DaveDeCaprio opened a new pull request #24028: [SPARK-26917][SQL] Further 
reduce locks in CacheManager
URL: https://github.com/apache/spark/pull/24028
 
 
   ## What changes were proposed in this pull request?
   
   Further load increases in our production environment have shown that even 
the read locks can cause some contention, since they contain a mechanism that 
turns a read lock into an exclusive lock if a writer has been starved out.  
This PR reduces the potential for lock contention even further than 
https://github.com/apache/spark/pull/23833.  Additionally, it uses more 
idiomatic scala than the previous implementation.
   
   @cloud-fan & @gatorsmile This is a relatively minor improvement to the 
previous CacheManager changes.  At this point, I think we finally are doing the 
minimum possible amount of locking.
   
   ## How was this patch tested?
   
   Has been tested on a live system where the blocking was causing major issues 
and it is working well.
   CacheManager has no explicit unit test but is used in many places internally 
as part of the SharedState.
   
   

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