Looking through the HRegion.RegionScannerImpl, I see various synchronized next(...) methods, same for StoreScanner.
Scanners are created for Get operations and these scanners are guaranteed to be only used from a single thread, so in that case all synchronization is pointless.
The client's next(...) operation is a bit more interesting.
Is anybody using scanners concurrently, such as calling next(...) on the *same* scanner from multiple threads concurrently?
If not, we could enforce this once at the RegionServer level and have the entire path down to HFileScannerVx unsynchronized.
Would this work? Any counterexamples?
A quick test shows that we could gain about 10-20% during a scan when everything is in cache.