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A comment on Bisected: The Unfortunate Reason Linux 4.20 Is Running Slower. Parent comment

Re:Bisected: The Unfortunate Reason Linux 4.20 Is Running Sl... by hermanradtke on Saturday November 17, 2018 @03:10PM (#18477124)
The article claims a 30% perf slowdown with PHP. The majority of PHP workloads are single threaded. That causes me to suspect this impacts single core workloads too.
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Re:Bisected: The Unfortunate Reason Linux 4.20 Is Running Sl... by bonzini on Saturday November 17, 2018 @03:32PM (#18477235)
Yes, on some processors basically it's as if all userspace was recompiled to use retpolines. Enabling STIBP promises that one thread cannot influence the indirect branch predictor's operation when the sibling runs; on those processors the indirect branch predictor is completely disabled, which is an inefficient but valid way to implement STIBP semantics.
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Re:Bisected: The Unfortunate Reason Linux 4.20 Is Running Sl... by monocasa on Sunday November 18, 2018 @12:29AM (#18479000)
That's interesting. 30% is generally the back of the envelope number for what hyperthreads get you in a OoOE core.
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