[cperciva]

FreeBSD 5.x was a very stressful time -- bringing full SMP to the entire kernel was a massive technical challenge, and it didn't help that a lot of the people who were expecting to work on it lost their jobs during the dot com crash.

5.x was a poor vintage because making the kernel SMP was hard, and social issues became problematic because SMP was hard, but Matt's departure was neither the result or cause of FreeBSD 5.x having issues.

[tiffanyh]

In hindsight, with it being now 20 years later, which approach to SMP do you think was the better technical choice?

The FreeBSD model or DragonflyBSD approach?

[cperciva]

Assuming "the DragonflyBSD approach" is the same as when it forked 20 years ago: FreeBSD's approach was definitely the right one, for two reasons.

First, adding locking is fundamentally easier to get right than rewriting everything into a message-passing model. It took ~5 years to get SMP right in FreeBSD (from when 4.0 was released to when 6.0 was released) but it would have taken double that to get things equally stable and performant with the more intrusive redesign Matt wanted to go with.

Second, pretty much everyone else went with the same locking approach in the end -- which has resulted in CPUs being optimized for that. Lock latencies dropped dramatically with microarchitectural changes between ~2005 and ~2015.

As a theoretical concept, I love the idea behind Dragonfly, but it simply wasn't practical in the real world.