Investigating Split Locks on x86-64
chipsandcheese.com26 points by ingve 3 days ago
26 points by ingve 3 days ago
Cool investigation. This part perplexes me, though:
> Games have apparently been using split locks for quite a while, and have not created issues even on AMD’s Zen 2 and Zen 5.
For the life of me I don't understand why you'd ever want to do an atomic operation that's not naturally aligned, let alone one split across cache lines....
> For the life of me I don't understand why you'd ever want to do an atomic operation that's not naturally aligned, let alone one split across cache lines....
I assume they force packed their structure and it's poorly aligned, but x86 doesn't fault on unaligned access and Windows doesn't detect and punish split locks, so while you probably would get better performance with proper alignment, it might not be a meaningful improvement on the majority of the machines running the program.
Ah, that's a great hypothesis. I wonder, then, how it works with x86 emulation on ARM. IIRC, atomic ops on ARM fault if the address isn't naturally aligned... but I guess the runtime could intercept that and handle it slowly.
An emulated x86 atomic instruction wouldn’t need to use atomic instructions on ARM.