How Stack and Heap is allocated for threads under the same process in Windows

One of the main reasons for using threads even in a completely CPU-bound process is to allow interaction or update progress output via a UI of some sort. While running the calculation in the foreground without allowing interaction would be more efficient, the slight cost of task switching and handling UI events usually outweighs the penalty of forcing a user to wait indefinitely for a process.

Another case where running multiple threads on a single core would be more efficient is when a multi-stage process involves one stage that produces a lot of temporary results that are processed by a later stage. Running these stages in series might exhaust available memory. Running them in parallel allows the second stage to free up memory as it processes results.

Finally, as we add more caches to our architectures, threads can often become idle when a cache miss occurs while accessing memory. This gives some time for another thread to activate and do some work.

The main benefits of multi-threading over multi-processing include

• shared memory
• less overhead for context-switching
• easier abstraction

Most modern operating systems provide some method of sharing memory across processes, too. Even still, allowing the OS or virtual machine to shift threads across multiple processors when available is very appealing. For example, when you move a multi-threaded Java program from a single-core machine to one with multiple cores, the JVM will automatically make use of those other cores.

The link Robert Harvey added as a comment does explain this more explicitly, but, by default, Windows allocates 1MB of virtual memory which is committed as physical memory as needed (as you described).