Windows® Internals, Sixth Edition, Part 1

System Worker Threads

During system initialization, Windows creates several threads in the System process, called system worker threads, which exist solely to perform work on behalf of other threads. In many cases, threads executing at DPC/dispatch level need to execute functions that can be performed only at a lower IRQL. For example, a DPC routine, which executes in an arbitrary thread context (because DPC execution can usurp any thread in the system) at DPC/dispatch level IRQL, might need to access paged pool or wait for a dispatcher object used to synchronize execution with an application thread. Because a DPC routine can’t lower the IRQL, it must pass such processing to a thread that executes at an IRQL below DPC/dispatch level.

Some device ...