> some workloads may experience up to 50% overhead

They word there are "some workloads", I suspect that performance hit is rare.

My guess is that HPC systems which run applications whose performance strongly depends on efficient data scatter gather will immediately disable the mitigation.

Key context you elided: "This depends on whether Gather is in the critical execution path of a program."

Well yes. That is the up to part.

Is that 50% overhead for "Gather" instructions? If that is the case, then if 10% of instructions are "gathers" in your workload then that would be 5% overall.

Something like that, yeah.

"Red Hat’s internal performance testing of a worst-case microbenchmark showed a significant slowdown. However, more realistic applications that utilize vector gathering showed only low single-digit percentage slowdowns."

https://access.redhat.com/solutions/7027704

Performance Impact

The performance impact of the microcode mitigation is limited to applications that use the gather instructions provided by Intel Advanced Vector Extensions (AVX2 and AVX-512) and the CLWB instruction. Actual performance impact will depend on how heavily an application uses those instructions. Red Hat’s internal performance testing of a worst-case microbenchmark showed a significant slowdown. However, more realistic applications that utilize vector gathering showed only low single-digit percentage slowdowns.

If the user decides to disable the mitigation after doing a thorough risk analysis (for example the system isn’t multi-tenant and doesn’t execute untrusted code), the user can disable the mitigation. After applying the microcode and kernel updates, the user can disable the mitigation by adding gather_data_samping=off to the kernel command line.Alternatively, to disable all CPU speculative execution mitigations, including GDS, use mitigations=off.

It is kind of like thinking that it is a huge sale at a store when they say "Up to 70% off". And there are like two things in the whole store which are 70% off.

I am always suspicious of "up to" claims.

I always translate such weasel words to their more straightforward equivalent. "Up to 70% off" means for me "Nothing in our store is discounted by more than 70%" :-)