Java memory management revolves around the garbage collector, which is the entity responsible for traversing the heap and freeing space that is being taken up by unreferenced objects. Garbage collection makes life easier for Java programmers, since it frees them from having to explicitly manage memory resources (this isn’t 100% true, but close enough). In the Java runtime environment, there are two types of collections that can occur. The first type of collection is referred to as minor collection. Minor collections are responsible for locating live objects in the young generation (eden), copying these objects to the inactive survivor space, and moving tenured objects from the active survivor space to the old (tenured) generation (this assumes that a generational collector is being used). The second form of collection is the major collection. This type of collection frees unreferenced objects in in the tenured generation, and optionally compacts the heap to reduce fragmentation.
When debugging performance problems, it is extremely useful to be able to monitor object allocations and frees in the new and old generations. The Java development kit comes with the jstat utility, which provides a ton of visibility into what the garbage collector is doing, as well as a slew of information on how each generation is being utilized. To use jstat to display garbage collection statistics for the new, old and permanent generations, jstat can be invoked with the “-gc” (print garbage collection heap statistics) option, the “-t” (print the total number of seconds the JVM has been up) option, the process id to retrieve statistics from, and an optional interval to control how often statistics are printed:
$ jstat -gc -tpgrep java 5000
Timestamp S0C S1C S0U S1U EC EU OC OU PC PU YGC YGCT FGC FGCT GCT
98772.0 1600.0 1600.0 0.0 1599.8 13184.0 5561.6 245760.0 201671.9 16384.0 6443.0 166683 2402.690 32411 110.564 2513.255
98777.0 1600.0 1600.0 1599.4 0.0 13184.0 9533.7 245760.0 156797.1 16384.0 6443.0 166690 2402.785 32414 110.573 2513.359
98782.0 1600.0 1600.0 1599.7 0.0 13184.0 10328.6 245760.0 166402.2 16384.0 6443.0 166698 2402.889 32416 110.580 2513.469
98787.0 1600.0 1600.0 0.0 1599.9 13184.0 2383.5 245760.0 195366.0 16384.0 6443.0 166707 2403.016 32416 110.580 2513.595
The output above contains the size of each survivor space (S0C && S1C), the utilization of each survivor space (S0U && S1U), the capacity of eden (EC), the utilization of eden (EU), the capacity of the old generation (OC), the utilization of the old generation (OU), the permanent generation capacity (PC), the permanent generation utilization (PU), the total number of young generation garbage collection events (YGC), the total amount of time spent collecting objects in the new generation (YGCT), the total number of old generation garbage collection events that have occurred (FGC), the total amount of time spent collecting objects in the old generation (FGCT), and the total time spent performing garbage collection.
If you prefer to view garbage collection events as percentages, you can use the “-gcutil” option:
$ jstat -gcutil -t -h5pgrep java 5000
Timestamp S0 S1 E O P YGC YGCT FGC FGCT GCT
99814.1 0.00 99.99 18.08 63.77 39.32 168551 2427.512 32800 111.800 2539.313
99819.1 99.96 0.00 66.29 78.18 39.32 168562 2427.649 32800 111.800 2539.449
99824.1 100.00 0.00 94.40 62.46 39.32 168572 2427.795 32803 111.815 2539.610
99829.2 100.00 0.00 60.25 65.08 39.32 168580 2427.888 32806 111.824 2539.713
The output above contains the utilization of each survivor space as a percentage of the total survivor space capacity (S0 && S1), the utilization of eden as a percentage of the total eden capacity (E), the utilization of the tenured generation as a percentage of the total tenured generation capacity (O), the utilization of the permanent generation as a percentage of the total permanent generation capacity (P), the total number of young generation garbage collection events (YGC), the total time spent collection objects in the young generation (YGCT), the total number of of old generation garbage collection events (FGC), the total amount of time spent collecting objects in the old generation (FGCT), and the total garbage collection time.
To get the time spent in garbage collection along with the reason the collection occurred, jstat can be run with the “-gccause” option:
$ jstat -gccause -tpgrep java 1000
Timestamp S0 S1 E O P YGC YGCT FGC FGCT GCT LGCC GCC
100157.3 99.96 0.00 66.27 63.82 39.32 169160 2435.394 32925 112.202 2547.595 CMS Initial Mark No GC
100158.3 0.00 99.99 32.14 67.72 39.32 169163 2435.430 32925 112.202 2547.631 unknown GCCause No GC
100159.3 0.00 99.97 50.22 65.10 39.32 169165 2435.454 32927 112.208 2547.662 CMS Initial Mark No GC
100160.3 99.97 0.00 6.02 62.46 39.32 169168 2435.493 32928 112.211 2547.704 unknown GCCause No GC
100161.3 99.97 0.00 32.14 62.46 39.32 169168 2435.493 32928 112.211 2547.704 unknown GCCause No GC
There are also options to print class loader activity and hotspot compiler statistics, and to break down utilization by generation (this is extremely useful when your trying to profile a specific memory pool). There are a number of incredibly useful opensource tools for visualizing garbage collection data, and I hope to talk about these in the near future.