JVM flags

📅 29 Dec 2018 🕑 4 min read

Recently, I needed to decrease the time of stop-the-world pause that happened while Garbage Collector worked. During working on the problem, I found out many interesting jvm flags. In order to keep the information in one place, I created the following table.

type	category	name	description
memory	common	-Xms<size>[k|m|G]	the initial heap size
		-Xmx<size>[k|m|G]	maximum heap size
		-Xss<size>[k|m|G]	execution stack size
		-XX:NewRatio=<N>	the ratio between old generation (tenured) and new generation
		-XX:MinHeapFreeRatio=<N>	minimum ratio between free space to decrease size of generation
		-XX:MaxHeapFreeRatio=<N>	minimum ratio between free space to increase size of generation
	new generation	-Xmn<size>[k|m|G]	size of new generation memory
		-XX:NewSize=<size>[k|m|G]	minimum size of young generation memory
		-XX:MaxNewSize=<size>[k|m|G]	maximum size of young generation memory
		-XX:SurvivorRatio=<N>	$$ ratio = \frac{\text{eden}}{\text{survivor} * 2} $$
		-XX:YoungGenerationSizeIncrement=<N>	percent of increasing young generation
	old generation	-XX:TenuredGenerationSizeIncrement=<N>	percent of increasing old generation
	metaspace	-XX:MaxMetaspaceSize=<size>[k|m|G]	size of metaspace
		-XX:+CMSClassUnloadingEnabled	unload unused classes from the memory
garbage collection	common	-XX:+ScavengeBeforeFullGC	perform GC for young generation area before Full GC
		-XX:+CMSScavengeBeforeRemark	perform GC for young generation area before remark> stage
		-XX:-DisableExplicitGC	disable System.gc()
		-XX:CMSInitiatingOccupancyFraction=<N>	load factor of Tenured> area to run GC
		-XX:InitiatingHeapOccupancyPercent=<N>	load factor of heap area to run mark stage
		-XX:GCHeapFreeLimit=<N>	free memory limit after full GC. In case if size of free memory is less, OutOfMemoryError occurs
		-XX:-UseGCOverheadFreeLimit	activity limit of GC. In case if GC takes more time, OutOfMemoryError occurs
		-XX:GCTimeLimit=<N>	time limit for GC. If GC time exceeds this limit, OutOfMemoryError occurs. This ratio is between summary time of execution app and GC time
	Serial GC	-XX:+UseSerialGC	enable Serial GC
	Parallel GC	-XX:+UseParallelGC	enable Parallel GC
		-XX:MaxGCPauseMillis=<N>	maximum time interval between GC
		-XX:GCTimeRatio=<N>	the ratio between GC time and time of execution application. $$ \text{GC} = \frac{1}{1+\text{N}} $$
	Concurrent Mark and Swap	-XX:+UseConcMarkSweepGC	enable Concurrent Mark and Swap GC
		-XX:+UseParNewGC	use parallelism during GC on new generation area
		-XX:ParallelGCThreads=<N>	count of threads for parallel GC execution
		-XX:ConcGCThreads=<N>	total count of threads for GC
		-XX:+CMSScavengeBeforeRemark	run remark stage after GC execution on new generation area
		-XX:CMSWaitDuration	run GC on tenured are after GC execution on new generation
	Garbage First	-XX:+UseG1GC	enable G1
		-XX:MaxGCPauseMillis=<N>	Maximum interval between GC pauses
		-XX:GCPauseIntervalMillis=<N>	Maximum interval between G1 executions
		-XX:G1HeapRegionSize=<size>[k|m|G]	heap memory size
		-XX:ParallelGCThreads=<N>	count of threads for parallel GC execution
		-XX:ConcGCThreads=<N>	total count of threads for GC
	GC logging	-verbose:gc	enable printing GC information
		-Xlog:gc	enable garbage collector logs
		-Xlog:gc*	enable garbage collector detailed logs
		-Xloggc:<filename>	save logs into a file
		-XX:+UseGCLogFileRotation>	enable log rotation
		-XX:+HeapDumpBeforeFullGC	create memory before full GC stage
		-XX:NumberOfGCLogFiles=<N>	count of log files
		-XX:+PrintGCDetails	print more details about GC
		-XX:+PrintGCDateStamps	add time to logs
		-XX:+PrintGCApplicationConcurrentTime	time that taken by GC in case when all treads of application were working
		-XX:+PrintGCApplicationStoppedTime	stop the world time
other		-XX:+PrintFlagsFinal	print passed jvm flags
		-XX:+HeapDumpOnOutOfMemoryError	make dump before unexpected termination due to OutOfMemoryError
		-XX:HeapDumpPath=dump.hprof	make dump before unexpected termination due to OutOfMemoryError
		-XX:+CompileThreshold=<N>	count of executions that have been done until JIT optimizations are applied
		-XX:+UseCompressedOops	compact references in order to decrease memory usage
		-XX:+OptimizeStringConcat	use StringBuilder instead of + operator
		-XX:+PrintCompilation	show optimization levels of JIT
		-XX:+AggressiveOpts	aggressive optimization level
		-XX:+UseStringCache	cache strings
jmap tool	run	jmap <pid>	get information about memory usage
	flags	-heap	information about a whole heap
		-histo	information about memory usage in terms of each classes
		-histo:live	information about live objects
		-dump:live,format=b,file=heap.hprof	save dump into a file

\(\text{heap} = \text{young generation} + \text{old generation}\)
\(\text{young generation} = \text{eden} + \text{survivor} * 2\)
\(\text{survivor ratio} = \frac{ \text{eden}}{\text{survivor} * 2}\)