# 问题

在`Postgresql`并发测试时，我们发现当并发psql数比较大时，会报告以下类似错误：

```
psql: error: could not connect to server: Resource temporarily unavailable
        Is the server running locally and accepting
        connections on Unix domain socket "/tmp/.s.PGSQL.5404"?
```

搜索Postgresql的源码推断，“Resource temporarily unavailable”错误不是Postgresql数据库报的错，推断是操作系统拦截了psql拦截。

# 复现方法

测试用例如下：

先修改Postgresql的`max_conections`参数值为500，并重启Postgresql服务：

```
max_conections = 500
```

编写测试脚本（test.sh）：

```
for ((i=1; i<300; i++)); do
        psql -p 5404 postgres -c "select pg_sleep(2);" &
done
```

执行脚本可能会报以下错误：

```
[yz@bogon ~]$ ./test.sh
psql: error: could not connect to server: Resource temporarily unavailable
        Is the server running locally and accepting
        connections on Unix domain socket "/tmp/.s.PGSQL.5404"?
psql:psql: error: could not connect to server: Resource temporarily unavailable
        Is the server running locally and accepting
        connections on Unix domain socket "/tmp/.s.PGSQL.5404"?
……
```

# 原因与解决

经查内核`net.core.somaxconn`值太小导致的（默认128）。

```
[yz@bogon ~]$ sysctl -a | grep somaxconn
net.core.somaxconn = 128
```

调大`net.core.somaxconn`值并重启Postgresql服务（**注意：修改该参数后务必重启服务**）。

```
[yz@bogon ~]$ sudo sysctl -w net.core.somaxconn=1024
net.core.somaxconn = 1024
[yz@bogon ~]$ sudo sysctl -a | grep somaxconn
net.core.somaxconn = 1024
```

`net.core.somaxconn`内核参数控制一个socket端口最大监听队列长度，因为上述测试中，psql连接的是同一个端口，并发数远远高于128，大概率会造成该队列超限。以下引自[华为云文档](https://support.huaweicloud.com/intl/en-us/bestpractice-cce/cce_bestpractice_00225.html)：

> `net.core.somaxconn` indicates the maximum number of half-open connections that can be backlogged in a listening queue. The default value is 128. If the queue is overloaded, you need to increase the listening queue length.