Viewing GC Information

Prerequisites

A real-time profiling task has been created and is in progress.

Procedure

1. In the Task/Report List area of the Java Profiler page, click the name of the analysis task under Real-Time Profiling.

   The Overview tab page is displayed.

2. Click the GC tab.
   View the GC event information. The GC analysis page is displayed by default. See Figure 1. For details about the parameters, see Table 1.

   

   • You can select GC Cause or Garbage Collector from the drop-down list and enter a keyword to quickly filter data.
   • You can click View Details in the upper part of the page to view the tuning suggestions for the current task.
   • You can click to export the current GC report.
   • Currently, Parallel Scavenge (PS), Concurrent Mark Sweep (CMS), and Garbage-First (G1) garbage collector logs can be parsed. Java 8 supports PS, CMS, and G1, Java 9 and later versions support only G1, and Java 17 and later versions support G1 and ZGC. For details, see Java Garbage Collector and Log Levels.
   Figure 1 GC analysis
   

   Table 1 Parameters on the GC analysis tab page

   Parameter	Description
   Timestamp	Time when a GC event occurred.
   GC Cause	Cause of GC.
   Garbage Collector	Name of the garbage collector that performs GC.
   Memory Applied for GC	Memory capacity allocated for GC.
   Memory Before GC	Memory capacity before GC.
   Memory After GC	Memory capacity after GC.
   GC Memory Size	Memory reclaimed by GC.
   GC Threads	Number of threads used during GC.
   Pause Time (ms)	Time when an application is paused due to GC.
   Collected GC Event Info
   GC Activities	Displays the ratio of the GC pause time to the unit time (1 second) in an area chart.
   Memory	Displays the memory for which the JVM applies, memory in use, and free memory in a stacked area chart.
   GC	Displays the memory reclaimed by GC in an area chart.
   Pause Time (ms)	Displays the application pause time caused by GC in an area chart.
   GC Threads	Displays the number of threads used during GC in an area chart.

3. Click the GC Logs tab.

   Click Collect GC Log to view the analysis of key GC metrics. See Figure 2. For details about the parameters, see Table 2.

   

   • During GC log collection, the collection progress and the Cancel Collection button are displayed. You can click Cancel Collection to cancel the collection event.
   • When you move the mouse pointer to the collection diagram, the detailed information for the corresponding time is displayed.
   • If GC log data is not obtained, enable the relevant log level. For details, see Java Garbage Collector and Log Levels.
   • Click to save the current GC log information. The same GC log information can be saved only once. You can view or download the GC log information in the GC log list on the main page.
   Figure 2 Analysis of key GC metrics
   

   Table 2 Parameters on the key GC metrics

   Parameter	Description
   GC throughput	Total duration except for GC.
   GC overhead	Percentage of resources occupied by GC.
   Linearity	CPU multi-core usage. The calculation formula is as follows: (User time + System time)/Actual time
   Average GC pause	Average GC pause duration.
   Maximum pause	Maximum pause duration of a process in GC.
   GC Pauses	Overall GC pause statistics.
   GC Linearity	Real-time linearity change within a collection period.

   Select GC Causes from the Display drop-down list to view the GC cause analysis data. See Figure 3.

   Figure 3 GC causes
   

   Select Detailed Analysis of GC Activities from the Display drop-down list to view the detailed GC statistics. See Figure 4. For details about the parameters, see Table 3.

   Figure 4 Detailed analysis of GC activities
   

   Table 3 Parameters of detailed GC activity analysis

   Parameter	Description
   G1 Collection Phase Statistics
   Total GC Time	Total duration of all GC tasks.
   Average GC Time	Average duration of GC for each category.
   Average Time Standard Deviation	Standard deviation of the average GC time.
   Minimum/Maximum Time	Minimum and maximum GC durations.
   Average Interval	Average interval between any two GC tasks.
   GC Pause & Memory Usage Change
   GC Pause Duration	GC pause duration of each mark.
   Heap Usage	Heap memory usage before and after GC.
   Young Zone Usage	Usage of the young generation before and after GC. This data is available only when the CMS collector is used.
   Old Zone Usage	Usage of the old generation before and after GC.
   Eden Zone Usage	Usage of the eden space before and after GC.
   Survivor Zone Usage	Usage of the survivor space before and after GC.
   Metadata Zone Usage	Usage of the metaspace before and after GC.