Major slowdowns caused by the new Linux 4.20 kernel have been traced to a mitigation for Spectre variant 2 that Linux founder Linus Torvalds now wants restricted.
As noted by Linux news site Phoronix, the sudden slowdowns have been caused by a newly implemented mitigation called Single Thread Indirect Branch Predictors (STIBP), which is on by default in the Linux 4.20 kernel for Intel systems with up-to-date microcode.
STIBP is one of three possible mitigations Intel added to its firmware updates in response to the Spectre v2 attacks. Others included Indirect Branch Restricted Speculation (IBRS), and Indirect Branch Predictor Barrier (IBPB), which could be enabled by operating-system makers.
STIBP specifically addresses attacks against Intel CPUs that have enabled Hyper Threading, its version of Simultaneous Multithreading (SMT)
Phoronix’s benchmarks comparing Linux 4.20 with STIPB enabled show that the mitigation on some application workloads has a severe impact on performance.
With STIBP enabled, Phoronix’s high-end Xeon Gold server also goes from being the fastest server to slower than AMD’s previously lower-performing EPYC-based server.
Because of these slowdowns Torvalds’ on Sunday posted a message demanding STIBP no longer be enabled by default in the kernel, especially since an existing option is to disable SMT.
Submitted by: Arnfried Walbrecht