如何檢查JVM正在使用的GC？

Question

我有服務器（oracle/sun）JVM正在運行，我該如何檢查它在運行時使用的GC？例如，如果它使用UseG1GC UseConcMarkSweepGC。 （jdk是1.7_79）

我知道我可以使用jinfo <pid>來檢查輸出中的VM標誌。但是，如果JVM在沒有這些參數的情況下啓動會怎樣。

我也無法在jvisualvm中找到此信息。

Answer 1

使用JMap。

jMap - 使用-heap選項啓動此工具並查找專用於垃圾收集器類型的字符串。像

$ jmap -heap 30166 
Attaching to process ID 30166, please wait... 
Debugger attached successfully. 
Server compiler detected. 
JVM version is 24.80-b11 

using parallel threads in the new generation. 
using thread-local object allocation. 
Concurrent Mark-Sweep GC 

Heap Configuration: 
    MinHeapFreeRatio = 40 
    MaxHeapFreeRatio = 70 
    MaxHeapSize  = 4294967296 (4096.0MB) 
    NewSize   = 1431633920 (1365.3125MB) 
    MaxNewSize  = 1431633920 (1365.3125MB) 
    OldSize   = 2863267840 (2730.625MB) 
    NewRatio   = 2 
    SurvivorRatio = 8 
    PermSize   = 21757952 (20.75MB) 
    MaxPermSize  = 85983232 (82.0MB) 
    G1HeapRegionSize = 0 (0.0MB) 

Heap Usage: 
New Generation (Eden + 1 Survivor Space): 
    capacity = 1288503296 (1228.8125MB) 
    used  = 339354840 (323.63399505615234MB) 
    free  = 949148456 (905.1785049438477MB) 
    26.337134026236903% used 
Eden Space: 
    capacity = 1145372672 (1092.3125MB) 
    used  = 313863536 (299.32359313964844MB) 
    free  = 831509136 (792.9889068603516MB) 
    27.402743550005006% used 
From Space: 
    capacity = 143130624 (136.5MB) 
    used  = 25491304 (24.310401916503906MB) 
    free  = 117639320 (112.1895980834961MB) 
    17.809818253849016% used 
To Space: 
    capacity = 143130624 (136.5MB) 
    used  = 0 (0.0MB) 
    free  = 143130624 (136.5MB) 
    0.0% used 
concurrent mark-sweep generation: 
    capacity = 2863333376 (2730.6875MB) 
    used  = 0 (0.0MB) 
    free  = 2863333376 (2730.6875MB) 
    0.0% used 
Perm Generation: 
    capacity = 23920640 (22.8125MB) 
    used  = 23801432 (22.698814392089844MB) 
    free  = 119208 (0.11368560791015625MB) 
    99.5016521297089% used 

10243 interned Strings occupying 814216 bytes.