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Merge request <a href="https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1164">!1164</a> was reviewed by <a href="https://gitlab.rtems.org/Chandanuvm">Chandan U</a>
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<a href="https://gitlab.rtems.org/Chandanuvm">Chandan U</a>
commented on a
<a href="https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1164#note_147578">discussion</a>:
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<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">Thank you for taking the time to review the draft and for the feedback.</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">I completely agree that a bitmap approach wouldn't scale for massive priority ranges like those in the EDF scheduler, and the memory overhead would be unreasonable in those cases.</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">The reason behind choosing this option was that, Since In RTEMS (via newlib), <code style="font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;">MQ_PRIO_MAX</code> is statically bounded to <code style="font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;">32</code> which i confirmed by crosschecking with the newlib source file and also from my local setup. For this small, fixed range, the memory overhead is limited to an array of 32 Chain_Control structures plus the Priority_bit_map_Control, which gives us deterministic O(1) insertions. As there is an advantage of the limit of 32 on the message priority value, which doesn't pose a memory overhead as well, priority bucket + bitmap which has a constant time complexity seemed more optimal compared to the Red black tree which is O(log(n)).</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">I also included a preprocessor guard <code style="font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;">(#if MQ_PRIO_MAX <= 255)</code> so that if a system ever defines a much larger priority range, it safely falls back to the existing linear traversal implementation.</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">I originally went down this path based on a recent Discourse discussion with Gedare and Joel, where the bitmap approach was also one of the suggestions as a good fit specifically because of this small POSIX limit <a href="https://users.rtems.org/t/gsoc-26-lower-message-priority-range-optimization/496/3?u=chandanuvm" rel="nofollow noreferrer noopener" target="_blank" style="margin-top: 0;">Discourse discussion</a></p>
<p dir="auto" style="color: #3a383f; margin: 0;" align="initial">It would be really helpful if you could take a look at my current proposal and guide me through this. <a href="https://docs.google.com/document/d/1WvJ4_waV9SRBtQFF5k7ljKqqKsEYgl6gm2zad5fv09g/edit?usp=drivesdk" rel="nofollow noreferrer noopener" target="_blank" style="margin-top: 0;">Draft Proposal</a></p>
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