AernaLingus, 1 year 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.