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From "Keith Turner (JIRA)" <j...@apache.org>
Subject [jira] [Created] (ACCUMULO-4164) Avoid copy of RFile Index blocks when in cache
Date Mon, 14 Mar 2016 17:55:33 GMT
Keith Turner created ACCUMULO-4164:
--------------------------------------

             Summary: Avoid copy of RFile Index blocks when in cache
                 Key: ACCUMULO-4164
                 URL: https://issues.apache.org/jira/browse/ACCUMULO-4164
             Project: Accumulo
          Issue Type: Improvement
    Affects Versions: 1.7.1, 1.6.5
            Reporter: Keith Turner
             Fix For: 1.6.6, 1.7.2, 1.8.0


I have been doing performance experiments with RFile.  During the course of these experiments
I noticed that RFile is not as fast at it should be in the case where index blocks are in
cache and the RFile is not already open.  The reason is that the RFile code copies and deserializes
the index data even though its already in memory.

I made the following change to RFile in a branch.

 * Avoid copy of index data when its in cache
 * Deserialize offsets lazily (instead of upfront) during binary search
 * Stopped calling lots of synchronized methods during deserialization of index info.  The
existing code use ByteArrayInputStream which results in lots of fine grained synchronization.
 Switching to an inputstream that offers the same functionality w/o sync showed a measurable
performance difference.  

These changes lead to performance in the following two situations  :
 * When an RFiles data is in cache, but its not open on the tserver.  
 * For RFiles with multilevel indexes with index data in cache.   Currently an open RFile
only keeps the root node in memory.   Lower level index nodes are always read from the cache
or DFS.   The changes I made would always avoid the copy and deserialization of lower level
index nodes when in cache.

I have seen significant performance improvements testing with the two cases above.  My test
are currently based on a new API I am creating for RFile, so I can not easily share them until
I get that pushed.  

For the case where a tserver has all files frequently in use already open and those files
have a single level index, these changes should not make a significant performance difference.

These change should result in less memory use for opening the same rfile multiple times for
different scans (when data is in cache).  In this case all of the RFiles would share the same
byte array holding the serialized index data.



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