4 ms·

This doesn't make a lot of sense off the bat, are there more details to the story that explain how CPU branch prediction performance depends on Windows behaviou

This doesn't make a lot of sense off the bat, are there more details to the story that explain how CPU branch prediction performance depends on Windows behaviour somehow?

pixl97 2y ago
Linux has different performance schedulers you can choose that can dramatically change performance dependant on workloads. It's very possible the CPU is being too smart for its own good and the way Windows is lining stuff up it's not executing branches or something like that. This is just shooting from the hip, but there is a ton of different stuff going on in both the OS and the CPU around this.
- deleted 2y ago
  [deleted]
jsheard 2y ago
The mitigations for Spectre/Meltdown involved deliberately breaking branch prediction across the user/kernel boundary, so it may be as simple as them disabling some mitigations which turned out to not be needed anymore on newer CPU architectures.
EricE 2y ago
Dave of Dave's Garage explained it here: https://www.youtube.com/watch?v=OBn3FbKusZI https://www.youtube.com/watch?v=OBn3FbKusZI Basically (according to him) they disabled the spectre/meltdown mitigation if you were an admin - because you were an admin and have full rights to the machine anyway. So now they are doing that for all users? He wasn't clear - or at least it wasn't clear to me. So the mitigations are no longer needed? I think I have more questions than before after listening to that.
- TiredOfLife 2y ago
  Dave lies. A lot. I would take anything he says with a bucket of salt.
  - ABraidotti 2y ago
    Interesting. I just found his channel. What does he lie about? Factual stuff or MS history?
    - TiredOfLife 2y ago
      Just the latest one. https://x.com/davepl1968/status/1812559735616577565 https://x.com/davepl1968/status/1812559735616577565 He claims he wrote realtime rendering of vertical text in start menu. Turns out he didn't. No released or unreleased version or source code show that.
- mey 2y ago
  That logic doesn't pass a sniff test at all.

This doesn't make a lot of sense off the bat, are there more details to the story that explain how CPU branch prediction performance depends on Windows behaviour somehow?

pixl97 2y ago

Linux has different performance schedulers you can choose that can dramatically change performance dependant on workloads. It's very possible the CPU is being too smart for its own good and the way Windows is lining stuff up it's not executing branches or something like that. This is just shooting from the hip, but there is a ton of different stuff going on in both the OS and the CPU around this.

deleted 2y ago
[deleted]

jsheard 2y ago

The mitigations for Spectre/Meltdown involved deliberately breaking branch prediction across the user/kernel boundary, so it may be as simple as them disabling some mitigations which turned out to not be needed anymore on newer CPU architectures.

EricE 2y ago

Dave of Dave's Garage explained it here: https://www.youtube.com/watch?v=OBn3FbKusZI https://www.youtube.com/watch?v=OBn3FbKusZI Basically (according to him) they disabled the spectre/meltdown mitigation if you were an admin - because you were an admin and have full rights to the machine anyway. So now they are doing that for all users? He wasn't clear - or at least it wasn't clear to me. So the mitigations are no longer needed? I think I have more questions than before after listening to that.

TiredOfLife 2y ago
Dave lies. A lot. I would take anything he says with a bucket of salt.
- ABraidotti 2y ago
  Interesting. I just found his channel. What does he lie about? Factual stuff or MS history?
  - TiredOfLife 2y ago
    Just the latest one. https://x.com/davepl1968/status/1812559735616577565 https://x.com/davepl1968/status/1812559735616577565 He claims he wrote realtime rendering of vertical text in start menu. Turns out he didn't. No released or unreleased version or source code show that.
mey 2y ago
That logic doesn't pass a sniff test at all.