Java Platform, Standard Edition Troubleshooting Guide
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2.17 The jstat Utility

The jstat utility uses the built-in instrumentation in the Java HotSpot VM to provide information about performance and resource consumption of running applications. The tool can be used when diagnosing performance issues, and in particular issues related to heap sizing and garbage collection. The jstat utility does not require the VM to be started with any special options. The built-in instrumentation in the Java HotSpot VM is enabled by default. This utility is included in the JDK download for all operating system platforms supported by Oracle.


Note:

The instrumentation is not accessible on a FAT32 file system.

For more details on the jstat utility, see the jstat command man page.

The jstat utility uses the virtual machine identifier (VMID) to identify the target process. The documentation describes the syntax of the VMID, but its only required component is the local virtual machine identifier (LVMID). The LVMID is typically (but not always) the operating system's PID for the target JVM process.

The jstat tool provides data similar to the data provided by the tools vmstat and iostat on Oracle Solaris and Linux operating systems.

For a graphical representation of the data, you can use the visualgc tool. See The visualgc Tool.

Example 2-32 illustrates the use of -gcutil option where the jstat utility attaches to LVMID number 2834, takes seven samples at 250-millisecond intervals.

The output of this example shows that a young generation collection occurred between the third and fourth samples. The collection took 0.017 seconds and promoted objects from the eden space (E) to the old space (O), resulting in an increase of old space utilization from 46.56% to 54.60%.

Example 2-33 illustrates the use of the -gcnew option where the jstat utility attaches to LVMID number 2834, takes samples at 250-millisecond intervals, and displays the output. In addition, it uses the -h3 option to display the column headers after every three lines of data.

In addition to showing the repeating header string, this example shows that between the fourth and fifth samples, a young generation collection occurred, whose duration was 0.02 seconds. The collection found enough live data that the survivor space 1 utilization (S1U) would have exceeded the desired survivor size (DSS). As a result, objects were promoted to the old generation (not visible in this output), and the tenuring threshold (TT) was lowered from 15 to 1.

Example 2-34 illustrates the use of the -gcoldcapacity option where the jstat utility attaches to LVMID number 21891 and takes three samples at 250-millisecond intervals. The -t option is used to generate a time stamp for each sample in the first column.

The Timestamp column reports the elapsed time in seconds since the start of the target JVM. In addition, the -gcoldcapacity output shows the old generation capacity (OGC) and the old space capacity (OC) increasing as the heap expands to meet allocation or promotion demands. The OGC has grown from 11696 KB to 13820 KB after the 81st full generation capacity (FGC). The maximum capacity of the generation (and space) is 60544 KB (OGCMX), so it still has room to expand.

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