I now added a #lockfree version of that MPMC job queue which is picked when the system headers claim that pointers are lockfree. Doesn't give any measurable performance gain 😞. Of course the #semaphore needs to stay, the pool threads need something to wait on. But I think the reason I can't get more than 3000 requests per second with my #jmeter stress test for #swad is that the machine's CPU is now completely busy 🙈.

Need to look into actually saving CPU cycles for further optimizations I guess...

I finally eliminated the need for a dedicated #thread controlling the pam helper #process in #swad. 🥳

The building block that was still missing from #poser was a way to await some async I/O task performed on the main thread from a worker thread. So I added a class to allow exactly that. The naive implementation just signals the main thread to carry out the requested task and then waits on a #semaphore for completion, which of course blocks the worker thread.

Turns out we can actually do better, reaching similar functionality like e.g. #async / #await in C#: Release the worker thread to do other jobs while waiting. The key to this is user context switching support like offered by #POSIX-1.2001 #getcontext and friends. Unfortunately it was deprecated in POSIX-1.2008 without an obvious replacement (the docs basically say "use threads", which doesn't work for my scenario), but still lots of systems provide it, e.g. #FreeBSD, #NetBSD, #Linux (with #glibc) ...

The posercore lib now offers both implementations, prefering to use user context switching if available. It comes at a price: Every thread job now needs its private stack space (I allocated 64kiB there for now), and of course the switching takes some time as well, but that's very likely better than leaving a task idle waiting. And there's a restriction, resuming must still happen on the same thread that called the "await", so if this thread is currently busy, we have to wait a little bit longer. I still think it's a very nice solution. 😎

In any case, the code for the PAM credential checker module looks much cleaner now (the await "magic" happens on line 174):
https://github.com/Zirias/swad/blob/57eefe93cdad0df55ebede4bd877d22e7be1a7f8/src/bin/swad/cred/pamchecker.c

#C #coding

Nice, #threadpool overhaul done. Removed two locks ( #mutex) and two condition variables, replaced by a single lock and a single #semaphore. 😎 Simplifies the overall structure a lot, and it's probably safe to assume slightly better performance in contended situations as well. And so far, #valgrind's helgrind tool doesn't find anything to complain about. 🙃

Looking at the screenshot, I should probably make #swad default to two threads per CPU and expose the setting in the configuration file. When some thread jobs are expected to block, having more threads than CPUs is probably better.

https://github.com/Zirias/poser/commit/995c27352615a65723fbd1833b2d36781cbeff4d