March 10th, 2008

ULE happenings, context switch surprise.

Lately I've been able to spend a bunch of time on ULE thanks to Nokia. They use it in one of their networking products. I've been doing all of this work in 8.0-CURRENT and backporting it for them at the same time. It's a great model for both parties because users on -CURRENT shake out bugs that they'd have to find in testing otherwise and we get new development paid for.

I finished and committed the topology aware cpu scheduling that I discussed in earlier posts. I also implemented a mechanism for CPU provisioning that you can use to restrict groups of processes to sets of cpus which can be dynamically migrated. This will be useful for restricting jails to certain CPUs or dedicating some CPUs to real-time special-purpose tasks for example.

Over the last few days I cleaned up my cpu switch optimizations and got those in. The results are 25% faster context switching in a yield benchmark. Even faster than linux on the same hardware. Some day I'll put open solaris on so I have something else to compare to.

Separate from the other switch benchmarks I've been working on reimplementing amd64's context switching routine almost entirely in C. I just wanted to do it because we're putting more complex things in and it was getting hard to find registers but it turns out you can make it much faster too. The yield benchmark is another 10% faster with the C switch routine. Mostly due to enabling more complex checks, like not setting MSR_FSBASE/GSBASE if they haven't changed, and getting uncommon code out of the fast path.