Eliminate Excessive MySQL TIME_WAIT Connections with Simple Kernel Tweaks

Problem Identification

When logging into the server, running netstat -an|grep mysql reveals many connections stuck in the TIME_WAIT state, which can consume resources and degrade MySQL performance.

Linux Solution: Adjust Kernel Parameters

Edit /etc/sysctl.conf and add the following lines:

net.ipv4.tcp_syncookies = 1
net.ipv4.tcp_tw_reuse = 1
net.ipv4.tcp_tw_recycle = 1
net.ipv4.tcp_fin_timeout = 30

Then apply the changes with /sbin/sysctl -p. The parameters mean:

net.ipv4.tcp_syncookies = 1 : Enables SYN cookies to protect against SYN‑queue overflow.

net.ipv4.tcp_tw_reuse = 1 : Allows reuse of TIME_WAIT sockets for new connections.

net.ipv4.tcp_tw_recycle = 1 : Enables faster recycling of TIME_WAIT sockets.

net.ipv4.tcp_fin_timeout = 30 : Reduces the default FIN timeout to 30 seconds.

After the change, verify the reduction with netstat -ae|grep "TIME_WAIT" | wc -l. The number of TIME_WAIT sockets should drop dramatically, and MySQL’s CPU usage returns to normal.

Common Application‑Level Cause

Often a large number of TIME_WAIT sockets appear because application code does not properly close MySQL connections (missing mysql.close()), leaving sockets in the wait state.

Windows Alternative

On Windows servers, modify the registry key:

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters]
"TcpTimedWaitDelay"=dword:0000001e

This sets the maximum TIME_WAIT duration to 30 seconds (default is 240 seconds).

Understanding TIME_WAIT

TIME_WAIT lasts for twice the Maximum Segment Lifetime (MSL), typically 240 seconds. Its purpose is to ensure that delayed packets from a closed connection do not interfere with new connections on the same port. Reducing this interval speeds up port reuse but must be done carefully.

When many connections enter TIME_WAIT simultaneously, each connection’s TCP Control Block remains allocated, potentially exhausting memory.