jcmd VM.set_flag, which flags are writable?

Question

I was trying to experiment with jcmd VM.set_flag option. But came across one error saying "only 'writeable' flags can be set". What are writable flags ?

Getting my pid:

XXX@XXX-Air:~/javacode$ jcmd -l
6294 Test
6295 jdk.jcmd/sun.tools.jcmd.JCmd -l

Trying to change vm flags:

XXX@XXX-Air:~/javacode$ jcmd 6294 VM.set_flag ConcGCThreads 4
6294:
only 'writeable' flags can be set
XXX@XXX-Air:~/javacode$ jcmd 6294 VM.set_flag MaxNewSize 1G
6294:
only 'writeable' flags can be set

Edit: It worked for manageable flags, below are successful commands.

 XXXX@XXX-Air:~/javacode$ jcmd 11441 VM.flags -all | grep MinHeapFreeRatio
    uintx MinHeapFreeRatio                         = 40                                    {manageable} {default}
XXXX@XXX-Air:~/javacode$ jcmd 11441 VM.set_flag MinHeapFreeRatio 45
11441:
Command executed successfully
XXXX@XXX-Air:~/javacode$ jcmd 11441 VM.flags -all | grep MinHeapFreeRatio
    uintx MinHeapFreeRatio                         = 45    

Answer 1

Writable flags are labeled as {manageable}.

You can list all flags with jcmd 12345 VM.flags -all. You can then grep for manageable ones (the is on my Oracle jdk8 VM) :

$ jcmd 12345 VM.flags -all | grep manageable
     intx CMSAbortablePrecleanWaitMillis            = 100                                 {manageable}
     intx CMSTriggerInterval                        = -1                                  {manageable}
     intx CMSWaitDuration                           = 2000                                {manageable}
     bool HeapDumpAfterFullGC                       = false                               {manageable}
     bool HeapDumpBeforeFullGC                      = false                               {manageable}
     bool HeapDumpOnOutOfMemoryError                = false                               {manageable}
    ccstr HeapDumpPath                              =                                     {manageable}
    uintx MaxHeapFreeRatio                          = 100                                 {manageable}
    uintx MinHeapFreeRatio                          = 0                                   {manageable}
     bool PrintClassHistogram                       = false                               {manageable}
     bool PrintClassHistogramAfterFullGC            = false                               {manageable}
     bool PrintClassHistogramBeforeFullGC           = false                               {manageable}
     bool PrintConcurrentLocks                      = false                               {manageable}
     bool PrintGC                                   = false                               {manageable}
     bool PrintGCDateStamps                         = false                               {manageable}
     bool PrintGCDetails                            = false                               {manageable}
     bool PrintGCID                                 = false                               {manageable}
     bool PrintGCTimeStamps                         = false                               {manageable}