JVM Command Line Flags

-XX:+AlwaysPreTouch
-XX:CompressedClassSpaceSize=528482304
-XX:ErrorFile=/app/logs/my-biz/hs_err_%p.log
-XX:GCLogFileSize=1048576000
-XX:+HeapDumpBeforeFullGC # Added for test environment
-XX:+HeapDumpOnOutOfMemoryError
-XX:HeapDumpPath=/app/logs/my-biz
-XX:InitialHeapSize=17179869184
-XX:MaxHeapSize=17179869184
-XX:MaxMetaspaceSize=536870912
-XX:MetaspaceSize=268435456
-XX:NewRatio=1
-XX:NumberOfGCLogFiles=5
-XX:-OmitStackTraceInFastThrow
-XX:+PrintCommandLineFlags
-XX:+PrintFlagsFinal
-XX:+PrintGC
-XX:+PrintGCApplicationStoppedTime
-XX:+PrintGCDateStamps
-XX:+PrintGCDetails
-XX:+PrintGCTimeStamps
-XX:+PrintHeapAtGC
-XX:+UnlockDiagnosticVMOptions
-XX:+UseCompressedClassPointers
-XX:+UseCompressedOops
-XX:+UseGCLogFileRotation
-XX:+UseParallelGC # Uses Java 8 default Parallel GC, not CMS + ParNew
-XX:-UseAdaptiveSizePolicy # This parameter was not enabled

-XX:+AlwaysPreTouch

This JVM parameter instructs the JVM to pre-allocate and touch all pages of the Java heap memory during startup to lock them into physical memory. This eliminates potential runtime delays caused by on-demand memory allocation:

1. Pre-allocated Heap: The JVM reserves and locks the entire configured heap size on startup, so no dynamic heap allocation occurs during application runtime unless the heap is explicitly expanded.
2. Avoid Allocation Latency: Dynamic memory allocation at runtime can trigger OS virtual memory paging if physical memory is constrained, leading to significant performance overhead. Pre-touching avoids this.
3. Reduce GC Pauses: On-demand memory allocation can sometimes increase GC pause times; pre-allocating heap memory minimizes this impact.

Sample Parallel GC Log Analysis

Below are real-world Parallel GC logs from a running application, with annotations:

2024-02-20T17:30:45.436+0800: 21056.921: Total time for which application threads were stopped: 0.0039479 seconds, Stopping threads took: 0.0004651 seconds

# 1097th GC (GCLocker Initiated GC, no full GC occurred yet)
{Heap before GC invocations=1097 (full 0):
 PSYoungGen      total 6738432K, used 6732547K [0x00000005e0800000, 0x00000007e0800000, 0x00000007e0800000)
  eden space 6044672K, 99% used [0x00000005e0800000,0x0000000751142610,0x0000000751700000)
  from space 693760K, 99% used [0x00000007b6280000,0x00000007e07fe630,0x00000007e0800000)
  to   space 1171968K, 0% used [0x0000000751700000,0x0000000751700000,0x0000000798f80000)
 ParOldGen       total 8388608K, used 4992341K [0x00000003e0800000, 0x00000005e0800000, 0x00000005e0800000)
  object space 8388608K, 59% used [0x00000003e0800000,0x00000005113556b0,0x00000005e0800000)
 Metaspace       used 139136K, capacity 151259K, committed 151424K, reserved 1183744K
  class space    used 15678K, capacity 17502K, committed 17536K, reserved 1048576K
2024-02-20T17:30:45.459+0800: 21056.945: [GC (GCLocker Initiated GC) [PSYoungGen: 6732547K->1171955K(7216640K)] 11724888K->7372510K(15605248K), 0.2131267 secs] [Times: user=6.32 sys=1.22, real=0.22 secs] 
Heap after GC invocations=1097 (full 0):
 PSYoungGen      total 7216640K, used 1171955K [0x00000005e0800000, 0x00000007e0800000, 0x00000007e0800000)
  eden space 6044672K, 0% used [0x00000005e0800000,0x00000005e0800000,0x0000000751700000)
  from space 1171968K, 99% used [0x0000000751700000,0x0000000798f7cf40,0x0000000798f80000)
  to   space 1171968K, 0% used [0x0000000798f80000,0x0000000798f80000,0x00000007e0800000)
 ParOldGen       total 8388608K, used 6200554K [0x00000003e0800000, 0x00000005e0800000, 0x00000005e0800000)
  object space 8388608K, 73% used [0x00000003e0800000,0x000000055af3a950,0x00000005e0800000)
 Metaspace       used 139136K, capacity 151259K, committed 151424K, reserved 1183744K
  class space    used 15678K, capacity 17502K, committed 17536K, reserved 1048576K
}

