Hacker News new | ask | show | jobs
by Scramblejams 2927 days ago
I've never trusted hyperthreading for workloads I haven't tested. Sometimes it's faster, often it's slower. Beyond that, I've been suspicious of its security implications from day one. My first trip through the BIOS on a personal machine always includes turning it off.
2 comments
ailideex 2927 days ago
Can you give example of where it was slower for you with HT enabled?
link
Scramblejams 2927 days ago
Running finite element models with MSC NASTRAN, basically heavy matrix math. Matrices were NxN, N was around 10 million. This was on a server with 36 cores and a half terabyte of RAM, purchased in 2014.

Also seen Erlang workloads where you could get a bit of throughput increase with your VM scheduler scheduling more threads than your physical cores (so starting to use HT) but the latency would spike and become very unpredictable, which was a bad tradeoff for the use case.

link
gnufx 2927 days ago
HPC workloads normally at least won't benefit and probably take a hit from HT on Xeon-ish hardware at least. It's normally turned off on HPC compute nodes (perhaps in software so the resource manager can enable per-job if necessary). There are exceptions, particularly with KNC and, perhaps, KNL. The situation is likely different for POWER, but I don't have experience of it.
link
rythie 2927 days ago
You could just buy i5 based machines instead which don't have hyperthreading.
link
krylon 2927 days ago
From what I have seen, I think many dual-core i5 CPUs for notebooks support hyperthreading.
link
letsgetphysITal 2927 days ago
The U (ultra-low power) lines are indeed two-core with Hyperthreading. All others are 4 core without.
link
krylon 2927 days ago
Okay, that explains it. The only i5-based systems I have used (or still use) are notebooks.
link
berbec 2927 days ago
This changed with 8th-gen Intel. The low-powers are now dual-HT/quad/quad-HT i3/i5/i7 [1]/[2]/[3]

1: https://ark.intel.com/products/137977/Intel-Core-i3-8130U-Pr... 2: https://ark.intel.com/products/124969/Intel-Core-i5-8350U-Pr... 3: https://ark.intel.com/products/122589/Intel-Core-i7-8550U-Pr...

link
rythie 2926 days ago
Yes, you're right, I was thinking only about desktop CPUs, forgetting the laptop CPUs are different.
link