之前从jstat -gcuitl改到jstat -gccause ， gccause比gcutil多出来一个last gc reanson，可以看到上一次的gc由什么原因触发。但是实际在服务器上jdk6u45上使用时，新生代的结果全是 Unknown GCCause，导致我很困惑。
一度怀疑是jvm参数禁用了s区导致，所以动手实验了一下。发现s区正常的情况下，LGCC依然打印的是Unknown GCCause.
./jstat -gccause 15628 1000 1000
S0 S1 E O P YGC YGCT FGC FGCT GCT LGCC GCC
0.00 0.00 4.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 4.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 4.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 4.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 4.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 4.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 4.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 4.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 4.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 4.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 4.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 4.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 4.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 41.70 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 41.70 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 41.70 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 41.70 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 41.70 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 79.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 79.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 79.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 79.20 0.00 14.45 0 0.000 0 0.000 0.000 No GC No GC
0.00 19.56 37.50 30.01 14.45 1 0.005 0 0.000 0.005 unknown GCCause No GC
0.00 19.56 39.50 30.01 14.47 1 0.005 0 0.000 0.005 unknown GCCause No GC
0.00 19.56 39.50 30.01 14.47 1 0.005 0 0.000 0.005 unknown GCCause No GC
0.00 19.56 39.50 30.01 14.47 1 0.005 0 0.000 0.005 unknown GCCause No GC
0.00 19.56 39.50 30.01 14.47 1 0.005 0 0.000 0.005 unknown GCCause No GC
0.00 19.56 39.50 30.01 14.47 1 0.005 0 0.000 0.005 unknown GCCause No GC
0.00 19.56 78.05 30.01 14.47 1 0.005 0 0.000 0.005 unknown GCCause No GC
0.00 19.56 78.05 30.01 14.47 1 0.005 0 0.000 0.005 unknown GCCause No GC
0.00 19.56 78.05 30.01 14.47 1 0.005 0 0.000 0.005 unknown GCCause No GC
0.00 19.56 78.05 30.01 14.47 1 0.005 0 0.000 0.005 unknown GCCause No GC
0.00 19.56 78.05 30.01 14.47 2 0.005 0 0.000 0.009 unknown GCCause No GC
18.13 0.00 37.50 60.02 14.47 2 0.009 1 0.000 0.009 CMS Initial Mark No GC
18.13 0.00 37.50 60.02 14.47 2 0.009 1 0.000 0.009 CMS Initial Mark No GC
18.13 0.00 37.50 60.02 14.47 2 0.009 1 0.000 0.009 CMS Initial Mark No GC
18.13 0.00 37.50 60.02 14.47 2 0.009 1 0.000 0.009 CMS Initial Mark No GC
18.13 0.00 37.50 60.02 14.47 2 0.009 1 0.000 0.009 CMS Initial Mark No GC
18.13 0.00 75.69 60.02 14.47 2 0.009 2 0.001 0.010 CMS Final Remark No GC
18.13 0.00 75.69 60.02 14.47 2 0.009 2 0.001 0.010 CMS Final Remark No GC
18.13 0.00 75.69 60.02 14.47 2 0.009 3 0.001 0.010 CMS Initial Mark No GC
18.13 0.00 75.69 60.02 14.47 2 0.009 3 0.001 0.010 CMS Initial Mark No GC
18.13 0.00 75.69 90.03 14.47 3 0.013 3 0.001 0.014 CMS Final Remark No GC
0.00 19.83 37.50 90.03 14.47 3 0.013 4 0.001 0.014 CMS Final Remark No GC
0.00 19.83 37.50 90.03 14.47 3 0.013 5 0.002 0.015 CMS Initial Mark No GC
0.00 19.83 37.50 90.03 14.47 3 0.013 6 0.002 0.015 CMS Final Remark No GC
0.00 19.83 37.50 90.03 14.47 3 0.013 6 0.002 0.015 CMS Final Remark No GC
0.00 19.83 37.50 90.03 14.47 3 0.013 8 0.003 0.016 CMS Final Remark No GC
