Advanced

Optimize the shadowsocks server on Linux

First of all, upgrade your Linux kernel to 3.5 or later.

Step 1, increase the maximum number of open file descriptors

To handle thousands of concurrent TCP connections, we should increase the limit of file descriptors opened.

Edit the limits.conf

vi /etc/security/limits.conf

Add these two lines

* soft nofile 51200
* hard nofile 51200

Then, before you start the shadowsocks server, set the ulimit first

ulimit -n 51200

Step 2, Tune the kernel parameters

The priciples of tuning parameters for shadowsocks are

1. Reuse ports and conections as soon as possible.
2. Enlarge the queues and buffers as large as possible.
3. Choose the TCP congestion algorithm for large latency and high throughput.

Here is an example /etc/sysctl.conf of our production servers:

fs.file-max = 51200

net.core.rmem_max = 67108864
net.core.wmem_max = 67108864
net.core.netdev_max_backlog = 250000
net.core.somaxconn = 4096

net.ipv4.tcp_syncookies = 1
net.ipv4.tcp_tw_reuse = 1
net.ipv4.tcp_tw_recycle = 0
net.ipv4.tcp_fin_timeout = 30
net.ipv4.tcp_keepalive_time = 1200
net.ipv4.ip_local_port_range = 10000 65000
net.ipv4.tcp_max_syn_backlog = 8192
net.ipv4.tcp_max_tw_buckets = 5000
net.ipv4.tcp_fastopen = 3
net.ipv4.tcp_mem = 25600 51200 102400
net.ipv4.tcp_rmem = 4096 87380 67108864
net.ipv4.tcp_wmem = 4096 65536 67108864
net.ipv4.tcp_mtu_probing = 1
net.ipv4.tcp_congestion_control = hybla

Of course, remember to execute sysctl -p to reload the config at runtime.

How to verify your optimizations work

Use munin or any server monitor tools to generate the graph of your TCP connections. A well tuned server should look like this