The syntax is

-Xverbosegc usage: -Xverbosegc[:help]|[0|1][:file=[stdout|stderr|<filename>]]

    :help prints this message.

    0|1 controls the printing of heap information:
        0   Print after every Old Generation GC or Full GC
        1   (default) Print after every Scavenge and Old Generation GC 
            or Full GC


    :file=[stdout|stderr|<filename>] specifies output file
        stderr    (default) directs output to standard error stream
        stdout     directs output to standard output stream
        <filename> file to which the output will be written
 
 At every garbage collection, the following 20 fields are printed:
 <GC: %1 %2 %3 %4 %5 %6 %7 %8 %9 %10 %11 %12 %13 %14 %15 %16 %17 %18 %19 %20>

  %1:  Indicates the type of the garbage collection.
        1: represents a Scavenge (GC of New Generation only)
           %2: indicates if this is a parallel scavenge.
                0: non-parallel scavenge
                n(>0): parallel scavenge, n represents the number of parallel GC threads

        2: represents an Old Generation GC or a Full GC
           %2: indicates the GC reason:
                1: Allocation failure, followed by a failed scavenge, leading to a Full GC
                2: Call to System.gc
                3: Tenured Generation full
                4: Permanent Generation full
                5: Scavenge followed by a Train collection
                6: Concurrent-Mark-Sweep (CMS)eneration full
                7: Old generation expanded on last scavenge
                8: Old generation too full to scavenge
                9: FullGCAlot
               10: Allocation profiler triggered
               11: JVMTI force GC
               12: Adaptive Size Policy
               13: Last ditch collection

        3: represents a complete background CMS GC
           %2:  indicates the GC reason:
                1: Occupancy > initiatingOccupancy
                2: Expanded recently
                3: Incremental collection will fail
                4: Linear allocation will fail
                5: Anticipated promotion

        4: represents an incomplete background CMS GC
              (exited after yielding to foreground GC)
           %2:  n.m
                n indicates the GC reason:
                  1: Occupancy > initiatingOccupancy
                  2: Expanded recently
                  3: Incremental collection will fail
                  4: Linear allocation will fail
                  5: Anticipated promotion
                  6: Incremental CMS
                m indicates the background CMS state when yielding:
                  0: Resetting
                  1: Idling
                  2: InitialMarking
                  3: Marking
                  4: FinalMarking
                  5: Precleaning
                  6: Sweeping
                  7: AbortablePreclean

  %3:  Program time at the beginning of the collection, in seconds

  %4:  Garbage collection invocation. Counts of background CMS GCs
        and other GCs are maintained separately

  %5:  Size of the object allocation request that forced the GC,
        in bytes

  %6:  Tenuring threshold - determines how long the new born object
        remains in the New Generation

  The report includes the size of each space:
     Occupied before garbage collection (Before)
     Occupied after garbage collection (After)
     Current capacity (Capacity)
  All values are in bytes

  Eden Sub-space (within the New Generation)
    %7:  Before
    %8:  After
    %9:  Capacity

  Survivor Sub-space (within the New Generation)
    %10:  Before
    %11:  After
    %12:  Capacity

  Old Generation
    %13:  Before
    %14:  After
    %15:  Capacity

  Permanent Generation (Storage of Reflective Objects)
    %16:  Before
    %17:  After
    %18:  Capacity

    %19:  The total stop-the-world duration, in seconds.

    %20:  The total time used in collection, in seconds.

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