0.00 19.83 37.50 90.03 14.47 3 0.013 8 0.003 0.016 CMS Final Remark No GC
0.00 19.83 37.50 90.03 14.47 3 0.013 10 0.004 0.017 CMS Final Remark No GC
0.00 19.83 37.50 90.03 14.47 3 0.013 10 0.004 0.017 CMS Final Remark No GC
0.00 19.83 37.50 90.03 14.47 3 0.013 12 0.004 0.017 CMS Final Remark No GC
0.00 19.83 37.50 90.03 14.47 3 0.013 12 0.004 0.017 CMS Final Remark No GC
0.00 19.83 75.46 0.03 14.47 3 0.013 14 0.005 0.018 CMS Final Remark No GC
0.00 19.83 75.46 0.03 14.47 3 0.013 14 0.005 0.018 CMS Final Remark No GC
0.00 19.83 75.46 0.03 14.47 3 0.013 14 0.005 0.018 CMS Final Remark No GC
0.00 19.83 75.46 0.03 14.47 3 0.013 14 0.005 0.018 CMS Final Remark No GC
19.79 0.00 0.00 15.04 14.47 4 0.014 14 0.005 0.019 unknown GCCause No GC
19.79 0.00 37.50 15.04 14.47 4 0.014 14 0.005 0.019 unknown GCCause No GC
19.79 0.00 37.50 15.04 14.47 4 0.014 14 0.005 0.019 unknown GCCause No GC
19.79 0.00 37.50 15.04 14.47 4 0.014 14 0.005 0.019 unknown GCCause No GC
19.79 0.00 37.50 15.04 14.47 4 0.014 14 0.005 0.019 unknown GCCause No GC
19.79 0.00 39.50 15.04 14.51 4 0.014 14 0.005 0.019 unknown GCCause No GC
fgc的次数是我在sleep导致的，不清楚算不算jstat的另外一个bug
搜索了一下，发现果然是一个bug.JDK-7015169 : GC Cause not always set
这个bug的描述是这样的
The GC Cause is not always set up. This can be verified by running:
jstat -gccause <PID> 100
"unknown GCCause" is often listed in the LGCC column.
这个bug大概在 2011-02-11 修补，具体的修改在这里可以看到，之后的版本就没有了这个问题。
切换到jdk8，把相同的case跑了一下，果然就没问题了。
$ jstat -gccause 8964 1000 1000
S0 S1 E O M CCS YGC YGCT FGC FGCT GCT LGCC GCC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 12.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 49.51 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 49.51 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 49.51 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 49.51 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 49.51 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 87.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 87.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 87.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 87.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 0.00 87.01 0.00 17.18 19.75 0 0.000 0 0.000 0.000 No GC No GC
0.00 55.55 37.50 30.01 55.17 57.22 1 0.005 0 0.000 0.005 Allocation Failure No GC
0.00 55.55 37.50 30.01 55.17 57.22 1 0.005 0 0.000 0.005 Allocation Failure No GC
0.00 55.55 37.50 30.01 55.17 57.22 1 0.005 0 0.000 0.005 Allocation Failure No GC
0.00 55.55 37.50 30.01 55.17 57.22 1 0.005 0 0.000 0.005 Allocation Failure No GC
0.00 55.55 37.50 30.01 55.17 57.22 1 0.005 0 0.000 0.005 Allocation Failure No GC
0.00 55.55 76.95 30.01 55.17 57.22 1 0.005 0 0.000 0.005 Allocation Failure No GC
0.00 55.55 76.95 30.01 55.17 57.22 1 0.005 0 0.000 0.005 Allocation Failure No GC
0.00 55.55 76.95 30.01 55.17 57.22 1 0.005 0 0.000 0.005 Allocation Failure No GC
0.00 55.55 76.95 30.01 55.17 57.22 1 0.005 0 0.000 0.005 Allocation Failure No GC
0.00 55.55 76.95 30.01 55.17 57.22 1 0.005 0 0.000 0.005 Allocation Failure No GC
0.00 0.00 37.50 62.82 55.17 57.22 2 0.011 1 0.000 0.011 CMS Initial Mark No GC
0.00 0.00 37.50 62.82 55.17 57.22 2 0.011 1 0.000 0.011 CMS Initial Mark No GC
0.00 0.00 37.50 62.82 55.17 57.22 2 0.011 1 0.000 0.011 CMS Initial Mark No GC
0.00 0.00 37.50 62.82 55.17 57.22 2 0.011 1 0.000 0.011 CMS Initial Mark No GC
0.00 0.00 37.50 62.82 55.17 57.22 2 0.011 1 0.000 0.011 CMS Initial Mark No GC
0.00 0.00 76.62 62.76 55.17 57.22 2 0.011 2 0.002 0.013 CMS Final Remark No GC
