What would you like to be added:

It would be nice to pass a process a number that is a bit lower than the container memory limit so that its garbage collection or memory management can use that limit.

For example:

• setting GOMEMLIMIT or setting NODEMEMLIMIT
• setting NODE_OPTS=--max-old-space-size=$(NODEMEMLIMIT) would be useful.
• setting JAVA_MEM_LIMIT and passing -Xmx$(JAVA_MEM_LIMIT)M

Currently you can use envFrom.resourceFieldRef to "copy" a resource request or limit to an environment variable, but you can only multiply the value by some power of 1000 or 1024 by specifying divisor.

Why is this needed:

This would make it possible to pass a memory limit into a process inside the container that is based on the container resource request or limit but less.

For example, when specifying node's --max-old-space-size or Java's -Xmx parameter, setting it equal to the memory limit of the container will be a problem, because the process actually uses more memory than what you provided as a parameter.

Potential Solutions

• If we could specify a divisor like 1.5Mi then it would result in a number of megabytes equal to roughly 2 thirds of the available memory, leaving some extra for the other heaps.
• Add another field like scale or multiplier to enfFrom.resourceFieldRef which is multiplied by the input value to get the env var value
• Allow arithmetic in the existing env var expansion system - where currently you can only do $(SOME_VAR) in the command line, additionally support something like $(SOME_VAR - 100)
• Allow using a go template or some simple templating/expression language to actually fill out the env var with prefix and suffix, like --max-old-space-size={{ value * 800 }}M

Workarounds

• Create an admission webhook that does one of the above without changing the kubernetes core features
• If the container uses bash, you could run a command like bash -c 'NODE_OPTIONS=---max-old-space-size=$(($MEMORY_LIMIT-500)) ... or create your own entrypoint wrapper (maybe a shell script) that calculates the env vars before running the actual command