Class sharing is a feature in Eclipse OpenJ9 that improves your application’s startup performance. It also reduces the memory footprint if the same cache shared across multiple JVMs. Traditionally, this feature is turned on by explicitly specifying option -Xshareclasses in the command line.
Default Class Sharing
Class data sharing is enabled by default for bootstrap classes from release 0.16.0 of OpenJ9 onwards (unless you are running in a container). When the JVM starts up, it will attempt to create or connect to a default shared cache. If an error occurred during the start-up process of the default shared classes cache, the JVM treats the error as non-fatal, silently ignoring it and goes back to normal start-up process without using the default shared classes cache.
We introduced a new sub-option bootClassesOnly. If the default shared cache failed to start up, user can use option -Xshareclasses:bootClassesOnly,fatal,verbose in the command line to show information regarding the start-up error. The default class sharing is equivalent to specifying -Xshareclasses:bootClassesOnly,nonfatal,silent in the command line.
Command line utilities on default shared cache
You can use all -Xshareclasses command line utilities on the default shared cache as usual. For example, you can use -Xshareclasses:listAllCaches to find the default shared cache.
java -Xshareclasses:listAllCaches Listing all caches in cacheDir /root/javasharedresources/ Cache name level cache-type feature layer OS shmid OS semid last detach time Compatible shared caches sharedcc_root Java11 64-bit persistent cr 0 Wed Oct 9 11:47:34 2019
Use -Xshareclasses:printStats to show the cache statistics.
java -Xshareclasses:printStats Current statistics for cache "sharedcc_root": Cache created with: -Xnolinenumbers = false BCI Enabled = true Restrict Classpaths = false Feature = cr Cache contains only classes with line numbers base address = 0x00007F1975459000 end address = 0x00007F1988000000 allocation pointer = 0x00007F197563DC68 cache layer = 0 cache size = 314572192 softmx bytes = 67108864 free bytes = 64208696 Reserved space for AOT bytes = -1 Maximum space for AOT bytes = -1 Reserved space for JIT data bytes = -1 Maximum space for JIT data bytes = -1 Metadata bytes = 29104 Metadata % used = 1% Class debug area size = 25133056 Class debug area used bytes = 477352 Class debug area % used = 1% ROMClass bytes = 1985640 AOT bytes = 42624 JIT data bytes = 904 Zip cache bytes = 0 Startup hint bytes = 0 Data bytes = 363936 # ROMClasses = 610 # AOT Methods = 24 # Classpaths = 1 # URLs = 0 # Tokens = 0 # Zip caches = 0 # Startup hints = 0 # Stale classes = 0 % Stale classes = 0% Cache is 4% soft full Cache is accessible to current user = true
If the default shared cache is obsolete, you can use -Xshareclasses:destroy to delete it.
java -Xshareclasses:destroy JVMSHRC806I Compressed references persistent shared cache "sharedcc_root" has been destroyed. Use option -Xnocompressedrefs if you want to destroy a non-compressed references cache.
The default shared cache is a normal cache under default cache directory (user home) with the default cache name (sharedcc_username). Another JVM explicitly specifying -Xshareclasses option can connect to the default shared cache as usual.
You can use -Xsharedclasses:none in the command line to completely turn off class data sharing, but disabling it hurts your applications startup. Visit here to find out more about class sharing in Eclipse OpenJ9.