precondition: 

caffeine cache only set expire after write 10m.

appearance:

 Many of  expired cache store in Old-Generation.

case1.  The cache survive from 15 ygc,  Upgrade  into  Old-Generation.

case2.  Young-Generation  don't have enough space to shore this. 

Analysis: 

The average of  YGC, interval is 12m.    ps:  

 1. jstat -gcutil  <pid>    (gather the num of ygc)   

 2. top    (gather the old of process)   

But it may exists that  16 times  Ygc in 10minutes.  it exists big calculation in special time.

For case1. 

We need  make the cache  can be  release before ygc. 

add spec param    weakValues .  it's  work!!!.  if  use the parm  <softValues>.  the space will  be  release  after FullGC.   

to be  continue.