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[[!meta title="I heard that context-switch on Mach is really slow?"]]

It is not, there is no real reason why it would be particularly slow, it is just
about switching virtual addresses and registers, which all OS have to perform
anyway.

A quick-and-dirty benchmark:

        #include <fcntl.h>
        #include <semaphore.h>
        #include <stdio.h>
        #include <time.h>
        #include <unistd.h>
        #include <sys/mman.h>
        
        sem_t *sem1, *sem2;
        
        void worker1(void) {
        	time_t last;
        	int n = 0;
        	last = time(NULL);
        	while(1) {
        		time_t new = time(NULL);
        		if (new != last) {
        			printf("%d\n", n);
        			n = 0;
        			last = new;
        		}
        		n++;
        		sem_wait(sem1);
        		sem_post(sem2);
        	}
        }
        
        void worker2(void) {
        	while(1) {
        		sem_post(sem1);
        		sem_wait(sem2);
        	}
        }
        
        int fd;
        void get_sems(void) {
        	void *ptr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
        	sem1 = ptr;
        	sem2 = sem1+1;
        }
        
        int main(void) {
        	fd = open("/tmp/foo", O_CREAT|O_TRUNC|O_RDWR, 0666);
        	ftruncate(fd, 4096);
        
        	get_sems();
        	sem_init(sem1, 1, 0);
        	sem_init(sem2, 1, 0);
        
        	if (fork())
        		worker1();
        	else {
        		get_sems();
        		worker2();
        	}
        }

run on my current Linux system (a Core i5-10210U), gets about 300k switches per second on Linux. Running it on Hurd-in-kvm (which would supposedly be slower) gets about 400k switches per second.