Hi, Upcoming versions of the linux kernel (and apparently also windows and others), will include new feature that apparently has been implemented with haste to work around an intel hardware bug.
https://lwn.net/SubscriberLink/741878/eaff7b24627c41a2/ The fix, split userland / kernel pagetables, is going to be merged in the next version of the linux kernel and is being backported to older point releases. The backports of a complex invasive new feature signals that this concerns a significant issue. There's plenty speculation about details about what exactly the vulnerability is. Don't want to go into that here. The fix will unfortunately cause performance regressions. Depending on the hardware version and kernel version (will not be backported for every version) hardware features (PCID / ASID) will be used to reduce the impact. pti is the workaroud, page table isolation, which can be enabled/disabled via boot parameters. nopcid disables the use of the hardware feature that reduces the impact of workaround. PCID support readonly pgbench (tpch-like), 16 clients, i7-6820HQ CPU (skylake): pti=off: tps = 236629.778328 pti=on: tps = 220791.228297 (~0.93x) pti=on, nopcid: tps = 198959.801459 (~0.84x) To get closer to the worst case, I've also measured: pgbench SELECT 1, 16 clients, i7-6820HQ CPU (skylake): pti=off: tps = 420490.162391 pti=on: tps = 350746.065039 (~0.83x) pti=on, nopcid: tps = 324269.903152 (~0.77x) Note that real-world scenarios probably will see somewhat smaller impact, as this was measured over a loopback unix sockets which'll have smaller overhead itself than proper TCP sockets + actual network. The rumor mill has it that details about the vulnerability will be un-embargoed in the next few days. Greetings, Andres Freund
