why stack size for the main thread is 1mb in iOS?

The stack size for the main thread in iOS is set to 1MB primarily due to the nature of stack memory allocation and its usage patterns. Stack memory is used for local variables and function call frames, which are typically short-lived and scoped to the function's execution. This makes stack memory very efficient for these purposes, as it can be allocated and deallocated quickly by simply adjusting the stack pointer.

In the session Explore Swift performance, it is explained that allocating space on the stack can be done by just subtracting from the stack pointer, which is a very fast operation. The stack size is kept relatively small (1MB) to ensure that the system can manage multiple threads efficiently without running into memory pressure issues. This size is a balance between providing enough space for typical function call frames and local variables while avoiding excessive memory usage that could impact the overall performance and stability of the system.

For more details on memory allocation and stack usage, you can refer to the chapter on Memory allocation in the "Explore Swift performance" session.