-XX MaxPermSize with or without -XX PermSize

Question

We ve run into a Java lang OutOfMemoryError  PermGen space error and looking at the tomcat JVM params  other than the -Xms and -Xmx params we also specify -XX MaxPermSize 128m  After a bit of profiling I can see occasionally garbage collection happening on the PermGen space saving it from running full   My question is  other than increasing the -XX MaxPermSize what would be the difference if I specify as well -XX PermSize  I know the total memory then would be Xmx maxPermSize but is there any other reason why -XX PermSize should not be there when -XX MaxPermSize is specified   Please do share if you have real-world experience dealing with these JVM parameters   ps  The JVM is HotSpot 64bit Server VM build 16 2-b04

User · Answer

-XX PermSize specifies the initial size that will be allocated during startup of the JVM  If necessary  the JVM will allocate up to -XX MaxPermSize

User · Answer

If you re doing some performance tuning it s often recommended to set both -XX PermSize and -XX MaxPermSize to the same value to increase JVM efficiency   Here is some information    Support for large page heap on x86 and amd64 platforms Java Support for Large Memory Pages Setting the Permanent Generation Size   You can also specify -XX  CMSClassUnloadingEnabled to enable class unloading option if you are using CMS GC  It may help to decrease the probability of Java lang OutOfMemoryError  PermGen space

User · Answer

By playing with parameters as -XX PermSize and -Xms you can tune the performance of - for example - the startup of your application  I haven t looked at it recently  but a few years back the default value of -Xms was something like 32MB  I think   if your application required a lot more than that it would trigger a number of cycles of fill memory - full garbage collect - increase memory etc until it had loaded everything it needed  This cycle can be detrimental for startup performance  so immediately assigning the number required could improve startup   A similar cycle is applied to the permanent generation  So tuning these parameters can improve startup  amongst others    WARNING The JVM has a lot of optimization and intelligence when it comes to allocating memory  dividing eden space and older generations etc  so don t do things like making -Xms equal to -Xmx or -XX PermSize equal to -XX MaxPermSize as it will remove some of the optimizations the JVM can apply to its allocation strategies and therefor reduce your application performance instead of improving it   As always  make non-trivial measurements to prove your changes actually improve performance overall  for example improving startup time could be disastrous for performance during use of the application

[java] -XX:MaxPermSize with or without -XX:PermSize

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