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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).

Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.

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The Linux kernel has been accidentally hardcoded to a maximum of 8 cores for the past 15 years and nobody noticed (thehftguy.com)

submitted 11 months ago by [email protected] to c/[email protected]

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"The min_granularity setting was renamed to base_slice in this commit in v6 kernel.

The comment says it scales with CPU count and the comment is incorrect. I wonder whether kernel developers are aware of that mistake as they are rewriting the scheduler!

Official comments in the code says it’s scaling with log2(1+cores) but it doesn’t.
All the comments in the code are incorrect.
Official documentation and man pages are incorrect.
Every blog article, stack overflow answer and guide ever published about the scheduler is incorrect."

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[–] [email protected] 66 points 11 months ago* (last edited 11 months ago) (1 children)

Wish this BS article would stop getting posted everywhere.

This article is clickbait and in no way has the kernel been hardcoded to a maximum of 8 cores. If you read the commit [0], you can see, that a /certain/ scaling factor for scheduling can scale linearly or logarithmically with the number of cores and for calculating this scaling factor, the number is capped to 8. This has nothing to do with the number of cores that can actually be used.

[0] https://github.com/torvalds/linux/commit/acb4a848da821a095ae9e4d8b22ae2d9633ba5cd

[–] [email protected] 1 points 11 months ago (2 children)

unsigned int cpus = min(num_online_cpus(), 8);

doesn't that mean it's actually at least 8, as in if you have 4 cores cpus will be assigned 8, if you have 20 cores cpus will be assigned 20.

[–] [email protected] 5 points 11 months ago

No, I think min() returns the lower of two arguments. If you had 4 cores, min(4, 8) == 4, and if you had 20 cores, min(20, 8) == 8

[–] [email protected] 1 points 11 months ago

Yeah I was wondering this too - is 8 the floor, 8 cores or below the value is always the same, but above 8 cores you then get your log progression? I don't know enough about this though.