The previous `ForkableSharedMutex` using `std::shared_mutex` was showing
up as significant CPU time on arm64ec. In particular it was showing up
upwards of 700ms/S of CPU time for read-contented workloads at only
~2700 locks per second. The libc++ implementation for arm64ec must be
particularly gnarly for this to be so slow.
With this swapped over, it's now only spending around 56ms/S on the
contended shared lock, but at ~8000 locks per second. So a significant
uplift.
Ryan Houdek
01a3ab6ca7