Varnish前面的Nginx有时会产生“上游过早关闭连接,同时从上游读

我在同一台服务器上的Varnish(4.1.0)前面有Nginx(1.9.9).

//nginx
upstream varnish {
    server 127.0.0.1:8391;
    keepalive 16;
}

location ~ .php${
     proxy_pass http://varnish;
     proxy_http_version 1.1; #for 1.0 varnish shows blank page
     proxy_set_header Connection "";
     proxy_redirect off;

     proxy_set_header Host $host:$server_port;
     proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
     proxy_set_header X-Forwarded-Proto $scheme;

     proxy_read_timeout 600;
     proxy_send_timeout 600;
     proxy_connect_timeout 600;
}

//varnish
DAEMON_OPTS="-a :8391 
             -T localhost:6082 
             -f /etc/varnish/default.vcl 
             -S /etc/varnish/secret 
             -s malloc,1024m"

对于0.001％的请求,nginx显示错误：

[error] 5331#5331: *7392847 upstream prematurely closed connection while reading response header from upstream,client: xx.xx.xx.xx,server: _,request: "GET /home HTTP/1.1",upstream: "http://127.0.0.1:8391/index.php?q=/home",host: "xxx",referrer: "xxx"

使用proxy_buffers没有帮助.