[Interest] Thread-To-CPU-Core distribution

[Konrad Rosenbaum]

Hi,

On 28/12/2022 11:08, Alexander Carôt wrote:
> in a special use case I launch audio- and video streaming classes in my Qt main thread and also a web browser as an interface (webview or webengine) but I want to have their operation consciously separated over available cpu cores such as
>
> Core 1: Audio Callback Thread
> Core 2: Video Callback Thread
> Core 3: Web browser
>
> and not let the system decide what runs on which core.

I can't hold back my curiosity, so I've got to ask: why? Why would you 
do that? Why do you assume you know better than the operating system?


There are subtleties in what core is assigned to which task. The kernel 
knows stuff like IRQ affinities, hardware bus connections, IO port 
assignments and so on that are fairly hard to guess for you in user 
space. Normally the kernel will find a good balance between cache 
affinity, short call paths into hardware and load distribution, so there 
is usually no need for you to meddle in this.


Unless you've found that rare unicorn of a real scheduling problem in 
your OS _and_ your program only needs to run on that one machine... 
don't meddle. Once you optimized for one machine, things will perform 
(much) worse on different machines.


Usually if your program does not perform well it is one of those problems:

a) you have unnecessarily complicated call paths in your program: you 
need to shorten them

b) bad math: sometimes it is worth spending hours simplifying an 
algorithm to save on a few microseconds - billions of microseconds in a 
loop are hours after all.

c) too many context switches: use FEWER threads, not more, reduce 
dependencies between threads

d) your hardware is not powerful enough for what you want to do: save 
some money, get better hardware

e) you are waiting for the hard disk or network: use a cache (big 
problem on Windows, Linux already does this for you)


> With audio and video this works fine already according to
>
> https://eli.thegreenplace.net/2016/c11-threads-affinity-and-hyperthreading
>
> because I am using pthreads for them anyways.
>
> Now I wonder if this is also possible for Qt and my web browser instance in particular.
You can always call sched_setaffinity with pid==0 from within that thread.



     Konrad


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