The Performance Impact To POWER9's Eager L1d Cache Flushing Fix

In terms of the real-world impact from this more frequent L1 data cache flushing on POWER9, database servers like MariaDB (MySQL) and PostgreSQL did record consistent minor performance impacts of this mitigation. At least not as bad as the early pre-hardware-mitigation Intel performance hits from their early issues.

Database workloads seem to be the main area we have benchmarked so far where there is a small but consistent slide in performance from running a mitigated kernel. But overall not too bad and can fortunately be easily disabled via the new kernel options if so desired. Over the Thanksgiving days I'll be running additional benchmarks of the more time consuming test cases on Blackbird and the Talos II to see if there are any other surprises that remain for the performance but at least to date I have not overall found anything too dramatic.