futex – Tarik Billa

What is the difference between FUTEX_WAIT and FUTEX_WAIT_PRIVATE?

August 9, 2023 by Tarik

This is an optimization done by linux/glibc to make futexes faster when they’re not shared between processes. Glibc will use the _PRIVATE versions of each of the futex calls unless the PTHREAD_PROCESS_SHARED attribute is set on your mutex It’s explained in more detail here: http://lwn.net/Articles/229668/ For the purposes of your debugging, you can just ignore … Read more

Why is a pthread mutex considered “slower” than a futex?

April 13, 2023 by Tarik

Futexes were created to improve the performance of pthread mutexes. NPTL uses futexes, LinuxThreads predated futexes, which I think is where the “slower” consideration comes. NPTL mutexes may have some additional overhead, but it shouldn’t be much. Edit: The actual overhead basically consists on: selecting the correct algorithm for the mutex type (normal, recursive, adaptive, … Read more