理解Keepalive(1)大家都听过keepalive，但是其实对于keepalive这个词还是很晦涩的，至少我一直都只知道一个大概，直到之前排查线上一些问题，发现keepalive还是有很多玄机的。其实keepalive有两种，一种是TCP层的keepalive，另一种是HTTP层的Keep-Alive。这篇文章先说说tcp层的keepalive tcp keepalive设想有一种场景：A和B两边通过三次握手建立好TCP连接，然后突然间B就宕机了，之后时间内B再也没有起来。如果B宕机后A和B一直没有数据通信的需求，A就永远都发现不了B已经挂了，那么A的内核里还维护着一份关于A&B之间TCP连接的信息，浪费系统资源。于是在TCP层面引入了keepalive的机制，A会定期给B发空的数据包，通俗讲就是心跳包，一旦发现到B的网络不通就关闭连接。这一点在LVS内尤为明显，因为LVS维护着两边大量的连接状态信息，一旦超时就需要释放连接。 Linux内核对于tcp keepalive的调整主要有以下三个参数 1. tcp_keepalive_time
the interval between the last data packet sent (simple ACKs are not considered data) and the first keepalive probe; after the connection is marked to need keepalive, this counter is not used any further
2. tcp_keepalive_intvl
the interval between subsequential keepalive probes, regardless of what the connection has exchanged in the meantime
3. tcp_keepalive_probes
the number of unacknowledged probes to send before considering the connection dead and notifying the application layer
Example $ cat /proc/sys/net/ipv4/tcp_keepalive_time
7200
$ cat /proc/sys/net/ipv4/tcp_keepalive_intvl
75
$ cat /proc/sys/net/ipv4/tcp_keepalive_probes
9
当tcp发现有tcp_keepalive_time(7200)秒未收到对端数据后，开始以间隔tcp_keepalive_intvl(75)秒的频率发送的空心跳包，如果连续tcp_keepalive_probes(9)次以上未响应代码对端已经down了，close连接
在socket编程时候，可以调用setsockopt指定不同的宏来更改上面几个参数 TCP_KEEPCNT: tcp_keepalive_probes
TCP_KEEPIDLE: tcp_keepalive_time
TCP_KEEPINTVL: tcp_keepalive_intvl
Nginx配置tcp keepaliveNginx对于keepalive的配置有一大堆，大伙每次看都迷茫了，其实Nginx涉及到tcp层面的keepalive只有一个：so_keepalive。它属于listen指令的配置参数，具体配置 so_keepalive=on|off|[keepidle]:[keepintvl]:[keepcnt]
so_keepalive=30m::10
will set the idle timeout (TCP_KEEPIDLE) to 30 minutes, leave the probe interval (TCP_KEEPINTVL) at its system default, and set the probes count (TCP_KEEPCNT) to 10 probes.
在Nginx的代码里可以看到 ./src/http/ngx_http_core_module.c
static ngx_command_t ngx_http_core_commands[] = {
...
// listen 指令解析 -->> call ngx_http_core_listen()
{ ngx_string('listen'),
NGX_HTTP_SRV_CONF|NGX_CONF_1MORE,
ngx_http_core_listen,
NGX_HTTP_SRV_CONF_OFFSET,
0,
NULL },
...
}
static char *
ngx_http_core_listen(ngx_conf_t *cf, ngx_command_t *cmd, void *conf){
...
// 下面就是 so_keepalive 后面的参数解析
if (ngx_strncmp(value[n].data, 'so_keepalive=', 13) == 0) {
if (ngx_strcmp(&value[n].data[13], 'on') == 0) {
lsopt.so_keepalive = 1;
} else if (ngx_strcmp(&value[n].data[13], 'off') == 0) {
lsopt.so_keepalive = 2;
} else {
// 自定义系统keepalive的相关设置
...
}
if (ngx_http_add_listen(cf, cscf, &lsopt) == NGX_OK) {
return NGX_CONF_OK;
}
}
./src/core/ngx_connection.c
if (ls[i].keepidle) {
value = ls[i].keepidle;
// 设置 tcp_keepalive_time
if (setsockopt(ls[i].fd, IPPROTO_TCP, TCP_KEEPIDLE,
(const void *) &value, sizeof(int))
== -1)
{
ngx_log_error(NGX_LOG_ALERT, cycle->log, ngx_socket_errno,
'setsockopt(TCP_KEEPIDLE, %d) %V failed, ignored',
value, &ls[i].addr_text);
}
}
if (ls[i].keepintvl) {
value = ls[i].keepintvl;
// 设置 tcp_keepalive_intvl
if (setsockopt(ls[i].fd, IPPROTO_TCP, TCP_KEEPINTVL,
(const void *) &value, sizeof(int))
== -1)
{
ngx_log_error(NGX_LOG_ALERT, cycle->log, ngx_socket_errno,
'setsockopt(TCP_KEEPINTVL, %d) %V failed, ignored',
value, &ls[i].addr_text);
}
}
if (ls[i].keepcnt) {
// 设置 tcp_keepalive_intvl
if (setsockopt(ls[i].fd, IPPROTO_TCP, TCP_KEEPCNT,
(const void *) &ls[i].keepcnt, sizeof(int))
== -1)
{
ngx_log_error(NGX_LOG_ALERT, cycle->log, ngx_socket_errno,
'setsockopt(TCP_KEEPCNT, %d) %V failed, ignored',
ls[i].keepcnt, &ls[i].addr_text);
}
}
总结这篇文章说了TCP层面的keepalive相关知识以及Nginx的支持tcp keepalive的配置。tcp层面的keepalive存在更多意义上是为了检测两端连接是否正常，注重点是在于连接的本身！要和HTTP层面的keepaplive区分开来，明白这点很重要。 |
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来自： waitingnothing > 《Nginx》
