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Comment by panic | Hacker News Reader

Since the VM system works at the granularity of pages, using N entirely different regions wastes up to $PAGE_SIZE * N bytes of memory just in the free space at the end of the region. You also make your fragmentation problem worse, since you can't place an allocation from one system within the free space of another.

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Not suggesting that everything go in separate regions, but let's say they have N major functions (where N < 10) .. maybe physics, AI, render, network, etc. that would each run in their own address space. If N were even, say, 20 this still only comes out to 20 * 4KB (PPC page size) = 80KB, which I would classify as trivial.

You have to manage memory for things like textures, models, and maps - each of which are generally going to be way larger individually than 80KB.

Here's an example of a pathological - but entirely reasonable to encounter - case:

You allocate several textures of various sizes. Your larger 3MB texture blocks might end up seperated by smaller allocations for, say, icons. Or other 3MB textures that can be reused on the next level (to reduce load times). Long story short, unless you do work to ensure textures are correctly grouped by lifetime (which in many cases will depend on and diverge based on user input) you're going to end up with lots of 3MB holes.

You switch levels, and release many/most of your 3MB environment textures. But the icons remain loaded for your UI, so you don't free those. So now you have 3MB holes everywhere.

You start to load the next level. It's using a lot more alpha textures in it's environment - lots of water and glass. The alpha channels adds another 33% to the size - these new textures are mostly 4MB. None of these new textures fit into the 3MB holes. You've already potentially just wasted about half your available memory (or address space), with a single texture size in two formats of note. Within a single system ("render"). Oops.

OK. So the issue is the GPU then. That same scenario with data that could use virtual memory would be a non-issue, since you could mash the available fragmented phsyical ram together into whatever size blocks were needed using the MMU.

> That same scenario with data that could use virtual memory would be a non-issue

Untrue I'm afraid - the case I'm basing this example off of could (and did) happen in virtual address space as well.

> since you could mash the available fragmented phsyical ram together into whatever size blocks were needed using the MMU.

Okay, so your physical memory is no longer fragmented. Unfortunately, your virtual address space still is. If I need to allocate a 3MB contiguous chunk of virtual address space (I do), I'm still SOL.

All the MMU gains you in this respect is a "free" memcpy - twiddle some MMU bits instead of copying data around from one buffer to another. It won't fix up any of your pointers.

Also of note is that fragmentation of physical RAM is completely irrelevant, aside from locality issues. They can map random physical pages into one nice, contiguous virtual space, and the only thing that matters is fragmentation in the virtual space. Since a each process gets its own entire 4GB virtual address space on a 32bit system, fragmentation nearly becomes a non-issue.

This is also why I suspect that they were mapping their virtual address space using block translation instead of a page table - if they had a page table, even in one process, it shouldn't have been a big issue since the virtual address space is many times larger than the available physical ram on the 360/PS3/Wii. The other possibility is that much of their RAM needs to be accessible by the GPU, for which fragmentation is an issue.

> Since a each process gets its own entire 4GB virtual address space

2GB of which is reserved for kernel shenanigans unless everyone enabled /3GB in their kernel boot parameters (they haven't.)

Default allocators start failing at around half that amount.

Meanwhile I'm running out of memory on the new generation of consoles where I have 5GB of physical memory and no virtual memory fragmentation to worry about.

Truth on the kernel. I'm used to working in an environment where we own the OS, the board design, and the design of many of the chips ... which means we can make it do whatever we need (downside is, you have to maintain it all too). However, our needs are unique enough to justify it -- computing is hard in an environment where atmospheric radiation can flip random bits for you (See: https://en.wikipedia.org/wiki/Single_event_upset).

I humbly suggest that if by some horrible twist of fate you had to work on games instead of "proper" software, you would quit programming.

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