377 questions for senior .NET engineer interviews, each with a 30-second answer, the full answer, a deep dive, the follow-ups interviewers ask next, and real-world production problems.
The kernel, user and kernel mode, system calls, processes and threads, scheduling, context switching, virtual memory, paging, page faults, file descriptors and handles.
The kernel's responsibilities, CPU privilege levels (rings on x64, exception levels on Arm64), why applications run in user mode, how and when the CPU switches to kernel mode, and what that means for .NET applications, drivers, containers and security.
The system call mechanism, the path from .NET APIs through SafeHandles and the PAL to Windows (IOCP) and Linux (epoll) interfaces, the cost of crossing into the kernel, and how buffering, batching and async I/O reduce that cost. Includes tracing system calls with strace and Process Monitor.
What a process owns (address space, handles, security context) versus what a thread owns (stack, registers, thread-local storage), how threads share memory, how .NET threads map to OS threads, how the thread pool and Task build on them, and when to use multiple processes instead of threads.
Preemptive scheduling, run queues, time slices, priorities and starvation, how Windows and Linux schedulers differ at a high level, CPU affinity and NUMA, and how cgroup CPU quotas throttle containers and interact with the .NET thread pool and GC.
What the OS saves and restores on a context switch, thread versus process switches, direct and indirect costs (cache and TLB pollution), voluntary versus involuntary switches, how blocking code and oversized thread counts multiply them, and how to measure and reduce them.
Why virtual memory exists, pages and page frames, multi-level page tables, the TLB, demand paging, copy-on-write, memory-mapped files, swap and the page file, overcommit, address space layout randomisation, and what all this means for .NET processes.
Page faults, minor vs major faults, what triggers them, and how to diagnose paging-related latency in a .NET service.
File descriptors and handles as the OS's per-process references to kernel resources, their limits, and how .NET wraps them with SafeHandle and IDisposable.
Diagnosing descriptor exhaustion and socket exhaustion in .NET on Linux: ulimit, lsof, ss, TIME_WAIT, ephemeral ports, and the HttpClient/IHttpClientFactory fixes.