CMS两个常见问题

CMS的问题

1. Memory Fragmentation（内存碎片过多，导致启动单线程serial垃圾回收器）

   -XX:+UseCMSCompactAtFullCollection -XX:CMSFullGCsBeforeCompaction 默认为0 指的是经过多少次FGC才进行压缩

2. Floating Garbage（浮动垃圾，并发回收垃圾的同时产生新的垃圾，但并未被回收）

   Concurrent Mode Failure 产生：if the concurrent collector is unable to finish reclaiming the unreachable objects before the tenured generation fills up, or if an allocation cannot be satisfiedwith the available free space blocks in the tenured generation, then theapplication is paused and the collection is completed with all the applicationthreads stopped

   解决方案：降低触发CMS的阈值

   PromotionFailed

   解决方案类似，保持老年代有足够的空间

   –XX:CMSInitiatingOccupancyFraction 92% 可以降低这个值，让CMS保持老年代足够的空间