需要注意的是在多进程模式下，需要设置accept_mutex off;
假设你已经会用keepalive模块，我们继续分析在fastcgi如何保持连接？
nginx连接fastcgi默认情况下，是这样的（即使你设置了上面keepalive）
[root@localhost ~]# tcpdump -i lo -s 1500 port 9000
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on lo, link-type EN10MB (Ethernet), capture size 1500 bytes
15:23:16.901004 IP localhost.localdomain.50867 > localhost.localdomain.9000: S 3482201970:3482201970(0) win 32767 <mss 16396,sackOK,timestamp 2296841391 0,nop,wscale 7>
15:23:16.901025 IP localhost.localdomain.9000 > localhost.localdomain.50867: S 3473410857:3473410857(0) ack 3482201971 win 32767 <mss 16396,sackOK,timestamp 2296841391 2296841391,nop,wscale 7>
15:23:16.901039 IP localhost.localdomain.50867 > localhost.localdomain.9000: . ack 1 win 256 <nop,nop,timestamp 2296841391 2296841391>
15:23:16.901150 IP localhost.localdomain.50867 > localhost.localdomain.9000: P 1:1377(1376) ack 1 win 256 <nop,nop,timestamp 2296841391 2296841391>
15:23:16.901170 IP localhost.localdomain.9000 > localhost.localdomain.50867: . ack 1377 win 256 <nop,nop,timestamp 2296841391 2296841391>
15:23:16.901214 IP localhost.localdomain.9000 > localhost.localdomain.50867: P 1:97(96) ack 1377 win 256 <nop,nop,timestamp 2296841391 2296841391>
15:23:16.901222 IP localhost.localdomain.50867 > localhost.localdomain.9000: . ack 97 win 256 <nop,nop,timestamp 2296841391 2296841391>
15:23:16.901236 IP localhost.localdomain.9000 > localhost.localdomain.50867: F 97:97(0) ack 1377 win 256 <nop,nop,timestamp 2296841391 2296841391>
15:23:16.901822 IP localhost.localdomain.50867 > localhost.localdomain.9000: F 1377:1377(0) ack 98 win 256 <nop,nop,timestamp 2296841392 2296841391>
15:23:16.901836 IP localhost.localdomain.9000 > localhost.localdomain.50867: . ack 1378 win 256 <nop,nop,timestamp 2296841392 2296841392>
可以看出是后端主动关闭了连接，所以直接在nginx.conf配置文件中设置keepalive无效。
查看ngx_http_fastcgi_module.c文件，发现ngx_http_fastcgi_begin_request_t数据结构中有flags字段，
这个字段用来控制后端是否要主动关闭连接，
102 typedef struct {
103 u_char role_hi;
104 u_char role_lo;
105 u_char flags; //通过这个来控制连接的关闭是否
106 u_char reserved[5];
107 } ngx_http_fastcgi_begin_request_t;
fastcgi协议说明如下：
Closing Transport Connections
The Web server controls the lifetime of transport connections. The Web server can close a connection when no requests are active. Or the Web server can delegate close authority to the application (see FCGI_BEGIN_REQUEST). In this case the application closes the connection at the end of a specified request.
再查看ngx_http_fastcgi_module.c文件中的设置：
475 static ngx_http_fastcgi_request_start_t ngx_http_fastcgi_request_start = {
476 { 1, /* version */
477 NGX_HTTP_FASTCGI_BEGIN_REQUEST, /* type */
478 0, /* request_id_hi */
479 1, /* request_id_lo */
480 0, /* content_length_hi */
481 sizeof(ngx_http_fastcgi_begin_request_t), /* content_length_lo */
482 0, /* padding_length */
483 0 }, /* reserved */
484
485 { 0, /* role_hi */
486 NGX_HTTP_FASTCGI_RESPONDER, /* role_lo */
487 0, /* NGX_HTTP_FASTCGI_KEEP_CONN */ /* flags */
488 { 0, 0, 0, 0, 0 } }, /* reserved[5] */
489
490 { 1, /* version */
491 NGX_HTTP_FASTCGI_PARAMS, /* type */
492 0, /* request_id_hi */
493 1 }, /* request_id_lo */
494
495 };
我们把487行的0改为1，确保后端不主动关闭connection。