Shared Processes with Hyper-Threading

Joel Fernandes wanted to speed up hyper-threading by making it possible for processes on the same CPU that trust each other to share hyper-threads. Hyper-threading is Intel's proprietary form of hardware-based multithreading. Generally in Linux and other operating systems (OSs), the OS is responsible for switching rapidly between processes, so everything on the system seems to be running at once. Hyper-threading does this at the hardware level, saving time for the OS. But the OS can still interact with Intel's hyper-threading features, so folks like Joel can try to eke out performance improvements.

Joel specifically wanted to improve Peter Zijlstra's "core-scheduling" patches, which Peter famously hates the way one hates slowly pulling out their own fingernails. He actually did this once. No, not really. However, the point of the core-scheduling patches is to make each CPU act like two. This way, for moments when one of the virtual CPUs has nothing to do, the other virtual CPU will keep chugging away, making sure the hardware "real" CPU is as fully utilized as possible.

Of course, processes running on those two hyper-threading virtual CPUs have to be treated like potential security threats, the same as all processes everywhere. If a hostile actor gets into a user process on a given system, the kernel wants to limit the amount of damage that actor can do. That's just part of standard Linux procedure. Keep everything isolated, and then nothing can hurt anything else too badly.

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