Context Switch

CPU Time as a Shared Resource

Modern operating systems juggle dozens or hundreds of active threads.
Only a few CPU cores exist, so most threads wait in queues while a small subset runs.

A context switch lets the scheduler pause one running thread and resume another.
This rapid switching creates the illusion of parallelism and also shapes what you see later inside crash dumps.

Inside a Thread’s Execution Context

Each thread carries more than just code and data.
It also has a complete execution context that the kernel must save and restore correctly.

That context includes:

• General-purpose CPU registers and flags

• Instruction pointer and stack pointer

• Segment registers and control registers where relevant

• Kernel and user stacks, plus some scheduler metadata

When the kernel switches away from a thread, it must record this state.
When the scheduler returns to that same thread, it restores everything so execution continues as if nothing paused.

Steps the Kernel Takes During a Context Switch

At a low level, a context switch follows a predictable sequence.
Details change across architectures, but the core actions stay similar.

1. The running thread reaches a scheduling point, such as a timer interrupt or a blocking system call.

2. The kernel saves CPU registers, stack pointers, and some control information to the thread’s kernel stack or control block.

3. The scheduler chooses the next runnable thread based on priority, fairness, and processor affinity.

4. The kernel loads the saved context for that next thread, including registers and stack pointers.

5. Control returns to user or kernel mode in the new thread, which continues at its previous instruction pointer.

This sequence happens millions of times per second on busy systems.
Any bug in these steps can cause data corruption, hangs, or a crash that produces a dump file.

Triggers That Cause Context Switches

Context switches do not occur randomly.
They arise from well-defined events inside the OS and hardware.

Common triggers:

• The running thread blocks on I/O, a lock, or a wait object

• A hardware timer interrupt indicates the thread has used its time slice

• A higher-priority thread becomes ready to run

• The scheduler receives a signal to rebalance work across cores

Crash dumps capture whichever thread happened to run when a fatal condition occurred or when the kernel detected a serious inconsistency during one of these transitions.

Crash Dumps and Thread States

When Windows hits a bug check, it freezes scheduling and writes a snapshot of memory to disk.
That snapshot usually includes details about several threads and the contexts that caused or detected the error.

Crash dumps typically record:

• The active thread on each CPU at the moment of failure

• Kernel stacks that show which functions ran just before the crash

• Structures that describe ready queues and wait states

By examining those threads and their last context switch events, analysts infer which driver, system call, or user action contributed to the failure.

Performance and Reliability Considerations

Context switching enables multitasking but carries overhead.
The kernel must move between privilege levels, flush or reload some CPU state, and disturb cache contents.

Too many context switches can:

• Increase CPU time spent in the scheduler

• Hurt cache and TLB locality

• Reduce throughput for I/O intensive applications

Unstable drivers or misbehaving user processes can create extreme switching patterns, which sometimes appear in crash dumps as long stacks of wait and wake operations.
In severe cases, bugs during those transitions cause memory corruption or deadlocks.

Practical Use of Context Information When Analyzing Dumps