To  achieve stability and low-latency the driver needs to process the audio  in real-time. While the driver itself is not CPU-intensive, it needs to  be scheduled for execution by the OS as soon as data is available. The  same applies for the DAWs. In order to guarantee that latency-sensitive  applications are scheduled as soon as possible, they run with high (or  even real-time) process and thread priority.
Windows provides a special service for multimedia applications – the  Multimedia Class Schedule Service (or MMCSS for short). Instead of  dealing with scheduling priorities manually, each thread in the  application can register itself to the MMCSS, and the service  automatically raises the thread priority. Most of the time this works  great. However, there is a catch – if the thread exceeds a given time  quota, the service drastically lowers the thread priority in order to  allow background tasks to run. This means that if you’re using lower  buffer sizes and there is another task running in the background, the  MMCSS might decide to drop the priority of the DAW and driver threads,  the background task will preempt them, and you’ll hear crackling.
    Different DAWs behave differently – some of them register with the  MMCSS, other manually manage the scheduling priority. We provide a  checkbox in the ASIO Settings Control Panel to select whether or not the  driver thread itself should register with the MMCSS. It should match  the behavior of the DAW.
    While it would be best if no background tasks are running during your  session, you can’t always guarantee that this is the case. In case you  are experiencing crackling due to background tasks taking over the CPU,  try toggling the MMCSS registration checkbox in the ASIO Settings  Control Panel, or try disabling the MMCSS altogether and manually set  the DAW to real-time priority. To disable MMCSS altogether
run “regedit”, go to Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MMCSS , change the value of “Start” to 4,  restart the computer, this will  permanently disable the service.

  

We’re  communicating the negative effects of the MMCSS with the maintainers of  ASIO Standard (Steinberg) and we’re hoping that more drivers and DAWs  can make the MMCSS registration optional.