IBM J9 VM 8

This page describes the Optimization Pack for Eclipse OpenJ9 (formerly known as IBM J9) Virtual Machine version 8.

Metrics

All metrics

Name	Unit	Description
jvm_heap_size	bytes	The size of the JVM heap memory
jvm_heap_used	bytes	The amount of heap memory used
jvm_heap_util	percent	The utilization % of heap memory
jvm_memory_used	bytes	The total amount of memory used across all the JVM memory pools
jvm_memory_used_details	bytes	The total amount of memory used broken down by pool (e.g., code-cache, compressed-class-space)
jvm_memory_buffer_pool_used	bytes	The total amount of bytes used by buffers within the JVM buffer memory pool
jvm_gc_time	percent	The % of wall clock time the JVM spent doing stop the world garbage collection activities
jvm_gc_time_details	percent	The % of wall clock time the JVM spent doing stop the world garbage collection activities broken down by type of garbage collection algorithm (e.g., ParNew)
jvm_gc_count	collections/s	The total number of stop the world JVM garbage collections that have occurred per second
jvm_gc_count_details	collections/s	The total number of stop the world JVM garbage collections that have occurred per second, broken down by type of garbage collection algorithm (e.g., G1, CMS)
jvm_gc_duration	seconds	The average duration of a stop the world JVM garbage collection
jvm_gc_duration_details	seconds	The average duration of a stop the world JVM garbage collection broken down by type of garbage collection algorithm (e.g., G1, CMS)
jvm_threads_current	threads	The total number of active threads within the JVM
jvm_threads_deadlocked	threads	The total number of deadlocked threads within the JVM
jvm_compilation_time	milliseconds	The total time spent by the JVM JIT compiler compiling bytecode

Parameters

Heap

Name	Type	Unit	Default	Domain	Restart	Description
j9vm_minHeapSize	integer	megabytes	You should select your own default value.	You should select your own domain.	yes	Minimum heap size (in megabytes)
j9vm_maxHeapSize	integer	megabytes	You should select your own default value.	You should select your own domain.	yes	Maximum heap size (in megabytes)
j9vm_minFreeHeap	real	percent	0.3	0.1 → 0.5	yes	Specify the minimum % free heap required after global GC
j9vm_maxFreeHeap	real	percent	0.6	0.4 → 0.9	yes	Specify the maximum % free heap required after global GC

Garbage Collection

Name	Type	Unit	Default	Domain	Restart	Description
j9vm_gcPolicy	categorical		gencon	gencon, subpool, optavgpause, optthruput, nogc	yes	GC policy to use
j9vm_gcThreads	integer	threads	You should select your own default value.	1 → 64	yes	Number of threads the garbage collector uses for parallel operations
j9vm_scvTenureAge	integer		10	1 → 14	yes	Set the initial tenuring threshold for generational concurrent GC policy
j9vm_scvAdaptiveTenureAge	categorical		blank	blank, -Xgc:scvNoAdaptiveTenure	yes	Enable the adaptive tenure age for generational concurrent GC policy
j9vm_newSpaceFixed	integer	megabytes	You should select your own default value.	You should select your own domain.	yes	The fixed size of the new area when using the gencon GC policy. Must not be set alongside min or max
j9vm_minNewSpace	integer	megabytes	You should select your own default value.	You should select your own domain.	yes	The initial size of the new area when using the gencon GC policy
j9vm_maxNewSpace	integer	megabytes	You should select your own default value.	You should select your own domain.	yes	The maximum size of the new area when using the gencon GC policy
j9vm_oldSpaceFixed	integer	megabytes	You should select your own default value.	You should select your own domain.	yes	The fixed size of the old area when using the gencon GC policy. Must not be set alongside min or max
j9vm_minOldSpace	integer	megabytes	You should select your own default value.	You should select your own domain.	yes	The initial size of the old area when using the gencon GC policy
j9vm_maxOldSpace	integer	megabytes	You should select your own default value.	You should select your own domain.	yes	The maximum size of the old area when using the gencon GC policy
j9vm_concurrentScavenge	categorical		concurrentScavenge	concurrentScavenge, noConcurrentScavenge	yes	Support pause-less garbage collection mode with gencon
j9vm_gcPartialCompact	categorical		nopartialcompactgc	nopartialcompactgc, partialcompactgc	yes	Enable partial compaction
j9vm_concurrentMeter	categorical		soa	soa, loa, dynamic	yes	Determine which area is monitored by the concurrent mark
j9vm_concurrentBackground	integer		0	0 → 128	yes	The number of background threads assisting the mutator threads in concurrent mark
j9vm_concurrentSlack	integer	megabytes	0	You should select your own domain.	yes	The target size of free heap space for concurrent collectors
j9vm_concurrentLevel	integer	percent	8	0 → 100	yes	The ratio between the amount of heap allocated and the amount of heap marked
j9vm_gcCompact	categorical		blank	blank, -Xcompactgc, -Xnocompactgc	yes	Enables full compaction on all garbage collections (system and global)
j9vm_minGcTime	real	percent	0.05	0.0 → 1.0	yes	The minimum percentage of time to be spent in garbage collection, triggering the resize of the heap to meet the specified values
j9vm_maxGcTime	real	percent	0.13	0.0 → 1.0	yes	The maximum percentage of time to be spent in garbage collection, triggering the resize of the heap to meet the specified values
j9vm_loa	categorical		loa	loa, noloa	yes	Enable the allocation of the large area object during garbage collection
j9vm_loa_initial	real		0.05	0.0 → 0.95	yes	The initial portion of the tenure area allocated to the large area object
j9vm_loa_minimum	real		0.01	0.0 → 0.95	yes	The minimum portion of the tenure area allocated to the large area object
j9vm_loa_maximum	real		0.5	0.0 → 0.95	yes	The maximum portion of the tenure area allocated to the large area object

