docs: add comparison table of real-time executors

docs/comparison_with_other_executors.md

@@ -0,0 +1,101 @@
+# Comparison of Real-Time Performance with Other ROS 2 Executors
+
+This document compares the real-time performance of the ROS 2 Default Executor and Executors proposed in academic literature from the following perspectives:
+
+**Immediate Enqueue of Ready Callbacks**
+
+In the ROS 2 Default Executor, a callback instance that becomes ready is enqueued only when the queue becomes empty. If the callback is enqueued immediately at the moment it becomes ready, ✔ is marked.
+
+**Per-Callback Priority Assignment**
+
+The ROS 2 Default Executor does not provide an API to assign priorities on a per-callback basis. If an Executor provides such an API, or allows users to assign priorities per callback through another mechanism, ✔ is marked.
+
+**Non-Nested Scheduling**
+
+In many Executors, the Executor maintains its own scheduling layer (e.g., a ready queue with priority ordering), and the OS independently schedules the threads that run callbacks. This two-level structure is referred to as nested scheduling.
+If the Executor's scheduling and the OS scheduling do not form such a nested structure, ✔ is marked.
+
+**Fully Preemptive**
+
+If a lower-priority callback can be preempted by any higher-priority callback during execution, ✔ is marked.
+
+**Support for Reentrant Callbacks**
+
+If, when a callback becomes ready while it is executing, it can be properly enqueued and executed, ✔ is marked.
+
+**ROS 2 Mainline**
+
+If the Executor has been merged into the ROS 2 mainline, or can be introduced without modifying the ROS 2 implementation, ✔ is marked.
+
+---
+
+## Comparison Table
+
+| Executor                                       | Immediate Enqueue | Per-Callback Priority | Non-Nested Scheduling | Fully Preemptive | Support for Reentrant Callbacks | ROS 2 Mainline |
+| ---------------------------------------------- | :---------------: | :-------------------: | :-------------------: | :--------------: | :-----------------------------: | :------------: |
+| SingleThreadedExecutor / MultiThreadedExecutor |                   |                       |                       |                  |                                | ✔              |
+| EventExecutor                                  | ✔                 |                       |                       |                  | ✔                              | ✔              |
+| PiCAS Single [1]                               |                   | ✔                     |                       |                  |                                |                |
+| PiCAS Multi [2]                                | ✔\*1              | ✔                     |                       |                  |                                |                |
+| Dynamic-Priority-based Executor [3,4]          | ✔\*1              | ✔                     |                       |                  |                                |                |
+| Extended EventExecutor [5]                     | ✔                 | ✔                     |                       |                  | ✔                              |                |
+| RTeX [6]                                       | ✔                 | ✔                     |                       |                  | ✔                              |                |
+| Budget-based Real-Time Executor [7]            | ✔\*1              | ✔                     | ✔                     | ✔                |                                |                |
+| Preemptive EDF Executor [8]                    | ✔\*1              | ✔                     | ✔                     | ✔                |                                |                |
+| Ros-RT [9]                                     | ✔                 | ✔                     | ✔                     | ✔                | ✔                              |                |
+| CallbackIsolatedExecutor [10]                  | ✔\*1              | ✔                     | ✔                     | ✔                | ✔                              | ✔              |
+
+\*1: The ready queue is updated each time a thread selects a callback to execute.
+
+---
+
+## References
+
+1. H. Choi, Y. Xiang and H. Kim,
+   “PiCAS: New Design of Priority-Driven Chain-Aware Scheduling for ROS2,”
+   *2021 IEEE 27th Real-Time and Embedded Technology and Applications Symposium (RTAS)*, Nashville, TN, USA, 2021, pp. 251–263.
+   doi: 10.1109/RTAS52030.2021.00028
+
+2. H. Sobhani, H. Choi and H. Kim,
+   “Timing Analysis and Priority-driven Enhancements of ROS 2 Multi-threaded Executors,”
+   *2023 IEEE 29th RTAS*, San Antonio, TX, USA, 2023, pp. 106–118.
+   doi: 10.1109/RTAS58335.2023.00016
+
+3. A. Al Arafat et al.,
+   “Response time analysis for dynamic priority scheduling in ROS2,”
+   *DAC ’22*, ACM, 2022, pp. 301–306.
+   doi: 10.1145/3489517.3530447
+
+4. A. Al Arafat et al.,
+   “Dynamic Priority Scheduling of Multithreaded ROS 2 Executor With Shared Resources,”
+   *IEEE Transactions on CAD*, vol. 43, no. 11, pp. 3732–3743, Nov. 2024.
+   doi: 10.1109/TCAD.2024.3445259
+
+5. H. Teper et al.,
+   “Reconciling ROS 2 with Classical Real-Time Scheduling of Periodic Tasks,”
+   *2025 IEEE 31st RTAS*, Irvine, CA, USA, 2025, pp. 177–189.
+   doi: 10.1109/RTAS65571.2025.00027
+
+6. S. Liu et al.,
+   “RTeX: An Efficient and Timing-Predictable Multithreaded Executor for ROS 2,”
+   *IEEE Transactions on CAD*, vol. 43, no. 9, pp. 2578–2591, Sept. 2024.
+   doi: 10.1109/TCAD.2024.3380551
+
+7. J. Staschulat et al.,
+   “Budget-based real-time Executor for Micro-ROS,”
+   arXiv:2105.05590, 2021.
+
+8. K. Wilson et al.,
+   “Physics-Informed Mixed-Criticality Scheduling for F1Tenth Cars with Preemptable ROS 2 Executors,”
+   *2025 IEEE 31st RTAS*, Irvine, CA, USA, 2025, pp. 215–227.
+   doi: 10.1109/RTAS65571.2025.00030
+
+9. S. Liu et al.,
+   “Ros-RT: Enabling Flexible Scheduling in ROS 2,”
+   *2025 IEEE Real-Time Systems Symposium (RTSS)*, Dec. 2025, pp. 42–54.
+   doi: 10.1109/RTSS66672.2025.00013
+
+10. T. Ishikawa-Aso et al.,
+    “Middleware-Transparent Callback Enforcement in Commoditized Component-Oriented Real-Time Systems,”
+    *2025 IEEE 31st RTAS*, Irvine, CA, USA, 2025, pp. 426–429.
+    doi: 10.1109/RTAS65571.2025.00017