Bisected: The Unfortunate Reason Linux 4.20 Is Running Slower

#1

Post by zrm » Sat, Nov 17, 2018, 5:59 PM UTC

Bisected: The Unfortunate Reason Linux 4.20 Is Running Slower
phoronix.com

Re: Bisected: The Unfortunate Reason Linux 4.20 Is Running Slower

#2

Post by schoen » Sat, Nov 17, 2018, 7:12 AM UTC

In case people don't like the headline style (leaving out the reference to the answer), the answer given at the end of the article is new Spectre mitigations.

We new that speculative-execution mitigations would sometimes reduce performance, and evidently they do. :-(

Re: Bisected: The Unfortunate Reason Linux 4.20 Is Running Slower

#3

Post by _cs2017_ » Sat, Nov 17, 2018, 9:13 AM UTC

1.3-1.4x slowdown is a lot more than I expected (I know it's for synthetic benchmarks but still...)

Can someone explain (or link to an article) how a tweak to HT branch prediction heuristic can have such a huge impact on performance?

Re: Bisected: The Unfortunate Reason Linux 4.20 Is Running Slower

#4

Post by kim0 » Sat, Nov 17, 2018, 10:35 AM UTC

Is Intel already selling CPUs which don't need these patches?

Re: Bisected: The Unfortunate Reason Linux 4.20 Is Running Slower

#5

Post by neopallium » Sat, Nov 17, 2018, 11:27 AM UTC

Is there a comparison table of CPU vulnerabilities? I would like to know which CPU has the least vulnerabilities or the best performance after patching.

From what I have read so far is seems that AMD CPUs have had the fewest vulnerabilities/slowdowns? But I can't be sure since I haven't seen a complete comparison (including these new vulnerabilities).

Re: Bisected: The Unfortunate Reason Linux 4.20 Is Running Slower

#6

Post by bonzini » Sat, Nov 17, 2018, 7:10 PM UTC

This was known. https://lwn.net/Articles/765837/ reports 21% performance impact and the plan was to handle STIBP as an opt-in security feature. I am not sure why that was not furthered in the patches that have actually been committed to Linux, as I did not follow them closely.