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Suggestions for memory management classes?

General and Gameplay Programming Programming

Started by Pieisgood October 30, 2013 05:27 PM

10 comments, last by Pieisgood 10 years, 5 months ago

Pieisgood

202

Author

October 30, 2013 05:27 PM

Hey guys,

I've been building a series of memory management classes for a small game engine. I currently have a memory pool class, stack allocator class, double buffer allocator, and a series of overloaded new and delete operators to handle memory dumps.

Do you guys have any suggestions for other classes or techniques that would be good to use for memory management? I'm thinking about wrapping this portion of the code up and releasing it on github but I want to be sure it's not lacking critical functionality or structural classes.

phil_t

8,087

October 30, 2013 08:47 PM

That's a pretty vague request. If you don't have any specific goals or requirements (i.e. a game) to drive your design, you're probably just going to end up building functionality that isn't of use to anyone.

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meeshoo

512

October 31, 2013 08:27 AM

Memory management is kind of an early optimization. Do you really need it? What I do in such situations is use my own functions as simple wrappers around the standard library functions. Then later on if I realize that things go slow in certain areas I implement my own stuff under those wrappers, but only where I do need them and in certain situations. As phil_t said, memory managers are pretty specific.

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Pieisgood

202

Author

October 31, 2013 07:03 PM

Memory managers can be pretty specific for particular games, sure. But, general memory management techniques and classes can at least be discussed.

When building a piece of software as general and large as a game engine; I think it's best practice to have a series of memory management, assert, and new/delete overloads and classes. For example, when building your software it's nice to know how memory is being allocated even before you begin writing specific game code. Like storing vertices, normals, UVs ect... it's nice to know through testing that you aren't having memory fragmentation so that you don't need to find this out later when you're actually running the game.

But I'm also just wondering if there's any type of memory management techniques you guys have seen come in handy. Like has there been a situation where organizing or monitoring memory has been critical to improving your game and allowed you to avoid cache misses.. ect.

meeshoo

512

October 31, 2013 07:44 PM

Well, I don't want to discourage you, but general game engines usually go nowhere, and there are no successful game engines out there that are general.

All have been built for a specific purpose (i.e. a specific game), and then continuously developed to adapt to other situations as well. So first you have to have a game which you are aiming for when developing the engine, and that will give you pretty specific situations where you need memory allocations optimized. If the game runs fine without any optimizations, then there is not reason for adding your own memory management to it.

As a general rule there should be nothing developed in your engine that doesn't serve a specific purpose of a specific game you are trying to build with it.

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Hodgman

52,717

November 01, 2013 12:36 AM

Look up "scope stack allocation" - it describes a Scope class that works with your stack allocator to make it much more useful in many situations.

As for the above optimization posts -- memory optimization is like multi-threading: it's hard to shoe-horn in a solution late in the project, so it does make sense to do some planning and come up with some strategies at the start.

Also, if you've written everything using a general-purpose allocator and later find that this allocator is slow, all you can do is replace it with a different general purpose allocator!
Different allocation strategies have different usage semantics; if you've written code for one class of allocators, you often can't switch to a different class of allocator without rewriting that code.

Lastly, memory optimization isn't always about having the fastest allocator, or the one with the least fragmentation. Often it's about making sure that the relative locations of your allocations are cache-friendly for later usage. If your allocator is causing bad locality if reference, it's hard to spot later on a profiler -- it certainly won't show up as an issue in any memory management code, instead you'll just have every other bit of code running 50% slower than it could be, which you don't know...

So yeah, big engines do plan out a lot of different allocation strategies at the start of the project ;)

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Pieisgood

202

Author

November 01, 2013 02:51 AM

Thanks Hodgman, I certainly wasn't thinking of locality.

My code so far has three unique allocation strategies, and I was looking to expand that. The scope stack allocation I'm looking into now.

I'm glad I wasn't on some sort of crazy train here. I figure creating memory management up-front is sort of a safety net, like unit testing.

Also, is locality as much of an issue on hardware that supports virtual memory? Does that cost really add up on windows, linux and OSX?

Hodgman

52,717

November 01, 2013 08:52 AM

Yep, virtual memory has no impact. If you're processing a batch of data, you ideally want that data to be as small as possible and to be stored continuously (contiguous in address space, either virtual or physical, depending on whether the address space is virtualized). Also, you ideally want to access each bit of data once in a linear order; no random accesses.
Working this way will allow the cache to actually function.

If you access memory randomly, unpredictably, and in areas that have not been used recently, then you get a cache miss - where you have to wait for the data to be moved from RAM to cache to CPU, which can waste hundreds of clock cycles.
e.g. On a Cell SPU, a cache miss is about 400 cycles, each cycle can issue two instructions, and each instruction can operate on 4 floats. That's 3200 FLOPs wasted each time you have to fetch a value from RAM!
On an x86 CPU, it's thankfully not that bad, but it's still a huge performance concern.

CPU speeds are increasing at a faster rate than RAM speeds, so every year this issue just gets worse and worse, not better!

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meeshoo

512

November 01, 2013 09:45 AM

I discovered while implementing "cache friendly" structures that usually they tend to dictate the code around them (access/storing code), so it is some kind of a data driven design approach. Typical OOP code with lists of objects and such doesn't map very well to this.

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Pieisgood

202

Author

November 01, 2013 08:33 PM

Data driven design IS engine design. Going as far back to Game Programming Gems 1 - Chapter 1 we can see that this has been a general goal in game development.

Also, inline function size is something else that you need to look out for in cache management.

While I'm not looking to write for the PS3, it's a nice example of the cost. It also seems like a good time to revisit memory management as whole new strategies are going to start cropping up on the PS4 and XBONE since they both have the homogeneous AMD cores.

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