Run worker pool at lowest thread priority

[ReillyBrogan]

On a Linux session with Plasma desktop I was having intermittent desktop and software freezes. These freezes were always associated with RSSGuard using 100% of CPU when it does a periodic feed check (~500 feeds).

In order to resolve this we can lower the thread priority of those background threads in order to not compete with interactive system threads. Unfortunately QThread priorities don't actually work on most Linux systems as it attempts to set the process priority which is ignored with SCHED_OTHER (the default scheduler for most systems), see here for more info. Instead we need to use nice values to achieve the same effect (confusingly the Linux documentation ALSO refers to this as process priority despite the fact that it's not the same thing as the other process priority).

Since Qt doesn't have any handy helpers for this I added a setThreadPriority function that does the same thing using libc functions. While this was only tested on glibc I didn't use any GNU extensions so in theory it should work on musl-based systems as well (unsure if those are actually supported by RSSGuard or not). I also changed the scheduling hint to SCHED_BATCH which means that the scheduler will consider the thread non-interactive and potentially CPU-intensive (which gives a slight penalty to wake-up scheduling).

This code is ifdefed to only run on Linux. To achieve the same effect QThread priority levels are defined which should work on Windows, Mac, and Linux systems where RSSGuard is not started with SCHED_BATCH or SCHED_OTHER.

Also in several places I noticed that the Qt thread handle was being converted to a number for logging or other purposes. While this did result in a unique per-thread identifier on Linux systems we can get the thread ID instead which is helpful since logging output can be correlated to other system tools (like ps -lcmT (pidof rssguard) which reports scheduling type and priority for all rssguard threads). I added a helper for that and switched all uses of currentThreadId() to it (I have no idea how to retrieve the actual thread ID on Win/Mac so I kept the previous behavior for those).

[martinrotter]

Hi @ReillyBrogan.

This is some amazing PR. Accepting. Will do some testing, but the code looks very good. Thanks.

[martinrotter]

Checked it on runtime, nice work.

Cleaned up a bit some bits of code, thanks once more.

c83ab23

[ReillyBrogan]

I see you added a Qt version check for the ThreadPool priority

#if QT_VERSION >= 0x060200 // Qt >= 6.2.0

I don't think this is necessary because you already have a Qt >= 6.3.0 check in the top-level CMakeLists.txt

set(QT6_MIN_VERSION 6.3.0)