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HT provides a significant benefit to many workloads. The use cases that benefit from actually disabling HT are likely working around pessimal OS scheduler or a

by loeg 1y ago

HT provides a significant benefit to many workloads. The use cases that benefit from actually disabling HT are likely working around pessimal OS scheduler or application thread use. (After all, even with it enabled, you're free to not use the sibling cores.) Otherwise, it is an overgeneralization to say that disabling it will benefit arbitrary workloads.

hedora 1y ago
There’s some argument that you should jam stuff on to as few hyperthread pairs as possible to improve energy efficiency and cache locality. Of course, if the CPU governor is set to “performance” or “game mode”, then the OS should use as many pairs as possible instead (unless thermal throttling matters; computers are hard).
robocat 1y ago
> use cases that benefit from actually disabling HT Other benefits: per-CPU software licencing sometimes, and security on servers that share CPU with multiple clients.
mkbosmans 1y ago
Especially in HPC there are lots of workloads that do not benefit from SMT. Such workloads are almost always bottlenecked on either memory bandwidth or vector execution ports. These are exactly the resources that are shared between the sibling threads. So now you have a choice of either disabling SMT in the bios, or make sure the application correctly interprets the CPU topology and only spawns one thread per physical core. The former is often the easier option, both from software development and system administration perspective.
- PunchyHamster 1y ago
  HT cores can still run OS stuff in that case as that isn't really in contention with those. Tho I can see someone not wanting to bother with pinning
- skeezyboy 1y ago
  >Especially in HPC there are lots of workloads that do not benefit from SMT...So now you have a choice of either disabling SMT in the bios Thats madness. Theyre cheaper than their all-core equivalent. Why even buy one in the first place if HT slows down the CPU? Youre still better off with them enabled.

by loeg 1y ago

hedora 1y ago

There’s some argument that you should jam stuff on to as few hyperthread pairs as possible to improve energy efficiency and cache locality. Of course, if the CPU governor is set to “performance” or “game mode”, then the OS should use as many pairs as possible instead (unless thermal throttling matters; computers are hard).

robocat 1y ago

> use cases that benefit from actually disabling HT Other benefits: per-CPU software licencing sometimes, and security on servers that share CPU with multiple clients.

mkbosmans 1y ago

Especially in HPC there are lots of workloads that do not benefit from SMT. Such workloads are almost always bottlenecked on either memory bandwidth or vector execution ports. These are exactly the resources that are shared between the sibling threads. So now you have a choice of either disabling SMT in the bios, or make sure the application correctly interprets the CPU topology and only spawns one thread per physical core. The former is often the easier option, both from software development and system administration perspective.

PunchyHamster 1y ago
HT cores can still run OS stuff in that case as that isn't really in contention with those. Tho I can see someone not wanting to bother with pinning
skeezyboy 1y ago
>Especially in HPC there are lots of workloads that do not benefit from SMT...So now you have a choice of either disabling SMT in the bios Thats madness. Theyre cheaper than their all-core equivalent. Why even buy one in the first place if HT slows down the CPU? Youre still better off with them enabled.