The Performance Impact Of MDS / Zombieload Plus The Overall Cost Now Of Spectre/Meltdown/L1TF/MDS

Written by Michael Larabel in Software on 18 May 2019. Page 7 of 10. 56 Comments

For this second round of tests is looking at the overall cost of all these CPU mitigations to date by making use of the new mitigations= kernel flag. Tests were done with mitigations=off that disable at run-time the relevant mitigations, mitigations=auto as the default behavior for the kernel's mitigations, and then mitigations=auto,nosmt that apply these mitigations but also disable SMT. With mitigations=auto,nosmt it's worth noting that only Intel Hyper Threading is disabled but if using the same parameter on AMD Zen hardware, SMT is left active.

In this comparison using Linux 5.0 on Ubuntu 19.04, various Intel and AMD desktop CPUs were tested for getting a broad look at how the mitigation costs are looking on different hardware. All systems were equipped with NVMe SSD storage and RAM to satisfy the maximum number of memory channels on each system and at their optimal rated frequency. Similar tests using Intel Xeon and AMD EPYC processors are forthcoming on Phoronix. The desktop CPUs tested in time for today's article included the Core i7 6800K, Core i7 8700K, Core i9 7980XE, AMD Ryzen 7 2700X, and AMD Ryzen 7 2700X.

With the CompileBench test case, the three Intel systems were seeing a larger drop in performance than the two AMD systems tested.

The PostMark test that mostly exercises fsync behavior saw a 20% drop in performance on the Core i7 8086K system while the Ryzen 7 2700X was lower by just 5%. Likewise, the Core i9 7980XE was lower by 20% as well with the Threadripper 2990WX seeing just a 7% hit from its Spectre mitigations.

With the Sockperf socket performance testing, the three Intel systems all saw a much greater hit from these CPU mitigations compared to the minor hit the AMD Ryzen systems saw with Spectre mitigations.

The mitigated Intel systems were seeing around three microseconds higher TCP latency with the default mitigations while the Ryzen boxes were just 1~2 microseconds.

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