multisocket plugin (#6882)

* multisocket plugin improves performance in multiprocessor systems

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* - refactoring
- update doc

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* remove port from reuseport plugin README

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* rename reuseport plugin to numsockets plugin

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* Add Recommendations to numsockets README

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* added numsockets test; made NUM_SOCKETS mandatory in doc

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* restart and whoami tests for numsockets plugin

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* default value for numsockets

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* caddy up

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* add numsockets to plugin.cfg

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* - rename numsockets plugin to multisocket
- default as GOMAXPROCS
- update README

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* resolve conflicts

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

---------

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

plugin/multisocket/README.md

@@ -0,0 +1,68 @@
+# multisocket
+
+## Name
+
+*multisocket* - allows to start multiple servers that will listen on one port.
+
+## Description
+
+With *multisocket*, you can define the number of servers that will listen on the same port. The SO_REUSEPORT socket 
+option allows to open multiple listening sockets at the same address and port. In this case, kernel distributes incoming 
+connections between sockets.
+
+Enabling this option allows to start multiple servers, which increases the throughput of CoreDNS in environments with a 
+large number of CPU cores.
+
+## Syntax
+
+~~~
+multisocket [NUM_SOCKETS]
+~~~
+
+* **NUM_SOCKETS** - the number of servers that will listen on one port. Default value is equal to GOMAXPROCS.
+
+## Examples
+
+Start 5 TCP/UDP servers on the same port.
+
+~~~ corefile
+. {
+	multisocket 5
+	forward . /etc/resolv.conf
+}
+~~~
+
+Do not define `NUM_SOCKETS`, in this case it will take a value equal to GOMAXPROCS.
+
+~~~ corefile
+. {
+	multisocket
+	forward . /etc/resolv.conf
+}
+~~~
+
+## Recommendations
+
+The tests of the `multisocket` plugin, which were conducted for `NUM_SOCKETS` from 1 to 10, did not reveal any side 
+effects or performance degradation.
+
+This means that the `multisocket` plugin can be used with a default value that is equal to GOMAXPROCS.
+
+However, to achieve the best results, it is recommended to consider the specific environment and plugins used in 
+CoreDNS. To determine the optimal configuration, it is advisable to conduct performance tests with different 
+`NUM_SOCKETS`, measuring Queries Per Second (QPS) and system load.
+
+If conducting such tests is difficult, follow these recommendations:
+1. Determine the maximum CPU consumption of CoreDNS server without `multisocket` plugin. Estimate how much CPU CoreDNS
+   actually consumes in specific environment under maximum load.
+2. Align `NUM_SOCKETS` with the estimated CPU usage and CPU limits or system's available resources.
+   Examples:
+   - If CoreDNS consumes 4 CPUs and 8 CPUs are available, set `NUM_SOCKETS` to 2.
+   - If CoreDNS consumes 8 CPUs and 64 CPUs are available, set `NUM_SOCKETS` to 8.
+
+## Limitations
+
+The SO_REUSEPORT socket option is not available for some operating systems. It is available since Linux Kernel 3.9 and 
+not available for Windows at all.
+
+Using this plugin with a system that does not support SO_REUSEPORT will cause an `address already in use` error.