Hi

We are using WAS 8.5.5.7 and JAVA 7.0.4.1

Verbose garbage collection has been enabled.
The following Generic JVM arguments which relates to GC has been added as per application vendor recommendation.
-XX:+UseParallelOldGC -XX:+UseAdaptiveSizePolicy -Xnoclassgc -Djava.nio.channels.spe.SelectorProvider=sun.nio.ch.DevPollSelectorProvider -XX:+UseSpinning -XX:+HeapDumpOnOutOfMemoryError -XX:+PrintGCTimeStamps -XX:+DisableExplicitGC -XX:+PrintGCDetails -XX:+PrintHeapAtGC -XX:+PrintGCDateStamps -XX:NewSize=1024m -XX:MaxNewSize=2048m

1. As Verbose garbage collection is enabled, I expect logging on stderr log, but this is not happening.  Can this be caused by additional generic arguments gc arguments? If yes.. which argument? Xnoclassgc
2. I have read Xnoclassgc is not recommended as it can cause OOM issues.  When is it advisable to use it?

Regards,
Sandile

------------------------------
Mziwandile Sithebe
IBM
------------------------------

Hi

Just discovered that WAS is running on Solaries, therefore the gc info in written on stdout.log not stderr.log.

Now my question is what is the effect of -XX:+DisableExplicitGC generic argument?
Does this completely disable any garbage collection?

Regards

------------------------------
Mziwandile Sithebe
IBM
------------------------------

Original Message:
Sent: 11-12-2018 06:58
From: Mziwandile Sithebe
Subject: GC Policy -Xnoclassgc

Hi

We are using WAS 8.5.5.7 and JAVA 7.0.4.1

Verbose garbage collection has been enabled.
The following Generic JVM arguments which relates to GC has been added as per application vendor recommendation.
-XX:+UseParallelOldGC -XX:+UseAdaptiveSizePolicy -Xnoclassgc -Djava.nio.channels.spe.SelectorProvider=sun.nio.ch.DevPollSelectorProvider -XX:+UseSpinning -XX:+HeapDumpOnOutOfMemoryError -XX:+PrintGCTimeStamps -XX:+DisableExplicitGC -XX:+PrintGCDetails -XX:+PrintHeapAtGC -XX:+PrintGCDateStamps -XX:NewSize=1024m -XX:MaxNewSize=2048m

1. As Verbose garbage collection is enabled, I expect logging on stderr log, but this is not happening.  Can this be caused by additional generic arguments gc arguments? If yes.. which argument? Xnoclassgc
2. I have read Xnoclassgc is not recommended as it can cause OOM issues.  When is it advisable to use it?

Regards,
Sandile

------------------------------
Mziwandile Sithebe
IBM
------------------------------

Hello,
-XX:+DisableExplicitGC does not completely disabled garbage collection (otherwise you JVM would quickly run OOM) but this option turns the System.gc() calls from the application to a no-op. So if you application does not use System.gc() calls it should not make a difference here.
Also be careful setting this option as the direct memory buffer code calls System.gc() as well before extending the buffers (unless you have -XX:MaxDirectMemorySize=<size> set).

Hope this helps ...

------------------------------
Hermann Huebler
------------------------------

Original Message:
Sent: 11-12-2018 13:40
From: Mziwandile Sithebe
Subject: GC Policy -Xnoclassgc

Hi

Just discovered that WAS is running on Solaries, therefore the gc info in written on stdout.log not stderr.log.

Now my question is what is the effect of -XX:+DisableExplicitGC generic argument?
Does this completely disable any garbage collection?

Regards

------------------------------
Mziwandile Sithebe
IBM

Original Message:
Sent: 11-12-2018 06:58
From: Mziwandile Sithebe
Subject: GC Policy -Xnoclassgc

Hi

We are using WAS 8.5.5.7 and JAVA 7.0.4.1

Verbose garbage collection has been enabled.
The following Generic JVM arguments which relates to GC has been added as per application vendor recommendation.
-XX:+UseParallelOldGC -XX:+UseAdaptiveSizePolicy -Xnoclassgc -Djava.nio.channels.spe.SelectorProvider=sun.nio.ch.DevPollSelectorProvider -XX:+UseSpinning -XX:+HeapDumpOnOutOfMemoryError -XX:+PrintGCTimeStamps -XX:+DisableExplicitGC -XX:+PrintGCDetails -XX:+PrintHeapAtGC -XX:+PrintGCDateStamps -XX:NewSize=1024m -XX:MaxNewSize=2048m

1. As Verbose garbage collection is enabled, I expect logging on stderr log, but this is not happening.  Can this be caused by additional generic arguments gc arguments? If yes.. which argument? Xnoclassgc
2. I have read Xnoclassgc is not recommended as it can cause OOM issues.  When is it advisable to use it?

