CODEWITHSUNDEEP
multithreading
Null
Null

Reputation: 414

Understanding how threads are implemented

I've been trying to understand the mechanics of implementing user-space threads. I'm not able to understand the mechanics of the stack and frames. I've come across two really great resources (here and here) that explain threading and how it's implemented but I still don't understand the following details:

  1. How does the machine context used in the thread execution? I know it consists of a stack pointer and bunch of register values. But how exactly does the OS use it to execute the thread?
  2. Why do we require a trampoline function (mctx_create_trampoline)? In the link #2, they setup a function as a signal handler that saves the machine context and starts the thread function (mctx_create_boot).
  3. Based on these function how does one implement a "yield" function that the threads can call? Also, how can we interrupt a running thread? I assume you'd have a timer and SIGALRM that calls a signal handler when it goes off. But if the yield function switches contexts then the signal handler wont return which would block further signals from being delivered.

Upvotes: 2

Views: 743

Answers (1)

usr
usr

Reputation: 171236

  1. Once a thread has been set off executing on a physical CPU the OS is no longer involved until either the time-slice expires or some other re-scheduling needs to take place. So the key question is: How are threads scheduled to physical CPUs? Well, the OS sets the physical CPU registers to the appropriate values and jumps to the location where the thread was last interrupted (which in effect sets the instruction pointer). At this point the OS has lost control and is no longer involved. It can only regain control if a hardware interrupt occurs or some other physical CPU core decides to take control of the CPU.
  2. Can't open that document right now.
  3. "yield" cannot be implemented in user-space. It generally is a kernel API that picks some other thread to schedule and schedules it to the current CPU.

Upvotes: 1

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