Andrew Doran posted some threading benchmark results to NetBSD's tech-kern mailing list, following up to some benchmarks he'd posted earlier. The results compared NetBSD -current with FreeBSD -current, and the Linux 2.6.21 kernel. Kris Kennaway was surprised by the results, and ran his own benchmarks with minimal configuration changes, summarizing, "this measurement shows that FreeBSD is performing 70-80% better than NetBSD in this 4 CPU configuration. This is in contrast to Andrew's findings which seem to show NetBSD performing 10% better than FreeBSD on a 4 CPU system (a very old one though)." He added, "the drop-off above 8 threads on FreeBSD is due to non-scalability of mysql itself. i.e. it comes from pthread mutex contention in userland."
Kris ran additional benchmarks with PostgreSQL instead of MySQL, showing much improved scalability above 8 threads, "postgresql is much more scalable than mysql on this workload and doesn't have silly scaling bottlenecks inside the application (cf the tail of the FreeBSD curve for mysql which is where pthread mutex contention kicked in)." He continued his testing, and found that on older 4CPU P3 hardware NetBSD did outperform FreeBSD, "but only by 3-4% (in particular I am not seeing the ~10% difference that Andrew observes on his 4*p3 700MHz). Given the age of the hardware and the fact that I am not seeing it on other workloads or on modern hardware it might just be due to a small scheduling difference on this configuration."