Regards,
Sandile

------------------------------
Mziwandile Sithebe
IBM
------------------------------

"What is the effect of -XX:+DisableExplicitGC generic argument? Does this completely disable any garbage collection?"

No. Developers can request an immediate garbage collection in Java code by calling System.gc(). This argument effectively disables that method on the System class and only runs garbage collection according to the algorithm and other options selected in the JVM arguments. Generally speaking, it is better to leave the timing of garbage collection to the JVM and not try to invoke it explicitly, so including this argument is really just imposing the best practice on developers.

------------------------------
Neil Weightman
------------------------------

Original Message:
Sent: 11-12-2018 13:40
From: Mziwandile Sithebe
Subject: GC Policy -Xnoclassgc

Hi

Just discovered that WAS is running on Solaries, therefore the gc info in written on stdout.log not stderr.log.

Now my question is what is the effect of -XX:+DisableExplicitGC generic argument?
Does this completely disable any garbage collection?

Regards

------------------------------
Mziwandile Sithebe
IBM

Original Message:
Sent: 11-12-2018 06:58
From: Mziwandile Sithebe
Subject: GC Policy -Xnoclassgc

Hi

We are using WAS 8.5.5.7 and JAVA 7.0.4.1

Verbose garbage collection has been enabled.
The following Generic JVM arguments which relates to GC has been added as per application vendor recommendation.
-XX:+UseParallelOldGC -XX:+UseAdaptiveSizePolicy -Xnoclassgc -Djava.nio.channels.spe.SelectorProvider=sun.nio.ch.DevPollSelectorProvider -XX:+UseSpinning -XX:+HeapDumpOnOutOfMemoryError -XX:+PrintGCTimeStamps -XX:+DisableExplicitGC -XX:+PrintGCDetails -XX:+PrintHeapAtGC -XX:+PrintGCDateStamps -XX:NewSize=1024m -XX:MaxNewSize=2048m

1. As Verbose garbage collection is enabled, I expect logging on stderr log, but this is not happening.  Can this be caused by additional generic arguments gc arguments? If yes.. which argument? Xnoclassgc
2. I have read Xnoclassgc is not recommended as it can cause OOM issues.  When is it advisable to use it?

Regards,
Sandile

------------------------------
Mziwandile Sithebe
IBM
------------------------------

Thank you Hermann and Neil, really appreciated.

One last question... what is the effect of -Xnoclassgc?

Regards,
Sandile
Original Message:
Sent: 11-13-2018 03:48
From: Neil Weightman
Subject: GC Policy -Xnoclassgc

"What is the effect of -XX:+DisableExplicitGC generic argument? Does this completely disable any garbage collection?"

No. Developers can request an immediate garbage collection in Java code by calling System.gc(). This argument effectively disables that method on the System class and only runs garbage collection according to the algorithm and other options selected in the JVM arguments. Generally speaking, it is better to leave the timing of garbage collection to the JVM and not try to invoke it explicitly, so including this argument is really just imposing the best practice on developers.

------------------------------
Neil Weightman

Original Message:
Sent: 11-12-2018 13:40
From: Mziwandile Sithebe
Subject: GC Policy -Xnoclassgc

Hi

Just discovered that WAS is running on Solaries, therefore the gc info in written on stdout.log not stderr.log.

Now my question is what is the effect of -XX:+DisableExplicitGC generic argument?
Does this completely disable any garbage collection?

