Threads: Who Am I?

I was curious as to what the cost is of retrieving "thread local storage" (per-thread variables). I am looking to move some variables from globals to per-thread, and I want to know how expensive access will be.

I went digging into the Linux thread library source. First: pthread_getspecific (which pulls out a specific variable) works by:

1. Finding the thread's "identity" as a pointer to a basic control block.
2. Thread-local storage is simply an array at the end of the control block.
3. The specific retrieval is just an array access.

So specific key retrieval isn't going to any worse than getting the thread itself. How does Linux do that?

In a way that I thought was quite clever: stacks are stored on a fixed granularity/page size. The calling thread has a stack pointer within its own stack. The thread control block is at the very beginning of the stack.

So all the calling code has to do is take a stack-local variable and round its address down to the thread stack granularity and there's the control block. That's pretty quick! No system calls needed.

Edit: here's an even cheaper way.