How much headroom to leave between Java Xmx and Docker container RAM size?

Question

I'm 100% aware of the pitfalls of the JVM and advances/strides that have been done in the JVM world for it to understand the ergonomics/resources of a container. Also, yes I know that Java by default tries to allocate 1/4 the RAM available in the environment it's running...

So I had some thoughts and questions. I have made a 50/50 rule. If my application needs 1GB of Xmx, then I create 2GB container which gives another 1GB of RAM for the JVM overhead and any container/swap stuff (though not sure how swap really works within a container).

So I was thinking if my application needs 6GB of Xmx do I really need to create 12GB container or can I get away with 7GB or 8GB container? How much headroom do we need to give inside the container when it comes to RAM?

Answer 1

This is a very complicated question. The heap is only a portion of Java process; which has a lot of native resources also, not tracked with -Xms and -Xmx. Here is a very nice summary about what these might be.

Then, there is the garbage collector algorithm that you use: the more freedom (extra-space) it has - the better. This has to do with so-called GC barriers. In very simple words, when GC is executed - all the heap manipulations will be "intercepted" and tracked separately. If the allocation rate is high, the space needed to track these changes (while an active GC happens) tends to grow. The more space you have - the better the GC will perform.

Then, it matters what java version you are using (because -Xmx and -Xms might mean different things under different java versions with respect to containers), for example take a look here.

So there is no easy answer here. If you can afford 12GB of memory - do so. Memory is a lot cheaper (usually) than debugging any of the problems above.

Answer 2

If your container is dedicated to the JVM, then you don't need to use a percentage.

You need to know how much java heap you need -- in this case 6GB -- and how much everything else needs, which it looks like you've already proven is less than 2GB.

Then just add them up. An 8GB container should be fine in this case. Note that stack and heap memory are separate, though, so if you will need a large number of threads running simultaneously, don't forget to add 1MB of stack space for each one (or whatever you set with -Xss)

ALSO, I really recommend setting the minimum and maximum Java heap size to the SAME value -- 6GB in this case. Then you're guaranteed that there will no surprises when Java tries to grow the heap.