tomcat - Should I set a MaxMetaspaceSize?

Question

So, after asking this question, it quickly became clear that the important question was not "how can I", but "should I"?

We have customers that we are moving from Java7 to Java8 (using Tomcat7). Java7 required setting the -XX:MaxPermSize, and some customers have increased their max above the default set by the installer due to individual needs and uses.

Should I set the -XX:MaxMetaspaceSize (to the previous -XX:MaxPermSize setting) for customers who have defined a custom max? What about new installs? Should we set -XX:MaxMetaspaceSize at all?

What are the pros and cons of such a decision?

Answer 1

As I commented on the previous answer the reasons for setting a limit on those memory pools is different.

If your users previously increased the MaxPermSize above the default that probably was either to avoid Full GCs / concurrent mode failures with CMS or because their applications genuinely needed a lot of perm gen space.

Decreasing the the metaspace limit from its effectively infinite default would serve an entirely different purpose: Avoiding unbounded metaspace growth.

The thing is that that's just an upper limit. The actually committed, i.e. current metaspace size will be smaller. In fact, there is a setting called MaxMetaspaceFreeRatio (default 70%) which means that the actual metaspace size will never exceed 230% of its occupancy.

And for it to grow it first would have to fill up, forcing a garbage collection (metaspace full) in an attempt to free objects and only when it cannot meet its MinMetaspaceFreeRatio (default 40%) goal it would expand the current metaspace to no more than 230% of the occupancy after the GC cycle.

So in practice the actual metaspace size should stabilize within a band close relative to its actual need unless the application is continuously leaking classloaders/classes or generating an enormous amount of dynamic code.

TL;DR: There may be reasons to restrict metaspace size, but they likely are different to the original reasons for setting the perm gen sizes. Therefore the need should be re-evaluated.