0.00 0.00 76.62 62.76 55.17 57.22 2 0.011 2 0.002 0.013 CMS Final Remark No GC
0.00 0.00 76.62 62.76 55.17 57.22 2 0.011 3 0.002 0.013 CMS Initial Mark No GC
0.00 0.00 76.62 62.76 55.17 57.22 2 0.011 3 0.002 0.013 CMS Initial Mark No GC
0.00 0.00 76.62 62.76 55.17 57.22 2 0.011 3 0.002 0.013 CMS Initial Mark No GC
0.00 0.00 37.50 92.76 55.17 57.22 3 0.015 4 0.004 0.020 CMS Final Remark No GC
0.00 0.00 37.50 92.76 55.17 57.22 3 0.015 4 0.004 0.020 CMS Final Remark No GC
0.00 0.00 37.50 92.76 55.17 57.22 3 0.015 6 0.006 0.021 CMS Final Remark No GC
0.00 0.00 37.50 92.76 55.17 57.22 3 0.015 6 0.006 0.021 CMS Final Remark No GC
0.00 0.00 37.50 92.76 55.17 57.22 3 0.015 8 0.008 0.023 CMS Final Remark No GC
0.00 0.00 37.50 92.54 55.18 57.22 3 0.015 11 0.017 0.033 CMS Final Remark No GC
0.00 0.00 37.50 92.54 55.18 57.22 3 0.015 11 0.017 0.033 CMS Final Remark No GC
0.00 0.00 37.50 92.54 55.18 57.22 3 0.015 13 0.020 0.035 CMS Final Remark No GC
0.00 0.00 37.50 92.54 55.18 57.22 3 0.015 13 0.020 0.035 CMS Final Remark No GC
0.00 0.00 37.50 92.54 55.18 57.22 3 0.015 15 0.021 0.036 CMS Final Remark No GC
0.00 0.00 76.74 92.54 55.18 57.22 3 0.015 15 0.021 0.036 CMS Final Remark No GC
0.00 0.00 76.74 2.54 55.18 57.22 3 0.015 17 0.023 0.038 CMS Final Remark No GC
0.00 0.00 76.74 2.54 55.18 57.22 3 0.015 17 0.023 0.038 CMS Final Remark No GC
0.00 0.00 76.74 2.54 55.18 57.22 3 0.015 17 0.023 0.038 CMS Final Remark No GC
0.00 0.00 76.74 2.54 55.18 57.22 3 0.015 17 0.023 0.038 CMS Final Remark No GC
0.00 0.00 37.50 17.55 55.18 57.22 4 0.017 17 0.023 0.040 Allocation Failure No GC
0.00 0.00 37.50 17.55 55.18 57.22 4 0.017 17 0.023 0.040 Allocation Failure No GC
0.00 0.00 37.50 17.55 55.18 57.22 4 0.017 17 0.023 0.040 Allocation Failure No GC
0.00 0.00 37.50 17.55 55.18 57.22 4 0.017 17 0.023 0.040 Allocation Failure No GC
0.00 0.00 37.50 17.55 55.18 57.22 4 0.017 17 0.023 0.040 Allocation Failure No GC
再贴一个源码的可能的gccause，这个代码是7u40版本的，因为手头正好没有其他版本源码了：
const char* GCCause::to_string(GCCause::Cause cause) {
switch (cause) {
case _java_lang_system_gc:
return "System.gc()";
case _full_gc_alot:
return "FullGCAlot";
case _scavenge_alot:
return "ScavengeAlot";
case _allocation_profiler:
return "Allocation Profiler";
case _jvmti_force_gc:
return "JvmtiEnv ForceGarbageCollection";
case _gc_locker:
return "GCLocker Initiated GC";
case _heap_inspection:
return "Heap Inspection Initiated GC";
case _heap_dump:
return "Heap Dump Initiated GC";
case _no_gc:
return "No GC";
case _allocation_failure:
return "Allocation Failure";
case _tenured_generation_full:
return "Tenured Generation Full";
case _permanent_generation_full:
return "Permanent Generation Full";
case _cms_generation_full:
return "CMS Generation Full";
case _cms_initial_mark:
return "CMS Initial Mark";
case _cms_final_remark:
return "CMS Final Remark";
case _cms_concurrent_mark:
return "CMS Concurrent Mark";
case _old_generation_expanded_on_last_scavenge:
return "Old Generation Expanded On Last Scavenge";
case _old_generation_too_full_to_scavenge:
return "Old Generation Too Full To Scavenge";
case _adaptive_size_policy:
return "Ergonomics";
case _g1_inc_collection_pause:
return "G1 Evacuation Pause";
case _g1_humongous_allocation:
return "G1 Humongous Allocation";
case _last_ditch_collection:
return "Last ditch collection";
case _last_gc_cause:
return "ILLEGAL VALUE - last gc cause - ILLEGAL VALUE";
default:
return "unknown GCCause";
}
ShouldNotReachHere();
}