The increment operation can release the rowlock before sync-ing the Hlog
Review Request #2116 - Created Sept. 29, 2011 and updated
The increment operation releases the rowlock before doing the sync to the HLog. This improves performance of increments on hot rows. Introduced method HLog.appendNoSync() that returns a txid. The increment method then release the rowlock and invokes HLog.sync(txid). The HLog.sync(txid) returns only if all the transactions upto the one identified by that txid has been successfully sycned to HDFS. There is a single test TestIncrement that creates a 100 threads and ensures that all the concurrent increments match the final expected result. There is a benchmark TestHLogBench that measures the performance of the appendNoSync call.
All unit tests pass now (expect TestDistributedLogSplitting, TestRollingRestart, TestHTablePool), but I am seeing the same test to fail on trunk, so these failures do not seem to be related to this patch.