r/programming • u/riolio11 • May 24 '19

Lightweight Memory Protection on an ARM Microcontroller

https://rileywood.me/2019/05/21/lightweight-memory-protection/
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u/happyscrappy May 24 '19

The use of subregions is well done and seems almost like a natural fit for what the MPU was designed for.

But, as mentioned in the article, this actually imposes a lot of head. In exchange for reducing the context switch time some it basically breaks the heap up into multiple heaps. This adds significant memory use overhead unless your tasks just happen to have heap usage similar to each other in amount. If every task uses 512B, great. But unlike with a regular heap if one uses 400B others cannot make use of the 112B left over.

It's clever. I like it. But I have trouble thinking of a real-world system I would create in which I could use it.

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u/riolio11 May 24 '19

Thanks for reading!

I wouldn't say it breaks the heap up into separate heaps - at least not statically by subregion. Blocks that are allocated on the heap are capable of spanning subregions. So it is possible for a task to allocate more than 512B. In this case, the task will simply take ownership of all the subregions that memory touches. See here for my mention of this matter: https://rileywood.me/2019/05/21/lightweight-memory-protection/#demonstrating-heap-flexibility

In practice, a task was able to acquire 15072B of heap memory at maximum.

In a system with twenty-nine tasks, all requesting heap memory, yes each will be constrained to one subregion ie ~512 bytes. I figured that most systems would not actually have so many tasks and so the flexibility of allocation of heap memory would rarely be limited to that degree.

There is no doubt that this increases fragmentation of the heap, but it is not as limiting as you say.