Application Servers(java) : Should adding RAM to server depend on each domain's -Xmx value?

We have Glassfish application server running in Linux servers. Each Glassfish installation hosts 3 domains. Each domain has a JVM configuration such as -Xms 1GB and -XmX 2GB. That means if all these three domains are running at max memory, server should be able to allocate total 6GB to the JVMs

With that math,each of our server has 8GB RAM (2 GB Buffer) First of all - is this a good approach? I did not think so, because when we analyzed memory utilization on this server over past few months, it was only up to 1GB;

Now there are requests to add an additional domain to these servers - does that mean to add additional 2 GB RAM just to be safe or based on trend, continue wi开发者_开发知识库th whatever memory the server has?

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A few rules of thumb:

• You really want the sum of your XmXs and the RAM needed for the applications you need to run on the box constantly (including the OS) to be lower than your physical RAM available. Otherwise, something will be swapped to disk, and when that "something" needs to run, things will slow down dramatically. If things go constantly in and out of swap, nothing will perform.

• You might be able to get by with lower XmX on some of your application servers (your question seems to imply some of your JVMs have too much RAM allocated) . I believe Tomcat can start with as few as 64mb of XmX, but many applications will run out of memory pretty quickly with that setup. Tuning your memory allocation and the garbage collector might be worth it. Frankly, I'm not very up to date with GC performance lately (I haven't had to tune anything to get decent performance in 4-5 years), so I don't know how valuable will that be. GC collection pauses used to be bad, and bigger heaps meant longer pauses...

• Any memory you spare will not be "wasted". The OS will use it for disk cache and that might be a benefit.

In any case, the answer is normally... you need to run tests. Being able to run stress tests is really invaluable, and I suggest you spend some time writing one and running it. It will allow you to take educated decisions in this matter.