Regards

------------------------------
Mziwandile Sithebe
IBM

Original Message:
Sent: 11-12-2018 06:58
From: Mziwandile Sithebe
Subject: GC Policy -Xnoclassgc

Hi

We are using WAS 8.5.5.7 and JAVA 7.0.4.1

Verbose garbage collection has been enabled.
The following Generic JVM arguments which relates to GC has been added as per application vendor recommendation.
-XX:+UseParallelOldGC -XX:+UseAdaptiveSizePolicy -Xnoclassgc -Djava.nio.channels.spe.SelectorProvider=sun.nio.ch.DevPollSelectorProvider -XX:+UseSpinning -XX:+HeapDumpOnOutOfMemoryError -XX:+PrintGCTimeStamps -XX:+DisableExplicitGC -XX:+PrintGCDetails -XX:+PrintHeapAtGC -XX:+PrintGCDateStamps -XX:NewSize=1024m -XX:MaxNewSize=2048m

1. As Verbose garbage collection is enabled, I expect logging on stderr log, but this is not happening.  Can this be caused by additional generic arguments gc arguments? If yes.. which argument? Xnoclassgc
2. I have read Xnoclassgc is not recommended as it can cause OOM issues.  When is it advisable to use it?

Regards,
Sandile

------------------------------
Mziwandile Sithebe
IBM
------------------------------

When the JVM loads a new class, it has to store the class object and information about the class in the JVM heap in a region called the "permanent generation" or "PermGen", which is relatively small - a few 10s of MB typically. This region is normally part of the regular GC process and will be tidied up, removing information on classes which are no longer required. If you use the -Xnoclassgc JVM command line option, this tidying up is disabled, so eventually space will run out and you'll get an OOM error. I don't know why you'd want that; IBM recommends that you only use the option when directed to by IBM Support.

I hope that helps.

------------------------------
Neil Weightman
------------------------------

Original Message:
Sent: 11-14-2018 02:01
From: Mziwandile Sithebe
Subject: GC Policy -Xnoclassgc

Thank you Hermann and Neil, really appreciated.

One last question... what is the effect of -Xnoclassgc?

Regards,
Sandile
Original Message:
Sent: 11-13-2018 03:48
From: Neil Weightman
Subject: GC Policy -Xnoclassgc

"What is the effect of -XX:+DisableExplicitGC generic argument? Does this completely disable any garbage collection?"

No. Developers can request an immediate garbage collection in Java code by calling System.gc(). This argument effectively disables that method on the System class and only runs garbage collection according to the algorithm and other options selected in the JVM arguments. Generally speaking, it is better to leave the timing of garbage collection to the JVM and not try to invoke it explicitly, so including this argument is really just imposing the best practice on developers.

------------------------------
Neil Weightman

Original Message:
Sent: 11-12-2018 13:40
From: Mziwandile Sithebe
Subject: GC Policy -Xnoclassgc

Hi

Just discovered that WAS is running on Solaries, therefore the gc info in written on stdout.log not stderr.log.

Now my question is what is the effect of -XX:+DisableExplicitGC generic argument?
Does this completely disable any garbage collection?

Regards

------------------------------
Mziwandile Sithebe
IBM

Original Message:
Sent: 11-12-2018 06:58
From: Mziwandile Sithebe
Subject: GC Policy -Xnoclassgc

Hi

We are using WAS 8.5.5.7 and JAVA 7.0.4.1

Verbose garbage collection has been enabled.
The following Generic JVM arguments which relates to GC has been added as per application vendor recommendation.
-XX:+UseParallelOldGC -XX:+UseAdaptiveSizePolicy -Xnoclassgc -Djava.nio.channels.spe.SelectorProvider=sun.nio.ch.DevPollSelectorProvider -XX:+UseSpinning -XX:+HeapDumpOnOutOfMemoryError -XX:+PrintGCTimeStamps -XX:+DisableExplicitGC -XX:+PrintGCDetails -XX:+PrintHeapAtGC -XX:+PrintGCDateStamps -XX:NewSize=1024m -XX:MaxNewSize=2048m

1. As Verbose garbage collection is enabled, I expect logging on stderr log, but this is not happening.  Can this be caused by additional generic arguments gc arguments? If yes.. which argument? Xnoclassgc
2. I have read Xnoclassgc is not recommended as it can cause OOM issues.  When is it advisable to use it?

Regards,
Sandile

------------------------------
Mziwandile Sithebe
IBM
------------------------------