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Date: Sat, 21 Feb 2015 06:37:11 +0000 (UTC)
From: "Lars Hofhansl (JIRA)" <jira@apache.org>
To: dev@hbase.apache.org
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Subject: [jira] [Created] (HBASE-13082) Coarsen StoreScanner locks to
 RegionScanner
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Lars Hofhansl created HBASE-13082:
-------------------------------------

             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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