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c++11boostspinlock

Spinlock implementation reasoning

I want to improve the performance of a program by replacing some of the mutexes with spinlocks. I have found a spinlock implementation in

which I intend to reuse. I believe this implementation is safer than simpler implementations in which threads keep trying forever like the one found here

But i need to clarify some things on the yield function found here

First of all I can assume that the numbers 4,16,32 are arbitrary. I actually tested some other values and I have found that I got best performance in my case by using other values.

But can someone explain the reasoning behind the yield code. Specifically why do we need all three

  • BOOST_SMT_PAUSE
  • sched_yield and
  • nanosleep
Solution

  • Yes, this concept is known as "adaptive spinlock" - see e.g. https://lwn.net/Articles/271817/.

    Usually the numbers are chosen for exponential back-off: https://geidav.wordpress.com/tag/exponential-back-off/

    So, the numbers aren't arbitrary. However, which "numbers" work for your case depend on your application patterns, requirements and system resources.

    The three methods to introduce "micro-delays" are designed explicitly to balance the cost and the potential gain:

    • zero-cost is to spin on high-CPU, but it results in high power consumption and wasted cycles
    • a small "cheap" delay might be able to prevent the cost of a context-switch while reducing the CPU load relative to a busy-spin
    • a simple yield might allow the OS to avoid a context switch depending on other system load (e.g. if the number of threads < number logical cores)

    The trade-offs with these are important for low-latency applications where the effect of a context switch or cache misses are significant.

    TL;DR

    All trade-offs try to find a balance between wasting CPU cycles and losing cache/thread efficiency.