-XX:+TraceClassUnloading enabled w/ -Xloggc ?

[Charlie Hunt]

Hi Bengt,

I also just noticed Bernd's reply as I was drafting this note to you.

It appears there was a change made possibly after the Java 7 fork.  I checked 6u33, (which isn't too old), and got:
$ /home/chunt/jdks/jdk1.6.0_33/bin/java -Xloggc:/tmp/gc.log -XX:+PrintFlagsFinal -version | grep TraceClassUnloading
     bool TraceClassUnloading                      := true            {product rw}
java version "1.6.0_33"
Java(TM) SE Runtime Environment (build 1.6.0_33-b03)
Java HotSpot(TM) 64-Bit Server VM (build 20.8-b03, mixed mode)

I didn't check any Java 6u* after 33.  But, the Java 7 versions I checked shows it disabled with -Xloggc, as you showed with what looks to be a fairly recent HotSpot, or from built directly from hotspot-main's repository.

Perhaps someone had already noticed this peculiarity previously and the change was integrated into HotSpot after the fork for Java 7?  Or, if it's in a 6u* after 6u33, then perhaps it got back ported to Java 6 since 6u33 came after the Java 7 fork.

Ooh, before I forget ... yeah, I noticed +PrintGC and +PrintGCDetails were auto-enabled with -Xloggc.  My question, which I may not have made very clear, was if -Xloggc was auto-enabling +TraceClassUnloading, then why wouldn't it be auto-enabled with +PrintGC or +PrintGCDetails.  That's a mute point now since a change has been made to to leave TraceClassUnloading disabled with -Xloggc. ;-)

Thanks (both you and Bernd) for the additional info and forensics.

charlie ...

On Oct 31, 2012, at 10:41 AM, Bengt Rutisson wrote:


Charlie,

I don't see any code that enables TraceClassUnloading when -Xloggc is
used. Are you sure that this happens?

When I run it also doesn't look like it gets enabled.

java -Xloggc:log.txt -XX:+PrintFlagsFinal -version | grep
TraceClassUnloading
     bool TraceClassUnloading                       = false
{product rw}
java version "1.8.0-ea"
Java(TM) SE Runtime Environment (build 1.8.0-ea-b60)
Java HotSpot(TM) Client VM (build 25.0-b04, mixed mode)

We do enable PrintGC and PrintGCDetails when -Xloggc is enabled:

    if (match_option(option, "-Xloggc:", &tail)) {
      // Redirect GC output to the file. -Xloggc:<filename>
      // ostream_init_log(), when called will use this filename
      // to initialize a fileStream.
      _gc_log_filename = strdup(tail);
      FLAG_SET_CMDLINE(bool, PrintGC, true);
      FLAG_SET_CMDLINE(bool, PrintGCTimeStamps, true);

    // JNI hooks
    }

Bengt


On 2012-10-29 16:06, Charlie Hunt wrote:
We stumbled across this last week ... and now curious.

With -Xloggc, -XX:+TraceClassUnloading is also enabled.  But, with other GC related command line options such as; -XX:+PrintGCDetails, -XX:+PrintGC, -XX:+PrintHeapAtGC, etc. -XX:-TraceClassUnloading remains disabled by default.

Anyone know the history that led to the decision to only enable +TraceClassUnloading with -Xloggc, and not with say +PrintGCDetails?

thanks,

charlie ...


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