This display shows a collection of statistical information about your garbage collector trace captured with the
-Xverbosegc option. A description of each item follows:
-
Duration of the measurement. This is the approximate elapsed wall time for the entire
data collection
-
Heap Capacity. This shows the
initial and final heap sizes, along with the sizes of the individual heap
spaces.
-
Capacity Utilization. This shows the highest percentage utilization of the heap at its final capacity
configuration. That is, given the final heap capacity shown in item 2
above, these are the highest percentages of those areas actually used
by the application. A rule of thumb for optimal utilization would
be to have the Eden space at 100% with the Survivor and Old spaces showing some
reserve capacity depending on the application.
-
Total Bytes Allocated. This is the total amount of space created for new
objects over the lifetime
of the application. Do not confuse this with the amount of memory actually allocated for
your program to use! Instead, it is the amount of memory
your program would have consumed had no garbage collection ever been performed. This
is an abstract measure of the total work done by the
application.
-
Residual Bytes. This is the heap usage at program
termination.
-
Number of GC Events. This is
the number of times the garbage collector was invoked during the program
run.
-
Time Spend in GC. This is the total amount of wall time spent in the garbage
collector during the program run.
-
Percentage of Time in GC. This is the percentage wall time spent in
the garbage collector during the program run.
-
Average Allocation Rate.
This is the average memory allocation rate for new Java objects.
-
Average Ideal Allocation
Rate. This is the average memory allocation rate for new Java
objects with the time spent in the garbage collector removed.
In other words, this is what the average allocation rate would have been had
no garbage collection been necessary. It is a theoretical limit of your
program's performance if GC time were driven to zero.
-
GC Activity Summary. This
table collects information broken out by garbage collection type. The
information in each column is as follows:
Last Occur
(s) |
Time at which this GC event last occurred. |
Count |
Total number of times this GC event
occurred. |
Interval
(s) |
Average amount of time between events of this
type. |
Duration
(s) |
Average amount of time spent in events of this
type. |
Average collection
rate (KB/s) |
Average rate of space recovered by events of
this type. |