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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 nearly 20 years (thehftguy.com)

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

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I'm not smart enough to verify the accuracy of this claim, nor exactly what the implications are, but it seems like it might improve performance if fixed.

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[–] [email protected] 18 points 11 months ago

There's a variable that contains the number of cores (called cpus) which is hardcoded to max out at 8, but it doesn't mean that cores aren't utilized beyond 8 cores--it just means that the scheduling scaling factor will not change in either the linear or logarithmic case once you go above that number:

code snippet

/*
 * Increase the granularity value when there are more CPUs,
 * because with more CPUs the 'effective latency' as visible
 * to users decreases. But the relationship is not linear,
 * so pick a second-best guess by going with the log2 of the
 * number of CPUs.
 *
 * This idea comes from the SD scheduler of Con Kolivas:
 */
static unsigned int get_update_sysctl_factor(void)
{
	unsigned int cpus = min_t(unsigned int, num_online_cpus(), 8);
	unsigned int factor;

	switch (sysctl_sched_tunable_scaling) {
	case SCHED_TUNABLESCALING_NONE:
		factor = 1;
		break;
	case SCHED_TUNABLESCALING_LINEAR:
		factor = cpus;
		break;
	case SCHED_TUNABLESCALING_LOG:
	default:
		factor = 1 + ilog2(cpus);
		break;
	}

	return factor;
}

The core claim is this:

It’s problematic that the kernel was hardcoded to a maximum of 8 cores (scaling factor of 4). It can’t be good to reschedule hundreds of tasks every few milliseconds, maybe on a different core, maybe on a different die. It can’t be good for performance and cache locality.

On this point, I have no idea (hope someone more knowledgeable will weigh in). But I'd say the headline is misleading at best.