-Xtgc

Provides garbage collection tracing options.

Syntax

    -Xtgc:<parameter>{,<parameter>}

Parameters

Specify one one or more of the following parameters in a comma-separated list:

allocation

    -Xtgc:allocation

Prints both per thread (TLH) and cumulative allocation cache statistics, such as total bytes allocated since last GC, current TLH sizing, and discarded TLH size.

For more information about the allocation cache, see Allocation caches.

backtrace

    -Xtgc:backtrace

Before a garbage collection, a single line is printed containing the name of the main thread for garbage collection, as well as the value of the osThread slot in the J9VMThread structure.

compaction

    -Xtgc:compaction

Prints extra information showing the relative time spent by threads in the "move" and "fixup" phases of compaction

concurrent

    -Xtgc:concurrent

Prints extra information showing the activity of the concurrent mark background thread

dump

    -Xtgc:dump

Prints a line of output for every free chunk of memory in the system, including "dark matter" (free chunks that are not on the free list for some reason, typically because they are too small). Each line contains the base address and the size in bytes of the chunk. If the chunk is followed in the heap by an object, the size and class name of the object is also printed. This argument has a similar effect to the terse argument.

file

    -Xtgc:file=<filename>

Directs the logs to a file. Otherwise they are directed to stderr.

freeList

    -Xtgc:freeList

Before a garbage collection, prints information about the free list and allocation statistics since the last garbage collection. Prints the number of items on the free list, including "deferred" entries (with the scavenger, the unused space is a deferred free list entry). For TLH and non-TLH allocations, prints the total number of allocations, the average allocation size, and the total number of bytes discarded during allocation. For non-TLH allocations, also included is the average number of entries that were searched before a sufficiently large entry was found.

parallel

    -Xtgc:parallel

Produces statistics on the activity of the parallel threads during each operation (mark, sweep, scavenge etc.) of a GC cycle.

rootscantime

    -Xtgc:rootscantime

Prints duration of strong and weak roots scanning of a GC cycle.

scavenger

    -Xtgc:scavenger

Prints extra information after each scavenger collection. A histogram is produced showing the number of instances of each class, and their relative ages, present in the survivor space. The information is obtained by performing a linear walk-through of the space.

terse

    -Xtgc:terse

Dumps the contents of the entire heap before and after a garbage collection. For each object or free chunk in the heap, a line of trace output is produced. Each line contains the base address, "a" if it is an allocated object, and "f" if it is a free chunk, the size of the chunk in bytes, and, if it is an object, its class name.