Why Multitasking on Windows Was So Hard (and How It Finally Worked)

Summary

In this engaging walkthrough, retired Microsoft engineer Dave Plamer takes viewers on a journey through the evolution of multitasking in computing, from 1960s punch card batch processing to modern multi-core, real-time systems. He explains the shift from inefficient single-job execution to time-sharing, preemptive multitasking, and the challenges of concurrent programming. Highlighting milestones like Unix, the Commodore Amiga, Windows 95, and Windows NT, Dave illustrates how operating systems manage threads, context switching, and scheduling. He concludes by exploring real-time OS applications in drones, medical devices, and industrial systems, showing how multitasking has transformed computing from CPU efficiency to seamless, parallel experiences for users.

Takeaways

• 🖥️ Early computing relied on batch processing, where jobs were executed one at a time with no interaction or immediate feedback.
• ⏱️ CPU efficiency was crucial in early systems; idle CPU time during slow operations was considered wasteful and expensive.
• ⌛ Time-sharing systems allowed multiple users to share CPU time, creating the illusion of having their own dedicated machine.
• 🐧 Unix popularized key multitasking concepts like processes, signals, input/output redirection, and schedulers.
• 🔄 Preemptive multitasking uses hardware timer interrupts and context switching to run multiple processes seemingly in parallel.
• 💾 Early personal computers like Apple II and IBM PC lacked multitasking, while the Commodore Amiga introduced preemptive multitasking on PCs.
• 🖱️ Cooperative multitasking in early Windows required programs to voluntarily yield CPU time, often causing system freezes if mismanaged.
• ⚡ Windows 95 introduced a hybrid model, mixing preemptive multitasking for 32-bit apps with cooperative multitasking for 16-bit apps, creating subtle bugs.
• 🛡️ Windows NT brought a fully preemptive, protected-memory OS with thread-level scheduling, asynchronous I/O, and multi-core support.
• 🕹️ Multi-core and multi-threaded systems introduce concurrency challenges like race conditions, deadlocks, and priority inversion.
• ⏲️ Real-time operating systems (RTOS) prioritize predictability over speed, ensuring tasks execute within strict time constraints for applications like drones, medical devices, and automotive systems.
• 🌐 Modern multitasking allows hundreds of applications to run simultaneously, creating the illusion of parallel execution while managing shared CPU resources efficiently.

Q & A

Who is Dave Palmer and what is his background?

Dave Palmer is a retired operating systems engineer from Microsoft, with experience going back to the MS-DOS and Windows 95 era.

What was the primary method of running programs in the early days of computing?

In the early days, programs were run using batch processing, where users submitted jobs via punch cards or magnetic tape and waited for the computer to execute them sequentially.

What problem did batch processing create for CPU efficiency?

Batch processing was wasteful because the CPU often sat idle while waiting for slow devices like tape drives or printers, even though CPU time was extremely expensive.

How did time-sharing improve computer usability?

Time-sharing allowed multiple users to log in concurrently, each getting a small slice of CPU time, creating the illusion of having a dedicated machine while keeping the CPU efficiently utilized.

What is preemptive multitasking and how does it work?

Preemptive multitasking uses hardware timer interrupts to suspend the current process, save its state, and allow the scheduler to run another process. This context switching is transparent to the programs, allowing multiple tasks to appear to run in parallel.

What was significant about the Commodore Amiga regarding multitasking?

The Amiga, released in 1985, had a preemptive multitasking operating system called AmigaDOS, which allowed simultaneous execution of tasks like dragging windows, copying files, and playing music, all with only 512K of RAM.

How did early versions of Windows handle multitasking?

Early Windows versions like Windows 3.x used cooperative multitasking, where applications had to voluntarily yield control of the CPU. Windows 95 introduced a hybrid model, preemptive for 32-bit apps but cooperative for 16-bit apps.

What features did Windows NT introduce to improve multitasking?

Windows NT introduced full 32-bit preemptive multitasking, thread-level scheduling, asynchronous I/O, memory-mapped files, kernel object models, and support for multiple processors running threads in parallel.

What challenges arise when multiple CPU cores execute threads simultaneously?

Concurrency issues like race conditions, priority inversion, deadlocks, and false sharing occur when multiple threads access shared memory at the same time, making multi-threaded programming complex.

What defines a real-time operating system, and where are they used?

Real-time operating systems prioritize predictability over speed, ensuring tasks execute within fixed time windows. They are used in drones, medical devices, automotive ECUs, avionics, and industrial robots where timing is critical.

Why is context switching important for multitasking?

Context switching allows the CPU to save the state of one process and load the state of another, enabling multiple applications to run seemingly simultaneously and ensuring efficient CPU utilization.

How did backward compatibility affect Windows 95 multitasking?

Backward compatibility required Windows 95 to support older 16-bit cooperative apps alongside 32-bit preemptive apps. This hybrid model sometimes caused subtle bugs and system instability when memory or GDI resources were low.