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flat assembler > OS Construction > Performance benefits of a custom OS

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coen



Joined: 13 Oct 2015
Posts: 14
Due to a strange performance issue I had recently I started thinking about the idea of building a custom OS and what kind of performance improvement would be possible over Linux or Windows. The reason would be to eliminate any kind over overhead that any of the existing operating systems have.

The OS/application would only have to do the following task (all in memory):

- Poll a server for files (every X seconds)
- Analyze file
- Push result back to server
- Repeat

By building an OS as simple as this you wouldn't need a scheduler to divide the CPU time over any other processes therefore eliminating the overhead of context switches. Also I imagine that memory access could be much faster because no protection is needed to prevent processes of reading each other's memory.

Does anyone have any experience building such an OS that only runs a single application and what the performance benefits are approximately?
Post 16 Oct 2015, 12:28
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 16787
Location: In your JS exploiting you and your system
I have done this a few times. It is easy to code and maintain. But please don't be driven to do this by the desire to have everything run "faster". The overhead of context switching is very minor, to almost negligible, in most well written OS kernels. Only for very fast switching regimes and/or very slow CPU/memory systems would you really start to see a significant difference.

BTW: the memory protection mechanisms are not a source of congestion in any modern CPU you can buy today. It is just not an issue at all, the overhead is literally zero. More likely, although still very minor, is the overhead from paging. It is possible to have really bad memory access patterns that manage to completely miss the TLBs and create awful delays.
Post 16 Oct 2015, 13:51
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coen



Joined: 13 Oct 2015
Posts: 14
If not for performance what were your reasons to take this approach?
Post 16 Oct 2015, 14:38
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 16787
Location: In your JS exploiting you and your system
There are many reasons to have simple noOS dedicated systems. For example: Instant start-up, run from ROM, constrained system, no UI, resistance to hacking, verifiability by customers, provable reliability, etc.

But to gain what is probably ~0.5% improvement in speed (if it is even that high) it would be easier to raise the clock by 0.5% and keep all the advantages of a full OS. And in practicality if your system is so sensitive to 0.5% speed differences then perhaps you should instead invest in a second system to take up the slack if the first system ever goes down.
Post 16 Oct 2015, 14:57
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coen



Joined: 13 Oct 2015
Posts: 14
Thanks for your response, those are all good reasons, but not for me in this case Smile

I must admit that I was kind of expecting (and hoping) for bigger performance bump than 0,5%.. I never would have tought that these huge operating systems had that little overhead.
Post 16 Oct 2015, 17:41
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Trinitek



Joined: 06 Nov 2011
Posts: 257
In Windows 10, the task manager shows the CPU usage percentages for all of the system services running in the background, including the interrupt handler and other low-level things. Most of those are typically at 0.0% usage on my system. So yeah, there isn't much overhead.
Post 16 Oct 2015, 18:37
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