Clients cause the WSUS App Pool to become unresponsive with HTTP 503 on Win2012 R2

1. On your WSUS Server, launch the IIS Manager
2. Open Application Pools
3. Right click 'WsusPool' and select 'Advanced Settings...'
4. To support the maximum SCCM Software Update Point clients, change 'Queue Length' from the default 1,000 to 25,000
   
5. If your server is NUMA aware, change 'Maximum Worker Processes' from the default 1 to 0. If you don't know if your server is NUMA aware, leave this value default
   
6. Change '"Service Unavailable" Response Type' from the default HttpLevel to TcpLevel
7. Change 'Failure Interval (minutes) from the default 5 to 30
8. Change 'Maximum Failures' from the default 5 to 60
   
9. Click 'OK' to save the App Pool changes
10. From an administrative command prompt, type IISRESET

Your clients should now be able to check-in to the Software Update Point / WSUS service.