I have read that hyperthreading is a "performance killer" when it comes to DBs. However, what I read didn't state which CPUs. Further, it mostly indicated that I/O was "cut to < 10% performance".
That logically doesn't make sense since I/O is primarily a function of controllers and disks, not CPUs. But then no one ever said bugs made sense.
What I read also stated that SQL Server could put two parallel query ops onto 1 logical core (2 threads), thereby degrading performance. I have a hard time believing SQL Server's architects would have made such an obvious miscalculation.
Does anyone have and data on how hyperthreading on current generation CPUs affects either of the RDBMSs I mentioned?