>But say in your OS now you want the thread to go to sleep until it’s available.. how is actually implemented without spinning and using CPU?
If you're facing such low-latency constraints that you need a spinlock, you wouldn't want the thread to sleep, instead you'd pin the core and have the dedicated core constantly spinning. That's how it's done in HFT. You can use a fancy network card like SolarFlare with efvi_drivers to bypass the kernel completely for network IO (spin waiting on the network card). Basically you need to treat the kernel like it's a paedophile trying to molest your children, and do everything you can to stop it touching your low-latency threads.
If you're facing such low-latency constraints that you need a spinlock, you wouldn't want the thread to sleep, instead you'd pin the core and have the dedicated core constantly spinning. That's how it's done in HFT. You can use a fancy network card like SolarFlare with efvi_drivers to bypass the kernel completely for network IO (spin waiting on the network card). Basically you need to treat the kernel like it's a paedophile trying to molest your children, and do everything you can to stop it touching your low-latency threads.