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In a blogpost on https://discourse.ubuntu.com the performance weakness of Gnome-Shell is explained as being caused – not by obviously detectable "hot spots", but being caused – by "cold spots":

"The thing is in the case of Gnome-Shell its biggest performance problems of late were not hot spots at all. They were better characterized as cold spots where it was idle instead of updating the screen smoothly. Such cold spots are only apparent when you look at the real time usage of a program, and in not the CPU or GPU time consumed."

Cited from https://discourse.ubuntu.com/t/boosting-the-real-time-performance-of-gnome-shell-3-34-in-ubuntu-19-10/13095

Since I have never heard of such "cold spots", I now wonder, if there is a more detailed definition of "cold spots" in program performance analysis.

Pokulo
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2 Answers2

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In general; "hot" means that it's frequently used or has been used recently; which has various implications depending on context - e.g. that any code or data is likely to be cached, that it's more beneficial to spend developer time optimizing that code, etc. "Cold" is the reverse (either not frequently used or not used yet).

The blogpost is saying that the performance problems aren't caused by "hot spots" (code that's executed often). It's saying is that the problems were mostly "things that don't consume CPU time at all" (e.g. miscalculation of how long to wait before updating the screen), which isn't the same as "cold spots" (code that's executed rarely) but is closer to "cold spots" than "hot spots".

Brendan
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  • I like the clear relation of "hot" been often used code, instead of "hot" meening resource intensive code. But I am not yet fully convinced that this is the literal definition of a "hot spot" simply and soleily being "very often used code lines". Is there a more precise "general" definition? – Pokulo Mar 04 '20 at 06:19
  • @Pokulo: I don't think there's a precise definition - it's more colloquial. – Brendan Mar 04 '20 at 07:48
  • Thanks brendan. I was hoping that some cs scientist defined it any more specific. Was trying to move this to cs.stackexchange.com though. – Pokulo Mar 06 '20 at 06:24
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The cited source speaks of that same weakness of Gnome-Shell just as if it would simply waste the users real time without even bothering CPU or GPU but looping in a single thread and stopping by for each disk IO or GPU IO. Which does not seem to be a good solution for a multitasking window manager and desktop running on Octa-Core CPUs. Anyways, scanning that detailed blogpost, I came to the conclusion that "cold spots" are only an extenuation of what could as well be called "hot spots" but are "only" occupying less important/responsive resources than the CPU, like disk-IO or GPU-IO. As the blog post suggests, such "cold spots" can be made visible as "hot spots" if one considers the actual real time performance including all resources that are needed by a program.

Hence "cold sport" here simply meens "off-CPU hot spot"?

Pokulo
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