New Mesa Patch To Improve CPU-Bound Applications

For those wondering what else Kristian Høgsberg is working on in his post-Wayland days, after tackling initial Skylake enablement in Mesa his latest achievement is a new Mesa performance patch.

Høgsberg wrote to the Mesa mailing list, "While modern pthread mutexes are very fast, they still incur a call to an
external DSO and overhead of the generality and features of pthread mutexes. Most mutexes in mesa only needs lock/unlock, and the idea here is that we can inline the atomic operation and make the fast case just two intructions...We implement "mutex3", which gives us a mutex that has no syscalls on uncontended lock or unlock. Further, the uncontended case boils down to a cmpxchg and an untaken branch and the uncontended unlock is just a locked decr and an untaken branch. We use __builtin_expect() to indicate that contention is unlikely so that gcc will put the contention code out of the main code flow."

In terms of the performance benefit for this generic Mesa patch, "The pthread mutex lock/unlock overhead shows up on benchmarks for CPU bound applications. Most CPU bound cases are helped and some of our internal bind_buffer_object heavy benchmarks gain up to 10%."

More details via the mailing list patch post.