2024-02-20T17:30:45.673+0800: 21057.158: Total time for which application threads were stopped: 0.2279374 seconds, Stopping threads took: 0.0038452 seconds
2024-02-20T17:30:46.057+0800: 21057.542: Total time for which application threads were stopped: 0.0123185 seconds, Stopping threads took: 0.0025314 seconds
# 1098th GC (Allocation Failure, no full GC occurred yet)
# Note: Young generation size changed because -XX:-UseAdaptiveSizePolicy was not enabled
{Heap before GC invocations=1098 (full 0):
 PSYoungGen      total 7216640K, used 7216627K [0x00000005e0800000, 0x00000007e0800000, 0x00000007e0800000)
  eden space 6044672K, 100% used [0x00000005e0800000,0x0000000751700000,0x0000000751700000)
  from space 1171968K, 99% used [0x0000000751700000,0x0000000798f7cf40,0x0000000798f80000)
  to   space 1171968K, 0% used [0x0000000798f80000,0x0000000798f80000,0x00000007e0800000)
 ParOldGen       total 8388608K, used 6440555K [0x00000003e0800000, 0x00000005e0800000, 0x00000005e0800000)
  object space 8388608K, 76% used [0x00000003e0800000,0x000000056999afd8,0x00000005e0800000)
 Metaspace       used 139141K, capacity 151323K, committed 151424K, reserved 1183744K
  class space    used 15678K, capacity 17502K, committed 17536K, reserved 1048576K
2024-02-20T17:30:46.712+0800: 21058.197: [GC (Allocation Failure) [PSYoungGen: 7216627K->1171953K(4494848K)] 13657183K->9195030K(12883456K), 0.1764428 secs] [Times: user=3.97 sys=2.57, real=0.17 secs] 
Heap after GC invocations=1098 (full 0):
 PSYoungGen      total 4494848K, used 1171953K [0x00000005e0800000, 0x00000007e0800000, 0x00000007e0800000)
  eden space 3322880K, 0% used [0x00000005e0800000,0x00000005e0800000,0x00000006ab500000)
  from space 1171968K, 99% used [0x0000000798f80000,0x00000007e07fc750,0x00000007e0800000)
  to   space 2532864K, 0% used [0x00000006ab500000,0x00000006ab500000,0x0000000745e80000)
 ParOldGen       total 8388608K, used 8023076K [0x00000003e0800000,0x00000005e0800000,0x00000005e0800000)
  object space 8388608K, 95% used [0x00000003e0800000,0x00000005ca309368,0x00000005e0800000)
 Metaspace       used 139141K, capacity 151323K, committed 151424K, reserved 1183744K
  class space    used 15678K, capacity 17502K, committed 17536K, reserved 1048576K
}

# 1099th GC: Full GC (Ergonomics), took ~690ms
2024-02-20T17:30:46.888+0800: 21058.374: [Heap Dump (before full gc): , 0.0000775 secs]
{Heap before GC invocations=1099 (full 1):
 PSYoungGen      total 4494848K, used 1171953K [0x00000005e0800000, 0x00000007e0800000, 0x00000007e0800000)
  eden space 3322880K, 0% used [0x00000005e0800000,0x00000005e0800000,0x00000006ab500000)
  from space 1171968K, 99% used [0x0000000798f80000,0x00000007e07fc750,0x00000007e0800000)
  to   space 2532864K, 0% used [0x00000006ab500000,0x00000006ab500000,0x0000000745e80000)
 ParOldGen       total 8388608K, used 8023076K [0x00000003e0800000, 0x00000005e0800000, 0x00000005e0800000)
  object space 8388608K, 95% used [0x00000003e0800000,0x00000005ca309368,0x00000005e0800000)
 Metaspace       used 139141K, capacity 151323K, committed 151424K, reserved 1183744K
  class space    used 15678K, capacity 17502K, committed 17536K, reserved 1048576K
2024-02-20T17:30:46.889+0800: 21058.374: [Full GC (Ergonomics) [PSYoungGen: 1171953K->0K(4494848K)] [ParOldGen: 8023076K->2460629K(8388608K)] 9195030K->2460629K(12883456K), [Metaspace: 139141K->137506K(1183744K)], 0.6823041 secs] [Times: user=4.57 sys=6.16, real=0.69 secs] 
Heap after GC invocations=1099 (full 1):
 PSYoungGen      total 4494848K, used 0K [0x00000005e0800000, 0x00000007e0800000, 0x00000007e0800000)
  eden space 3322880K, 0% used [0x00000005e0800000,0x00000005e0800000,0x00000006ab500000)
  from space 1171968K, 0% used [0x0000000798f80000,0x0000000798f80000,0x00000007e0800000)
  to   space 2532864K, 0% used [0x00000006ab500000,0x00000006ab500000,0x0000000745e80000)
 ParOldGen       total 8388608K, used 2460629K [0x00000003e0800000, 0x00000005e0800000, 0x00000005e0800000)
  object space 8388608K, 29% used [0x00000003e0800000,0x0000000476af55e8,0x00000005e0800000)
 Metaspace       used 137506K, capacity 148613K, committed 151424K, reserved 1183744K
  class space    used 15423K, capacity 17043K, committed 17536K, reserved 1048576K
}

Note: Some monitoring tools such as Prometheus may label Full GC (Ergonomics) events as Major GC (Ergonomics).

Terminology Discrepancy Explanation

Many monitoring tools use "full GC" and "major GC" interchangeably, as both refer to garbage collection events that clean the entire heap including both young and old generations. These events trigger when the JVM determines a full heap cleanup is needed, due to high heap occupancy, memory fragmentation, or explicit GC requests.

Terminology differences stem from varying conventions across tooling and JVM documentation, but both terms describe the same performance-impacting heap-wide colletcion event.

Additional Parallel GC Notes

Parallel GC is the default garbage collector for Java 8, distinct from the CMS + ParNew collector pair. Further analysis of Parallel GC bheavior and tuning remains a topic for additional exploration.