openHAB on Raspberry Pi 4 Model B (4GB): Why is -Xmx often limited to ~768MB?

Setup, Configuration and Use Beginners
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Hi everyone,

I’m currently tuning my openHAB setup on a Raspberry Pi 4 Model B with 4 GB RAM and noticed that many recommended configurations (including scripts and guides) use relatively conservative JVM heap settings, for example:

-Xms256m -Xmx768m

At first glance this feels low for a system with 4 GB of RAM, so I wanted to sanity-check the reasoning with the community.

I recently discussed this with ChatGPT as well, which also suggested 768 MB as a reasonable default heap size for a Raspberry Pi 4B, mainly with stability and responsiveness in mind rather than raw performance.

The arguments I’ve seen so far are:

  • openHAB typically doesn’t benefit much from a very large Java heap
  • larger heaps can increase GC pause times, especially on ARM CPUs
  • the JVM uses significant memory outside the heap (Metaspace, Code Cache, threads, native libs)
  • leaving RAM free benefits the Linux page cache and helps avoid swap, which is especially important on Raspberry Pi systems
  • overall system stability and responsiveness are usually more important than maximizing heap size

Based on this, the idea is that ~768 MB heap already covers most real-world openHAB installations on a Pi 4B, and increasing it further only makes sense if there are actual OutOfMemoryErrors or very large rule / persistence setups.

Does this match your real-world experience with openHAB on Raspberry Pi 4 Model B?
At what point do you usually recommend increasing -Xmx, and what symptoms do you look for?

Thanks in advance for your feedback!

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The reasoning is that Java per default uses 1/4th of physical memory.
That’s disadvantageous on boxes with >1 and <= 3GB so openHABian raises it to 768M.
(NOTE: openHABian does, not openHAB itself. It’s a Java startup parameter)

For 4+ GB systems that is less than the default but there’s not really any relevant drawbacks.
You might encounter a problem with that when you have a very very specific or weird setup with many video streams or image processing in Java, but in those cases you should be customizing your setup anyway.

I’ve recently removed setting any -Xmx parameter on 3+ GB boxes so on freshly installed 4/8 GB Raspis, you will be getting the defaults of 1/2 GB now.
You need to reinstall your box with latest openHABian or manually edit your Linux config files where it’s currently defined.

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