-XX:+UseConcMarkSweepGC会指定CMS收集器+ParNew收集器+Serial Old收集器组合，优先使用ParNew收集器+CMS收集器的组合，当出现ConcurrentMode Fail或者Promotion Failed时，则采用ParNew收集器+Serial Old收集器的组合。日志如下：
Java HotSpot(TM) 64-Bit Server VM (25.131-b11) for windows-amd64 JRE (1.8.0_131-b11), built on Mar 15 2017 01:23:53 by "java_re" with MS VC++ 10.0 (VS2010) Memory: 4k page, physical 8303556k(2846816k free), swap 16215992k(7664596k free) CommandLine flags: -XX:InitialHeapSize=29360128 -XX:MaxHeapSize=29360128 -XX:MaxNewSize=14680064 -XX:MaxTenuringThreshold=6 -XX:OldPLABSize=16 -XX:+PrintGC -XX:+PrintGCDateStamps -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseConcMarkSweepGC -XX:-UseLargePagesIndividualAllocation -XX:+UseParNewGC 2018-05-09T20:53:14.086+0800: 0.590: [GC (Allocation Failure) 2018-08-09T11:53:14.086+0800: 0.590: [ParNew: 11520K->1407K(12928K), 0.0034803 secs] 11520K->2254K(27264K), 0.0039082 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 2018-05-09T20:53:14.247+0800: 0.751: [Full GC (System.gc()) 2018-08-09T11:53:14.247+0800: 0.751: [CMS: 846K->1930K(14336K), 0.0103698 secs] 7165K->1930K(27264K), [Metaspace: 5963K->5963K(1056768K)], 0.0104529 secs] [Times: user=0.02 sys=0.00, real=0.01 secs] 2018-05-09T20:53:14.292+0800: 0.795: [GC (Allocation Failure) 2018-08-09T11:53:14.292+0800: 0.795: [ParNew: 11519K->1199K(12928K), 0.0085679 secs] 13450K->3129K(27264K), 0.0086244 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 2018-05-09T20:53:14.333+0800: 0.836: [GC (Allocation Failure) 2018-08-09T11:53:14.333+0800: 0.836: [ParNew: 12719K->300K(12928K), 0.0002620 secs] 14649K->2230K(27264K), 0.0003041 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 2018-05-09T20:53:14.364+0800: 0.867: [GC (Allocation Failure) 2018-08-09T11:53:14.364+0800: 0.867: [ParNew: 11820K->75K(12928K), 0.0002787 secs] 13750K->2005K(27264K), 0.0003223 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] # 并发标记周期开始，根区域扫描 2018-05-09T20:59:47.982+0800: 9.634: [GC concurrent-root-region-scan-start] 2018-05-09T20:59:47.982+0800: 9.652: [GC concurrent-root-region-scan-end, 0.0184308 secs] # 并发标记 2018-05-09T20:59:47.982+0800: 9.652: [GC concurrent-mark-start] 2018-05-09T20:59:47.982+0800: 9.693: [GC concurrent-mark-end, 0.0406187 secs] # 重新标记 2018-05-09T20:59:47.982+0800: 9.695: [GC remark 9.695: [Finalize Marking, 0.0005100 secs] 9.695: [GC ref-proc, 0.0003461 secs] 9.696: [Unloading, 0.0069466 secs], 0.0082011 secs] [Times: user=0.02 sys=0.00, real=0.01 secs] # 独占清理 2018-05-09T20:59:47.982+0800: 9.703: [GC cleanup 25M->21M(1024M), 0.0027119 secs] [Times: user=0.00 sys=0.01, real=0.00 secs] # 并发清理 2018-05-09T20:59:47.982+0800: 9.706: [GC concurrent-cleanup-start] 2018-05-09T20:59:47.982+0800: 9.706: [GC concurrent-cleanup-end, 0.0000167 secs] 2018-05-09T20:54:39.299+0800: 85.803: [Full GC (System.gc()) 2018-08-09T11:54:39.299+0800: 85.803: [CMS: 1930K->1832K(14336K), 0.0089015 secs] 12748K->1832K(27264K), [Metaspace: 6035K->6035K(1056768K)], 0.0089724 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] Heap par new generation total 12928K, used 227K [0x00000000fe400000, 0x00000000ff200000, 0x00000000ff200000) eden space 11520K, 1% used [0x00000000fe400000, 0x00000000fe438cd8, 0x00000000fef40000) from space 1408K, 0% used [0x00000000fef40000, 0x00000000fef40000, 0x00000000ff0a0000) to space 1408K, 0% used [0x00000000ff0a0000, 0x00000000ff0a0000, 0x00000000ff200000) concurrent mark-sweep generation total 14336K, used 1832K [0x00000000ff200000, 0x0000000100000000, 0x0000000100000000) Metaspace used 6045K, capacity 6252K, committed 6400K, reserved 1056768K class space used 691K, capacity 761K, committed 768K, reserved 1048576K第三行把当前使用的JVM参数打印出来，其中，-XX:MaxTenuringThreshold=6是指对象从新生代晋升到老年代需要对象年龄达到6岁，即经过6次GC。
第四行是新生代Young区的GC，首先是GC发生的时间。然后是GC发生的原因GC (Allocation Failure)，对象分配失败。[ParNew: 11520K->1407K(12928K), 0.0034803 secs]表示新生代回收前是11520K，回收后是1407K，新生代总大小12928K，回收耗时0.0034803 secs。11520K->2254K(27264K), 0.0039082 secs表示回收前堆大小11520K，回收后堆大小2254K，堆的总大小27264K。