JIT

Name	Type	Unit	Default	Domain	Restart	Description
j9vm_jitOptlevel	ordinal		noOpt	noOpt, cold, warm, hot, veryHot, scorching	yes	Force the JIT compiler to compile all methods at a specific optimization level
j9vm_compilationThreads	integer	integer	You should select your own default value.	1 → 7	yes	Number of JIT threads
j9vm_codeCacheTotal	integer	megabytes	You should select your own default value.	You should select your own domain.	yes	Maximum size limit in MB for the JIT code cache
j9vm_jit_count	integer		10000	0 → 1000000	yes	The number of times a method is called before it is compiled

Other parameters

Name	Type	Unit	Default	Domain	Restart	Description
j9vm_lockReservation	categorical		categorical	blank, -XlockReservation	no	Enables an optimization that presumes a monitor is owned by the thread that last acquired it
j9vm_compressedReferences	categorical		blank	blank, -Xcompressedrefs, -Xnocompressedrefs	yes	Enable/disable the use of compressed references
j9vm_aggressiveOpts	categorical		blank	blank, -Xaggressive	yes	Enable the use of aggressive performance optimization features, which are expected to become default in upcoming releases
j9vm_virtualized	categorical		blank	blank, -Xtune:virtualized	yes	Optimize the VM for virtualized environment, reducing CPU usage when idle
j9vm_shareclasses	categorical		blank	blank, -Xshareclasses	yes	Enable class sharing
j9vm_quickstart	categorical		blank	blank, -Xquickstart	yes	Run JIT with only a subset of optimizations, improving the performance of short-running applications
j9vm_minimizeUserCpu	categorical		blank	blank, -Xthr:minimizeUserCPU	yes	Minimizes user-mode CPU usage in thread synchronization where possible

Domains

The following parameters require their ranges or default values to be updated according to the described rules:

Memory

Parameter	Default value	Domain
j9vm_minNewSpace	25% of j9vm_minHeapSize	must not exceed j9vm_minHeapSize
j9vm_maxNewSpace	25% of j9vm_maxHeapSize	must not exceed j9vm_maxHeapSize
j9vm_minOldSpace	75% of j9vm_minHeapSize	must not exceed j9vm_minHeapSize
j9vm_maxOldSpace	same as j9vm_maxHeapSize	must not exceed j9vm_maxHeapSize
j9vm_gcthreads	number of CPUs - 1, up to a maximum of 64	capped to default, no benefit in exceeding that value
j9vm_compressedReferences	enabled for j9vm_maxHeapSize<= 57 GB

Notice that the value nocompressedreferences for j9vm_compressedReferences can only be specified for JVMs compiled with the proper --with-noncompressedrefs flag. If this is not the case you cannot actively disable compressed references, meaning:

• for Xmx <= 57GB is useless to tune this parameter since compressed references are active by default and it is not possible to explicitly disable it

• for Xmx > 57GB, since the by default (blank value) compressed references are disabled, Akamas can try to enable it. This requires removing the nocompressedreferences from the domain

Constraints

The following tables show a list of constraints that may be required in the definition of the study, depending on the tuned parameters:

Memory

Formula	Notes
jvm.j9vm_minHeapSize < jvm.j9vm_maxHeapSize
jvm.j9vm_minNewSpace < jvm.j9vm_maxNewSpace && jvm.j9vm_minNewSpace < jvm.j9vm_minHeapSize && jvm.j9vm_maxNewSpace < jvm.j9vm_maxHeapSize
jvm.j9vm_minOldSpace < jvm.j9vm_maxOldSpace && jvm.j9vm_minOldSpace < jvm.j9vm_minHeapSize && jvm.j9vm_maxOldSpace < jvm.j9vm_maxHeapSize
jvm.j9vm_loa_minimum <= jvm.j9vm_loa_initial && jvm.j9vm_loa_initial <= jvm.j9vm_loa_maximum
jvm.j9vm_minFreeHeap + 0.05 < jvm.j9vm_maxFreeHeap
jvm.j9vm_minGcTimeMin < jvm.j9vm_maxGcTime

Notice that

• j9vm_newSpaceFixed is mutually exclusive with j9vm_minNewSpace and j9vm_maxNewSpace

• j9vm_oldSpaceFixed is mutually exclusive with j9vm_minOldSpace and j9vm_maxOldSpace

• the sum of j9vm_minNewSpace and j9vm_minOldSpace must be equal to j9vm_minHeapSize, so it's useless to tune all of them together. Max values seem to be more complex.