Mikusa

Reputation: 15051

What buildpack is used to stage docker apps? I would assume a non-docker app would require a buildpack which would produce the container image during the staging process. For a docker based application a buildpack is not relevant since it is already a container image . Is this correct understanding ?

That sounds like a trick question. When you run Docker images on Cloud Foundry, no buildpacks run.

There are different app lifecycles to run each type of app. So when you have source code to build and run, the buildpacks app lifecycle takes that, runs the buildpacks, and generates a droplet from them. The droplet is then run.

With a Docker image, the docker app lifecycle is used. No build/stage happens since that's assumed to happen externally, and the image is just ready to be run.

When you allocate 1G to an app, how much memory does the application receive? What component determines this? I would assume, the actual application (For example: my_sample_spring_boot_app)would receive less memory than 1G , since some part of teh memory would be utilised for Infra + Stack + runtime etc. Is this correct understanding ?

Correct. If you allocate a 1G memory limit then everything running in the container must all fit within that 1G limit.

Your application will get most of that memory, but there are a couple of small processes that run in every container: an init process (i.e. PID1), an SSHD (for cf ssh) and in most CF versions there's also an envoy proxy (for TLS termination in the container). I haven't looked recently, but a while back this took up around 24M. If anyone has measured recently, please comment.

Everything has to fit under the memory limit. If your application forks, perhaps because it forks worker processes or if you fork and run sub-processes as part of the application. Everything you run has to all stay under the 1G limit.

Last note. For Java apps, 1G is the total memory size of the container. NOT your JVM heap. That's a common misconception. The entire JVM process and all its different memory regions (heap, metaspace, threads, direct memory, code cache, etc...) have to fit within the 1G limit.

Upvotes: 1

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