Failed to startup the Garbage Collector. When using optthruput, with special options.

[zheng-kai]

Java -version output

openjdk version "1.8.0_362" IBM Semeru Runtime Open Edition (build 1.8.0_362-b09) Eclipse OpenJ9 VM (build openj9-0.36.0, JRE 1.8.0 Windows 11 amd64-64-Bit Compressed References 20230207_599 (JIT enabled, AOT enabled) OpenJ9 - e68fb241f OMR - f491bbf6f JCL - eebde685ec based on jdk8u362-b09)

Summary of problem

I found a problem by accident. When I use optthruput GC with following options, jvm could not be created.

java  -Xgcpolicy:optthruput -Xgc:tlhIncrementSize=550000000 -Xgc:excessiveGCratio=80  -Xms512m -Xmx512m  -version

JVMJ9GC070E Failed to startup the Garbage Collector.
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.

But if I switch the GC, it's OK.

java  -Xgcpolicy:gencon -Xgc:tlhIncrementSize=550000000 -Xgc:excessiveGCratio=80  -Xms512m -Xmx512m  -version

openjdk version "1.8.0_362"
IBM Semeru Runtime Open Edition (build 1.8.0_362-b09)
Eclipse OpenJ9 VM (build openj9-0.36.0, JRE 1.8.0 Windows 11 amd64-64-Bit Compressed References 20230207_599 (JIT enabled, AOT enabled)
OpenJ9   - e68fb241f
OMR      - f491bbf6f
JCL      - eebde685ec based on jdk8u362-b09)

I would like to know why optthruput is the only one with exceptions. Thank you. And whether this information is sufficient to locate the problem?

[dmitripivkine]

I am able to reproduce the problem on xLinux with latest Java8. Thank you to let us know.

[dmitripivkine]

I understand failure scenario. The only problem with JVM itself is missing validation of entered value for -Xgc:tlhIncrementSize option. This option is used rarely, mostly for performance experiments.

• There are different heap geometries used for different GC policies. Optthruput operates with object heap as a one large Space. Gencon, however, has heap split to two major Spaces - Tenure and Nursery.
• Thread Local Heap (TLH) is an optimization to minimize heap memory contention during object allocation. An idea into give each java thread chunk of heap memory reserved for this only thread object allocation (and no extra checks necessary).
• The size of given TLH is calculated dynamically but can not exceed maximum TLH size. Growing if TLH is done by TLH increment size (set by -Xgc:tlhIncrementSize). There is no validation for entered value, so when very large number is provided first thread requested an increment took most of the object heap. Other threads struggle to allocate objects and call Garbage Collection often.
• TLH is used for entire heap for Optthruput (there is only one space) but for Nursery only for Gencon. Please note for Gencon if wrong TLH increment size block object allocation in Nursery practically, there is still be possibility to allocate in Tenure. For Optthruput, however there is not a option.
• Excessive GC is a condition to prevent endless cycle of GCs if progress is very small. In this case it was enforced by -Xgc:excessiveGCratio=80 option.
• An initialization of Garbage Collector includes creation of Worker GC threads. This operation requires allocation of java thread related objects in the heap.

So failure scenario is (with some simplifications): During GC initialization one of java threads reaches point where requests TLH increment. There is no validation for entered TLH increment size it is taken blindly. As a result size of requested TLH is much larger than maximum TLH size and take almost all memory in the heap for it. It means there is almost none left for other threads. And because this is Optthruput there is no other Space to try allocation. So, other threads struggle to allocate objects and call Garbage Collector. GC performes but with very little amount of memory to be freed. This operation repeats again and again until Excessive GC condition triggers OOM instead of next GC. This condition has reached before initialization of all GC Worker Threads is complete, so Failed to startup the Garbage Collector message is printed.

[dmitripivkine]

There is nothing wrong except missed input validation for rarely used option. Put it to Deep Backlog to be fixed in the future

[zheng-kai]

Thank you for your explanation, which has given me more understanding of GC.