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Vjekoslav Krajačić
9 posts
Dynamic arrays
2 months ago Edited by Vjekoslav Krajačić on May 15, 2020, 10:18 a.m.
Hi.
What's your solution when you need contiguous storage for data, but you don't know the size ahead of time. And I don't mean just while developing. It's dependent on the user input, so you have to ship it that way. My first attempt was to create some kind of sliced array, where each slice is a bigger chunk of memory. That way, data should still be cache friendly most of the time, except when it needs to jump to other slice. I want fast access and fast iteration time, so instead of linking the slices, I have another array of pointers to those slices. Each slice is same size, and all elements are same size. Here's my implementation using memory arena. What do you think? How would you improve it?

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struct SlicedArray
{
	MemArena arena;

	uptr *slices[128]; /* this can be dynamic too */
	sizet sliceCount;
	sizet itemCount;

	sizet effectiveItemSize;
	sizet itemsPerSlice;
};

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INLINE void initSlicedArray(SlicedArray *array, sizet sliceSize, sizet itemSize, u32 alignment)
{
	array->effectiveItemSize = ALIGN_POW_2(itemSize, alignment);
	/* ensure that at least one item fits */
	ASSERT(sliceSize >= (array->effectiveItemSize + sizeof(MemBlockFooter)));

	sizet effectiveSliceSize = sliceSize - sizeof(MemBlockFooter);
	array->itemsPerSlice = (effectiveSliceSize / array->effectiveItemSize);

	sizet arenaBlockSize = sliceSize;
	initArena(&array->arena, win32Allocate, win32Deallocate, arenaBlockSize);
	array->sliceCount = 0;
	array->itemCount = 0;
}

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INLINE void *pushItem(SlicedArray *array)
{
	sizet prevSliceCount = array->sliceCount;
	void *result = pushSize(&array->arena, array->effectiveItemSize, noAlignNoClear());
	++array->itemCount;
	array->sliceCount = array->arena.blockCount;
	if (prevSliceCount != array->sliceCount)
	{
		array->slices[array->sliceCount - 1] = (uptr *)array->arena.base;
	}
	return result;
}

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INLINE void *getItem(SlicedArray *array, sizet index)
{
	sizet sliceIndex = (index / array->itemsPerSlice);
	sizet itemIndex = (index % array->itemsPerSlice);
	void *slice = array->slices[sliceIndex];
	void *result = (u8 *)slice + (itemIndex * array->effectiveItemSize);
	return result;
}

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struct Item; /* some data type */

SlicedArray array;
initSlicedArray(&array, MEGABYTES(1), sizeof(Item), 4);

Item *item = pushItem(&array); /* adding */
Item *firstItem = getItem(&array, 0); /* fetching */

/* iteration */
sizet itemIndex;
for (itemIndex = 0; itemIndex < array.itemCount; ++itemIndex)
{
	Item *item = getItem(&array, itemIndex);
}

Simon Anciaux
828 posts
Dynamic arrays
2 months ago
There are GingerBill's articles on memory allocation, you can also search for stb stretchy buffer.

One thing that is also possible is to reserve memory without committing it. You just reserve a huge chunk of address space (1GB for example) but only commit 1MB. When you reach the end of the first MB you commit a second one... That way the memory is contiguous, it's just a big array.

Vjekoslav Krajačić
9 posts
Dynamic arrays
1 month, 4 weeks ago
I've read both of those resources before. I couldn't find solution to this problem on GingerBill's articles. Closest to this was pool based allocator, but it still uses preallocated memory buffer. Stretchy buffer is just syntactic sugar for using realloc. And if you want to just reserve the memory you still need to know how much of it, right?

To rephrase the question, is there anything better for this kind of situations than using realloc? It feels to me that, as the memory allocated using realloc becomes bigger you're in nastier situation because you start copying a lot of data if OS can't reallocate using old address. Realloc also has the problem that you cant use it for storage where multiple threads write. I just wanted to know is there some kind of best practice when you don't know how much memory you will need, but you want to keep the data contiguous?
Simon Anciaux
828 posts
Dynamic arrays
1 month, 4 weeks ago
I don't think there is a best practice. It always depends on the problem you're trying to solve.

In my (small) experience if you don't know the amount of memory that will be used, you can generally guess or dictate an upper bound that will cover most cases, and then either fail or handle special case with a performance penalty.
Dumitru Frunza
24 posts

Apprentice
github.com/dfrunza/hoc

Dynamic arrays
1 month, 3 weeks ago
I've researched this problem a while back, and the basic idea outlined in your solution is an approach being explored in the literature, and thus could qualify as "best practice".

The main focus of the variants to this solution are on the optimization of the time it takes to access an array item and the minimization of wasted space in the (last) slice.
Here's a paper that presents an implementation with a constant access time and O(sqrt(n)) storage consumption, in case you need to go that far:
Resizable Arrays in Optimal Time and Space

Concerning the alternative, which is to reserve 1GB of memory from the OS, and commit pages when the array needs to grow - the problem I have is this - the reserved memory becomes address space that is unaccessible to other parts of the program.

Programming is hard, let's go shopping!
Simon Anciaux
828 posts
Dynamic arrays
1 month, 3 weeks ago
There is 8 TB of process address space available on Windows 7 and 8; and 128 TB on Windows 10. So there is plenty for everybody. Or do you mean something else ?
Miles
67 posts / 2 projects
Dynamic arrays
1 month, 3 weeks ago
I frequently see people on HMN claim that it's fine to reserve huge portions of address space in 64-bit programs, but the figures given to support that claim are always for Windows, or very occasionally for macOS or Linux. What about people who may want to ship programs on other platforms, like iOS or Android, or a game console? Does anyone know of a good information source for what the virtual memory situation is like on those platforms?
Mārtiņš Možeiko
2091 posts / 1 project
Dynamic arrays
1 month, 3 weeks ago
It's documented in Linux kernel docs:
https://www.kernel.org/doc/html/latest/x86/x86_64/mm.html
https://www.kernel.org/doc/html/latest/arm64/memory.html

So 128TB or 64PB for x86_64. And 256TB or 4PB for ARM64.
I'd assume this is same for Android

No idea about macOS or iOS. But I would be very surprised if they have less than many TBs of virtual memory space.


Rémy Roy
6 posts
Dynamic arrays
1 month, 2 weeks ago
The Linear/Arena allocator seems like a solution that could be interesting for you. https://www.gingerbill.org/articl...memory-allocation-strategies-002/