API to provide GC parameters to JFR

[dmitripivkine]

This issue is created to be main discussion place for:

• find equivalents in OpenJ9
• specify unique internal OpenJ9 parameter name
• parameters details: initial vs current, availability for current GC policy etc.
• API implementation details

There is list of GC parameters required to be provided to JFR:

============================
GC General

Field: youngCollector
Type: GCName
Description: Young Garbage Collector: The garbage collector used for the young generation
OpenJ9 equivalent: Scavenger for Gencon, ? for other GC policies

Field: oldCollector
Type: GCName
Description: Old Garbage Collector: The garbage collector used for the old generation
OpenJ9 equivalent: ?

Field: parallelGCThreads
Type: uint
Description: Parallel GC Threads: Number of parallel threads to use for garbage collection
OpenJ9 equivalent: maximum? current?

Field: concurrentGCThreads
Type: uint
Description: Concurrent GC Threads: Number of concurrent threads to use for garbage collection
OpenJ9 equivalent: ?

Field: usesDynamicGCThreads
Type: boolean
Description: Uses Dynamic GC Threads: Whether a dynamic number of GC threads are used or not
OpenJ9 equivalent: Uses Dynamic GC Threads

Field: isExplicitGCConcurrent
Type: boolean
Description: Concurrent Explicit GC: Whether System.gc() is concurrent or not
OpenJ9 equivalent: ?

Field: isExplicitGCDisabled
Type: boolean
Description: Disabled Explicit GC: Whether System.gc() will cause a garbage collection or not
OpenJ9 equivalent: Disabled Explicit GC

Field: pauseTarget
Type: long: millis
Description: Pause Target: Target for GC pauses
OpenJ9 equivalent: ?

Field: gcTimeRatio
Type: uint
Description: GC Time Ratio: Target for runtime vs garbage collection time
OpenJ9 equivalent: ?

============================
Object Heap

Field: minSize
Type: ulong: bytes
Description: Minimum Heap Size
OpenJ9 equivalent: Initial Heap Size ?

Field: maxSize
Type: ulong: bytes
Description: Maximum Heap Size
OpenJ9 equivalent: Maximum Heap Size

Field: initialSize
Type: ulong: bytes
Description: Initial Heap Size
OpenJ9 equivalent: Initial Heap Size

Field: usesCompressedOops
Type: boolean
Description: Uses Compressed Oops: If Compressed Oops Are Used
OpenJ9 equivalent: Uses Compressed Oops

Field: compressedOopMode
Type: NarrowOopMode
Description: Compressed Oops Mode
OpenJ9 equivalent: ?

Field: objectAlignment
Type: ulong: bytes
Description: Object alignment (in bytes) on the heap
OpenJ9 equivalent: Object alignment (in bytes) on the heap

Field: heapAddressBits
Type: ubyte
Description: Heap address size (in bits)
OpenJ9 equivalent: Heap address size (in bits)

============================
Generational Collector

Field: minSize
Type: ulong: bytes
Description: Minimum Young Generation Size
OpenJ9 equivalent: Initial Nursery Size ?

Field: maxSize
Type: ulong: bytes
Description: Maximum Young Generation Size
OpenJ9 equivalent: Maximum Nursery Size

Field: newRatio
Type: uint
Description: New Ratio: The size of the young generation relative to the tenured generation
OpenJ9 equivalent: The size of the Nursery relative to the size of the Tenure ?

============================
TLABs

Field: usesTLABs
Type: boolean
Description: TLABs Used: If Thread Local Allocation Buffers are in use
OpenJ9 equivalent: TLHs Used: If Thread Local Buffers are in use

Field: minTLABSize
Type: ulong: bytes
Description: Minimum TLAB Size
OpenJ9 equivalent: Minimum TLH size

Field: tlabRefillWasteLimit
Type: ulong: bytes
Description: TLAB Refill Waste Limit
OpenJ9 equivalent: ?

============================
Tenuring Details

Field: maxTenuringThreshold
Type: ubyte
Description: Maximum Tenuring Threshold: Upper limit for the age of how old objects to keep in the survivor area
OpenJ9 equivalent: Maximum Tenuring Threshold

Field: initialTenuringThreshold
Type: ubyte
Description: Initial Tenuring Threshold: Initial age limit for how old objects to keep in the survivor area
OpenJ9 equivalent: Initial Tenuring Threshold ?

[dmitripivkine]

@TobiAjila @amicic

[dmitripivkine]

Implementation details:

Currently I see two reasonable ways to implement this feature:

We do have API visible externally to GC returns initial configuration details:

UDATA
j9gc_modron_getConfigurationValueForKey(J9JavaVM *javaVM, UDATA key, void *value)

This API can be stretched for JFR support.
Pros: there is single function to all parameters, list can be expanded by adding new keys.
Cons: Strech of current use, unification of outputs for all parameters (please note despite output parameter value has void * type - I guess was designed in expectation of flexible output - current practical use is limited to return UDATA only. I am not sure do we want to change this or unify output for all parameters - for example to return predefined number instead of string)

An alternative common used way is creation if individual external GC function for each of parameters.
Pros: Unlimited flexibility
Cons: Potentially large number of functions needs to be added to the table.