Separate from last week in uncovering a big performance regression on Linux 6.15 affecting workloads like Nginx and that regression getting fixed, I unfortunately discovered another heavy-hitting regression on Linux 6.15. This latest performance regression has been bisected and a possible fix is being thought through by the relevant party, but for the moment has yet to be fixed upstream and affects modern AMD processors.
When running some new Linux 6.15 Git kernel benchmarks toward the end of last week I unfortunately began noticing a number of performance regressions relative to Linux 6.14 stable. This round of testing was on a dual AMD EPYC 9755 “Turin” server.
In a number of different workloads I encountered lower performance with Linux 6.15 Git on this high-end AMD EPYC server than when using Linux 6.14 stable:
Code compilation benchmarks were noticeably slower when running atop the Linux 6.15 Git kernel… 10% slower code compilation jobs from a kernel upgrade? Uh oh!
But it wasn’t just code compilation tasks that were slower on the new kernel but also high performance computing (HPC) workloads, video encoding/transcoding, image encoding, digital signal processing, and a variety of other workloads.
At that point I knew my weekend would be over and spent exploring this performance regression. Now to figure out what the hell was going on.
