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Nibble Stew: Millennium prize problems but for Linux

 4 years ago
source link: https://nibblestew.blogspot.com/2021/02/millennium-prize-problems-but-for-linux.html
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Millennium prize problems but for Linux

Wednesday, February 24, 2021

Millennium prize problems but for Linux

There is a longstanding tradition in mathematics to create a list of hard unsolved problems to drive people to work on solving them. Examples include Hilbert's problems and the Millennium Prize problems. Wouldn't it be nice if we had the same for Linux? A bunch of hard problems with sexy names that would drive development forward? Sadly there is no easy source for tens of millions of euros in prize money, not to mention it would be very hard to distribute as this work would, by necessity, be spread over a large group of people.

Thus it seems is unlikely for this to work in practice, but that does not prevent us from stealing a different trick from mathematicians' toolbox and ponder how it would work in theory. In this case the list of problems will probably never exist, but let's assume that it does. What would it contain if it did exist? Here's one example I came up with. it is left as an exercise to the reader to work out what prompted me to write this post.

The Memory Depletion Smoothness Property

When running the following sequence of steps:
  1. Check out the full source code for LLVM + Clang
  2. Configure it to compile Clang and Clang-tools-extra, use the Ninja backend and RelWithDebInfo build type, leave all other settings to their default values
  3. Start watching a video file with VLC or a browser
  4. Start compilation by running nice -19 ninja
The outcome must be that the video playback works without skipping a single frame or audio sample.

What happens currently?

When Clang starts linking, each linker process takes up to 10 gigabytes of ram. This leads to memory exhaustion, flushing active memory to swap and eventually crashing the linker processes. Before that happens, however, every other app freezes completely and the entire desktop remains frozen until things get swapped back in to memory, which can take tens of seconds. Interestingly all browser tabs are killed before linker processes start failing. This happens both with Firefox and Chromium.

What should happen instead?

The system handles the problematic case in a better way. The linker processes will still die as there is not enough memory to run them all but the UI should never noticeably freeze. For extra points the same should happen even if you run Ninja without nice.

The wrong solution

A knee-jerk reaction many people have is something along the lines of "you can solve this by limiting the number of linker processes by doing X". That is not the answer. It solves the symptoms but not the underlying cause, which is that bad input causes the scheduler to do the wrong thing. There are many other ways of triggering the same issue, for example by copying large files around. A proper solution would fix all of those in one go.

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