Sometimes that’s true, though you’ve stated it imprecisely. RR emulates pthreads in userspace, converting your multithreaded programs to run in a single thread

Sometimes that’s true, though you’ve stated it imprecisely. RR emulates pthreads in userspace, converting your multithreaded programs to run in a single thread. It does context switching between your threads much as the OS would if you had only one CPU core, but there is no true concurrency. This can hide some problems that are the result of race conditions, making them never show up in any of your recordings. If the problem doesn’t show up in any recordings, then RR isn’t going to help you debug them. However, RR does have a mitigation for this, called chaos mode. In chaos mode it doesn’t schedule your threads fairly. It randomly starves some threads, not allowing them to be scheduled, or interrupts them before their usual time slice is up. This can make it easier to capture race conditions in a recording, though it isn’t perfect. You should give it a try some time.