理解Keepalive(1)大家都聽過keepalive，但是其實(shí)對于keepalive這個詞還是很晦澀的，至少我一直都只知道一個大概，直到之前排查線上一些問題，發(fā)現(xiàn)keepalive還是有很多玄機(jī)的。其實(shí)keepalive有兩種，一種是TCP層的keepalive，另一種是HTTP層的Keep-Alive。這篇文章先說說tcp層的keepalive tcp keepalive設(shè)想有一種場景：A和B兩邊通過三次握手建立好TCP連接，然后突然間B就宕機(jī)了，之后時間內(nèi)B再也沒有起來。如果B宕機(jī)后A和B一直沒有數(shù)據(jù)通信的需求，A就永遠(yuǎn)都發(fā)現(xiàn)不了B已經(jīng)掛了，那么A的內(nèi)核里還維護(hù)著一份關(guān)于A&B之間TCP連接的信息，浪費(fèi)系統(tǒng)資源。于是在TCP層面引入了keepalive的機(jī)制，A會定期給B發(fā)空的數(shù)據(jù)包，通俗講就是心跳包，一旦發(fā)現(xiàn)到B的網(wǎng)絡(luò)不通就關(guān)閉連接。這一點(diǎn)在LVS內(nèi)尤為明顯，因?yàn)長VS維護(hù)著兩邊大量的連接狀態(tài)信息，一旦超時就需要釋放連接。 Linux內(nèi)核對于tcp keepalive的調(diào)整主要有以下三個參數(shù) 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
當(dāng)tcp發(fā)現(xiàn)有tcp_keepalive_time(7200)秒未收到對端數(shù)據(jù)后，開始以間隔tcp_keepalive_intvl(75)秒的頻率發(fā)送的空心跳包，如果連續(xù)tcp_keepalive_probes(9)次以上未響應(yīng)代碼對端已經(jīng)down了，close連接
在socket編程時候，可以調(diào)用setsockopt指定不同的宏來更改上面幾個參數(shù) TCP_KEEPCNT: tcp_keepalive_probes
TCP_KEEPIDLE: tcp_keepalive_time
TCP_KEEPINTVL: tcp_keepalive_intvl
Nginx配置tcp keepaliveNginx對于keepalive的配置有一大堆，大伙每次看都迷茫了，其實(shí)Nginx涉及到tcp層面的keepalive只有一個：so_keepalive。它屬于listen指令的配置參數(shù)，具體配置 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的代碼里可以看到 ./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 后面的參數(shù)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 {
// 自定義系統(tǒng)keepalive的相關(guān)設(shè)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;
// 設(shè)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;
// 設(shè)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) {
// 設(shè)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);
}
}
總結(jié)這篇文章說了TCP層面的keepalive相關(guān)知識以及Nginx的支持tcp keepalive的配置。tcp層面的keepalive存在更多意義上是為了檢測兩端連接是否正常，注重點(diǎn)是在于連接的本身！要和HTTP層面的keepaplive區(qū)分開來，明白這點(diǎn)很重要。 |
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