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by fathyb 614 days ago
> On macOS memory can be paged to/from disk. On iOS it isn’t and applications must free memory when asked or be terminated

Not sure what you meant by that, you always could `mmap` files into memory on iOS. Back in the 32 bits days there was a ~700 MB limit due to the address space, but there aren't anymore nowadays with 64 bits. If `didReceiveMemoryWarning` is called on your app, then you need to free resident memory but the kernel will take care of dumping file-backed memory pages for you.
2 comments
umanwizard 614 days ago
> Back in the 32 bits days there was a ~700 MB limit due to the address space, but there aren't anymore nowadays with 64 bits.

Not true, unless something changed recently (definitely more recently than the 32->64 transition). All iPhones have a virtual memory limit (although the limit is higher on phones with more physical RAM).

I know this for sure because several years ago I was the main person in charge of reducing OOM kills on the Facebook iPhone app and virtual memory exhaustion on 64-bit phones was definitely an issue.

See here for where this is enforced in XNU: https://github.com/apple-oss-distributions/xnu/blob/xnu-1121...

I assume Apple does this specifically because they want to prevent apps from simulating swap space by mapping a big file and allocating from it.

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kuhsaft 614 days ago
That's memory mapping. This is memory paging [1]. I.e. Windows pagefile.sys, Linux swap, macOS swap files. iOS does not have swap files, only memory compression. If you're on a Mac, open up Activity Monitor, go to Memory, and at the bottom there is `Swap Used`. That doesn't exist on iOS. So, if more memory is used than available, applications will need to free memory or be terminated. Unlike macOS, where some used memory will be swapped to disk to allow other stuff to be loaded into memory.

[1] https://en.wikipedia.org/wiki/Memory_paging

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Rohansi 614 days ago
It's most likely just disabled rather than being completely different/non-existent. But yes, the application model is built around limited to no multitasking.
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kuhsaft 614 days ago
> It's most likely just disabled rather than being completely different/non-existent.

As evident by the limited Virtual Memory Swap enabled on iPadOS 16, but not iOS.

All Apple devices use the XNU kernel. But, as the parent blog post shows, the kernel configuration, device tree, and drivers are different.

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