Re: RFR: 8224479: New diagnostic command: VM.get_flag

[Stefan Karlsson]

Hi Thomas,

On 2019-05-21 12:37, Thomas Stüfe wrote:
I think this is useful. 

Thanks.

I have a vague preference for reusing VM.flags
though - giving it the option to only print one flag - instead of adding 
a new command.

So, we would have three different modes for VM.flags:
$ jcmd HelloSleep VM.flags
371:
-XX:CICompilerCount=15 -XX:ConcGCThreads=6 
-XX:G1ConcRefinementThreads=23 -XX:G1HeapRegionSize=4194304 
-XX:GCDrainStackTargetSize=64 -XX:InitialHeapSize=2113929216 
-XX:MarkStackSize=4194304 -XX:MaxHeapSize=32178700288 
-XX:MaxNewSize=19306381312 -XX:MinHeapDeltaBytes=4194304 
-XX:NonNMethodCodeHeapSize=8182140 -XX:NonProfiledCodeHeapSize=121738050 
-XX:ProfiledCodeHeapSize=121738050 -XX:ReservedCodeCacheSize=251658240 
-XX:+SegmentedCodeCache -XX:+UseCompressedClassPointers 
-XX:+UseCompressedOops -XX:+UseFastUnorderedTimeStamps -XX:+UseG1GC

$ /localhome/java/jdk9/bin/jcmd HelloSleep VM.flags -all
371:
[Global flags]
ccstrlist AOTLibrary                               = 
                      {product} {default}
      int ActiveProcessorCount                     = -1 
                       {product} {default}
    uintx AdaptiveSizeDecrementScaleFactor         = 4 
                       {product} {default}
    uintx AdaptiveSizeMajorGCDecayTimeScale        = 10 
                       {product} {default}
    uintx AdaptiveSizePolicyCollectionCostMargin   = 50 
                       {product} {default}

and a new mode:
$ /localhome/java/jdk9/bin/jcmd HelloSleep VM.flags -name=UseSerialGC
371:
     bool UseSerialGC                              = false 
                       {product} {default}

Let's see if anyone else has some feedback regarding this.

Thanks,
StefanK


Just my 5c

.. Thomas

On Tue, May 21, 2019, 12:14 Stefan Karlsson <stefan.karls...@oracle.com 
<mailto:stefan.karls...@oracle.com>> wrote:

    Hi all,

    Please review this patch to introduce a new diagnostic command:
    VM.get_flag.

    http://cr.openjdk.java.net/~stefank/8224479/webrev.01/
    https://bugs.openjdk.java.net/browse/JDK-8224479

    Today we have:

    - VM.set_flag - which allows the user to set a manageable flag
    - VM.flags - which allows the user to print all set flags or print
    similar output as -XX:+PrintFlagsFinal

    I propose that we add a new command to print the value of one flag.

    Output from help:
    ==========
    $ jcmd HelloSleep help VM.get_flag
    1060:
    VM.get_flag
    Prints the value of a VM flag option.

    Impact: Low

    Permission: java.lang.management.ManagementPermission(monitor)

    Syntax : VM.get_flag  <flag name>

    Arguments:
             flag name :  The name of the flag we want to get (STRING,
    no default value)
    ==========

    Output from usage:
    ==========
    $ jcmd HelloSleep VM.get_flag UseSerialGC
    1060:
    false
    ==========

    I'll create a CSR if others also thinks this is a worthwhile feature.

    Thanks,
    StefanK