describe solution to xvfb problem

source/guides/guides/continuous-integration.md

@@ -482,12 +482,40 @@ return server.start()
 node scripts/run-cypress-tests.js
 ```
 
-# Known Issues
+# Common problems and solutions
 
 ## In Docker
 
 If you are running long runs on Docker, you need to set the `ipc` to `host` mode. {% issue 350 'This issue' %} describes exactly what to do.
 
+## Xvfb
+
+When running on Linux, Cypress needs an X11 server; otherwise it spawns its own X11 server during the test run. When running several Cypress instances in parallel, the spawning of multiple X11 servers at once can cause problems for some of them. In this case, you can separately start a single X11 server and pass the server's address to each Cypress instance using `DISPLAY` variable.
+
+First, spawn the X11 server in the background at some port, for example `:99`. If you have installed `xvfb` on Linux or if you are using one of our Docker images from {% url cypress-docker-images https://github.com/cypress-io/cypress-docker-images %}, the tools below should be available.
+
+```shell
+Xvfb :99 &
+```
+
+Second, set the X11 address in an environment variable
+
+```shell
+export DISPLAY=:99
+```
+
+Start headless Cypress as usual
+
+```shell
+npx cypress run
+```
+
+After all tests across all Cypress instances finish, kill the Xvfb background process using `pkill`
+
+```shell
+pkill Xvfb
+```
+
 # See also
 
 - {% url cypress-example-kitchensink https://github.com/cypress-io/cypress-example-kitchensink#ci-status %} is set up to run on multiple CI providers.