I have been looking into reported performance regressions in HBase 2 relative to HBase 1. Depending on the test scenario, HBase 2 can demonstrate significantly better microbenchmarks in a number of cases, and usually shows improvement in whole cluster benchmarks like YCSB.
To assist in debugging I added methods to RpcServer for updating per-call metrics that leverage the fact it puts a reference to the current Call into a thread local and that all activity for a given RPC is processed by a single thread context. I then instrumented ScanQueryMatcher (in branch-1) and its various friends (in branch-2.2), StoreScanner, HFileReaderV2 and HFileReaderV3 (in branch-1) and HFileReaderImpl (in branch-2.2), HFileBlock, and DefaultMemStore (branch-1) and SegmentScanner (branch-2.2). Test tables with one family and 1, 5, 10, 20, 50, and 100 distinct column-qualifiers per row were created, snapshot, dropped, and cloned from the snapshot. Both 1.6 and 2.2 versions under test operated on identical data files in HDFS. For tests with 1.6 and 2.2 on the server side the same 1.6 PE client was used, to ensure only the server side differed.
The results for pe --filterAll were revealing. See attached.
It appears a refactor to ScanQueryMatcher and friends has disabled the ability of filters to provide meaningful SKIP hints, which disables an optimization that avoids reseeking, leading to a serious and proportional regression in reseek activity and time spent in that code path. So for queries that use filters, there can be a substantial regression.
Other test cases that did not use filters did not show this regression. If filters are not used the behavior of ScanQueryMatcher between 1.6 and 2.2 was almost identical, as measured by counts of the hint types returned, whether or not column or version trackers are called, and counts of store seeks or reseeks. Regarding micro-timings, there was a 10% variance in my testing and results generally fell within this range, except for the filter all case of course.