tcp_rmem - 取值是：3个整数向量: min, default, max

min

TCP sockets使用的最小接收缓存大小。这个最小值的作用是在，内存处在轻微压力下时，也会给socket分配的内存大小。
Default: 4K

default：

TCP sockets 初始时分配的接收缓存大小。这个值会覆盖net.core.rmem_default这个值。 默认是：87380 bytes。这个值也是计算窗口的输入值，This value results in window of 65535 with default setting of tcp_adv_win_scale and tcp_app_win:0 and a bit less for default tcp_app_win. See below about these variables.

max

允许TCP socket自动选择接收缓存的最大值。不会覆盖net.core.rmem_max。调用setsockopt()设置SO_RCVBUF会将自动调整缓存大小的方法关闭，这样这个值就没有任何作用了。默认值，在87380字节到6M之间，具体值依赖于RAM的大小。

rmem_default

The default setting of the socket receive buffer in bytes.

rmem_max

The maximum receive socket buffer size in bytes.

具体的用法

创建socket一开始的recv buff的大小是rmem_default；后续tcp层会重新设置这个receivebuffer到tcp_rmem:default;
然后，socket的用户标记为非SOCK_RCVBUF_LOCK；就会调用tcp_fixup_rcvbuf函数进行接收缓存大小的重新计算。
sk->sk_userlocks |= SOCK_RCVBUF_LOCK;

相关的方法

void sock_init_data(struct socket *sock, struct sock *sk)
{
	sk_init_common(sk);
	sk->sk_send_head	=	NULL;

	timer_setup(&sk->sk_timer, NULL, 0);

	sk->sk_allocation	=	GFP_KERNEL;
	sk->sk_rcvbuf		=	sysctl_rmem_default;
	
	
{
	.procname	= "rmem_default",
	.data		= &sysctl_rmem_default,
	.maxlen		= sizeof(int),
	.mode		= 0644,
	.proc_handler	= proc_dointvec_minmax,
	.extra1		= &min_rcvbuf,
},



/* 3. Tuning rcvbuf, when connection enters established state. */
static void tcp_fixup_rcvbuf(struct sock *sk)
{
	u32 mss = tcp_sk(sk)->advmss;
	int rcvmem;

	rcvmem = 2 * SKB_TRUESIZE(mss + MAX_TCP_HEADER) *
		 tcp_default_init_rwnd(mss);

	/* Dynamic Right Sizing (DRS) has 2 to 3 RTT latency
	 * Allow enough cushion so that sender is not limited by our window
	 */
	if (sock_net(sk)->ipv4.sysctl_tcp_moderate_rcvbuf)
		rcvmem <<= 2;

	if (sk->sk_rcvbuf < rcvmem)
		sk->sk_rcvbuf = min(rcvmem, sock_net(sk)->ipv4.sysctl_tcp_rmem[2]);
}





	case SO_RCVBUF:
		/* Don't error on this BSD doesn't and if you think
		 * about it this is right. Otherwise apps have to
		 * play 'guess the biggest size' games. RCVBUF/SNDBUF
		 * are treated in BSD as hints
		 */
		val = min_t(u32, val, sysctl_rmem_max);
set_rcvbuf:
		/* Ensure val * 2 fits into an int, to prevent max_t()
		 * from treating it as a negative value.
		 */
		val = min_t(int, val, INT_MAX / 2);
		sk->sk_userlocks |= SOCK_RCVBUF_LOCK;
		/*
		 * We double it on the way in to account for
		 * "struct sk_buff" etc. overhead.   Applications
		 * assume that the SO_RCVBUF setting they make will
		 * allow that much actual data to be received on that
		 * socket.
		 *
		 * Applications are unaware that "struct sk_buff" and
		 * other overheads allocate from the receive buffer
		 * during socket buffer allocation.
		 *
		 * And after considering the possible alternatives,
		 * returning the value we actually used in getsockopt
		 * is the most desirable behavior.
		 */
		sk->sk_rcvbuf = max_t(int, val * 2, SOCK_MIN_RCVBUF);