yes.asm

; nasm -f elf64 yes.asm
; gcc -o yes yes.o
; ./yes | pv -a > /dev/null

global main

section .text
main:
	mov rdi, 1
	mov rsi, msg
	mov rdx, msg.len

loop:
	mov rax, 1
	syscall
	jmp loop

section .data
msg: times 16384 db "y",10
.len: equ $ - msg

yes.c

// gcc -O3 -pipe -march=native -mtune=native yes.c -o yes
// ./yes | pv -a > /dev/null

#include <unistd.h>

#define LEN 1 << 15

int main() {
	char str[LEN];

	for (int i = 0; i < LEN; i += 2) {
		str[i] = 'y';
		str[i+1] = '\n';
	}

	while (write(1, str, LEN));
}

I fixed the asm version. Should work now.