BUIP033: Parallel Validation
Proposer: Peter Tschipper
Submitted on: 10/22/2016

Summary:

Essentially Parallel Validation is a simple concept. Rather than validating each block within the main processing thread, we instead create a separate thread to do the block validation. If more than one block arrives to be processed then we create yet another thread. There are currently up to 4 parallel block processing threads available making a big block DDOS attack impossible. Furthermore, if any attacker were somehow able to jam all 4 processing threads and another block arrived, then the processing for the largest block would be interrupted allowing the smaller block to proceed, unless the larger block or blocks have most proof of work. So only the most proof of work and smallest blocks will be allowed to finish in such
as case.

If there are multiple blocks processing at the same time, when one of the blocks wins the race to complete, then the other threads of processing are interrupted and the winner will be able to update the UTXO and advance the chain tip. Although the other blocks that were interrupted will still be stored on disk in the event of a re-org.