Java之CMS GC Causes
Allocation Failure
Allocation Failure happens when there isn't enough free space to create new objects in Young generation. Allocation failures triggers Young GC.
On Linux, the JVM can trigger a GC if the kernel notifies there isn't much memory left via mem_notify.
- 產生原因：新生代沒有足夠的空間分配物件。
- 觸發GC型別：Young GC。
GCLocker Initiated GC
The GC locker prevents GC from occurring when JNI code is in a critical region. If GC is needed while a thread is in a critical region, then it will allow them to complete, i.e. call the corresponding release function. Other threads will not be permitted to enter a critical region. Once all threads are out of critical regions a GC event will be triggered
- 產生原因：如果執行緒執行在JNI臨界區時，剛好需要進行GC，此時GC locker將會阻止GC的發生，同時阻止其他執行緒進入JNI臨界區，直到最後一個執行緒退出臨界區時觸發一次GC。
- 觸發GC型別：GCLocker Initiated GC。
Promotion Failure
Promotion Failure happens when there is no continuous memory space to promote larger object, even though total free memory is large enough. This is problem is called as heap fragmentation. Promotion Failure typically triggers Full GC
- 產生原因：老年代沒有足夠的連續空間分配給晉升的物件（即使總可用記憶體足夠大）。
- 觸發GC型別：Full GC。
- 解決方法：增加堆記憶體，特別是新生代記憶體，儘可能讓物件在新生代被回收掉。減少物件的建立，縮短物件的生命週期。
Concurrent Mode Failure
The CMS collector uses one or more garbage collector threads that run simultaneously with the application threads with the goal of completing the collection of the tenured generation before it becomes full. In normal operation, the CMS collector does most of its tracing and sweeping work with the application threads still running, so only brief pauses are seen by the application threads. However, if the CMS collector is unable to finish reclaiming the unreachable objects before the tenured generation fills up, or if an allocation cannot be satisfied with the available free space blocks in the tenured generation, then the application is paused and the collection is completed with all the application threads stopped. The inability to complete a collection concurrently is referred to as concurrent mode failure and indicates the need to adjust the CMS collector parameters. Concurrent mode failure typically triggers Full GC.
- 產生原因：CMS GC執行期間，老年代預留的空間不足以分配給新的物件。
- 觸發GC型別：Full GC。
- 解決方法：適當調低-XX:CMSInitiatingOccupancyFraction，或者增加老年代記憶體。
關於GCLocker Initiated GC
bugs.openjdk.java.net/browse/JDK-…
轉載請註明來源：http://zhanjia.iteye.com/blog/2434083
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