Lars Hofhansl created HBASE-13082:
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Summary: Coarsen StoreScanner locks to RegionScanner
Key: HBASE-13082
URL: https://issues.apache.org/jira/browse/HBASE-13082
Project: HBase
Issue Type: Bug
Reporter: Lars Hofhansl
Continuing where HBASE-10015 left of.
We can avoid locking (and memory fencing) inside StoreScanner by deferring to
the lock already held by the RegionScanner.
In tests this shows quite a scan improvement and reduced CPU (the fences make
the cores wait for memory fetches).
There are some drawbacks too:
* All calls to RegionScanner need to be remain synchronized
* Implementors of coprocessors need to be diligent in following the locking
contract. For example Phoenix does not lock RegionScanner.nextRaw() and
required in the documentation (not picking on Phoenix, this one is my fault as
I told them it's OK)
* possible starving of flushes and compaction with heavy read load.
RegionScanner operations would keep getting the locks and the
flushes/compactions would not be able finalize the set of files.
I'll have a patch soon.
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