First of all, upgrade your Linux kerenl 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 open file descriptors.
Add these two lines
* soft nofile 51200 * hard nofile 51200
Then, before you start the shadowsocks server, make sure to set the ulimit first
ulimit -n 51200
Step 2, Optimize the kernel parameters
There are several kernel parameters needed to be tuned for shadowsocks. The priciples are
- Reuse ports and conections as soon as possible.
- Enlarge the queues and buffers as large as possible.
- Choose the TCP congestion algorithm for large latency and high throughput.
Here is an example
/etc/sysctl.conf of our production servers:
net.core.wmem_max = 12582912 net.core.rmem_max = 12582912 net.ipv4.tcp_rmem = 10240 87380 12582912 net.ipv4.tcp_wmem = 10240 87380 12582912 net.ipv4.ip_local_port_range = 18000 65535 net.ipv4.netfilter.ip_conntrack_tcp_timeout_time_wait = 1 net.ipv4.tcp_window_scaling = 1 net.ipv4.tcp_max_syn_backlog = 3240000 net.core.somaxconn = 3240000 net.ipv4.tcp_max_tw_buckets = 1440000 net.ipv4.tcp_congestion_control = hybla net.ipv4.tcp_tw_reuse = 1 net.ipv4.tcp_fin_timeout = 15 net.ipv4.tcp_syn_retries = 2 net.ipv4.tcp_synack_retries = 2 net.ipv4.tcp_tw_recycle = 1
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
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