When sketching out a new low level system I always start with the data layout, because that's the most important part. And I do that with two main goals:
- Make sure that memory is laid out and accessed linearly.
- Minimize the number of individual memory allocations.
The importance of the first goal should be obvious to anybody who has been following the data-oriented design movement. Modern CPUs are often memory bound. Writing cache friendly code is paramount. Etc.
The advantages of the second goal are less obvious.
Too some extent, it goes hand-in-hand with the first. For the cache to work efficiently you need to avoid pointer chasing, which naturally leads to fewer allocations. But there are other advantages as well.
Data with fewer individual allocations puts less pressure on the memory system. It gives you less fragmentation, more control over memory usage, allocation patterns that are easier to profile and optimize, etc.
It also makes the data easier to move and copy, since it requires less pointer patching, which let's you do lots of cool tricks with it. In general, it's just neater.
For static data, such as resources loaded from disk, both these goals can be easily achieved, by using the blob method.
For dynamic data, things get trickier. If you just use STL as you are told to, you end up with scary stuff like std::map< std::string, std::vector<std::string> > where the cache and memory usage is all over the place.
So let's try and see how we can escape that monster's lair by looking at a component and improving it step-by-step until we get to something nicer.
Introducing the DataComponent
The DataComponent is a part of our entity system that is used to store small amounts of arbitrary dynamic data for an entity. For example, it can be used to store a character sheet:
name = "The One"
stats = {
health = 100
mana = 200
}
status_effects = {
drunk = true
delirious = true
}As data format we use a restricted form of JSON where the only allowed values are:
- booleans
- floats
- strings
- objects
- arrays of numbers
Note that arbitrary arrays are not allowed. Arrays can only be lists of numbers, i.e. [0 0.5 0.5 0.7]. If you want to have sets of other items, you need to use an object with unique values (possibly GUIDs) as the keys.
The reason for these restrictions is that we want to make sure that all operations on the data merge in simple and well-defined ways. This makes it easier to use the data in collaborative workflows.
Strings and float arrays are regarded as monolithic with respect to merging. This means that all data operations can be reduced to setting object keys, which makes them easy to reason about.
We assume that the amount of data stored for each entity will be small, but that lots of entities will have data and that it is important to keep memory use low and performance high.
The naive approach
Representing a tree structure that can include both strings and arrays is pretty complicated and if we just start to sketch the structure using our regular STL tools it quickly becomes pretty messy:
enum class DataType {BOOL, FLOAT, STRING, OBJECT, ARRAY};
struct DataValue
{
DataType type;
union {
bool b;
float f;
std::string *s;
std::vector<float> *array;
std::map<std::string, DataValue> *object;
};
};That doesn't look very fun at all. There are tons of allocations everywhere, both explicit (std::string *) and implicit (inside string, vector, and map). So let's start working.
1: Hash the keys
If we consider how we will use this data structure, we will always be setting and getting the values corresponding to certain keys. We don't have a pressing need to extract the key strings themselves. This means we can use hashed strings instead of the strings themselves for lookup.
Since we assume that the number of keys will be small, it is probably enough to use an unsigned rather than an uint64_t for the hash value:
std::map<unsigned, DataValue> *object;Great start. That's a lot of allocations already gone and as a bonus we also got rid of a lot of string comparisons.
2: Flatten the structure
If we also sacrifice the ability of being able to enumerate all the (hashed) keys in a particular object we can go even further and flatten the entire structure.
That is, instead of representing the data as:
name = "The One"
stats = {
health = 100
mana = 200
}
status_effects = {
drunk = true
delirious = true
}We will use:
name = "The One"
stats.health = 100
stats.mana = 200
status_effects.drunk = true
status_effects.delirious = trueJust as before we represent these keys as hashed values, so "stats.health" is represented by an unsigned containing the hashed value of "stats.health".
(Note: we won't actually hash the string "stats.health" directly, because that could lead to problems if some of our keys contained a "." character. Instead we hash each sub key separately and then hash them all together.)
With this approach, the user will still be able to look up a particular piece of data, such as stats.health, but she won't be able to enumerate all the keys and values under stats, since the hierarchy information is lost. But that's ok.
Getting rid of the tree structure allows us to store our data in a flat array:
enum class DataType {BOOL, FLOAT, STRING, OBJECT, ARRAY};
struct Entry
{
unsigned key;
DataType type;
union {
bool b;
float f;
std::string *s;
std::vector<float> *array;
};
};
std::vector<Entry> data;Note that we no longer have a lookup hierarchy for the entries. To find a particular key we have to linearly search std::vector<Entry> for a match. Luckily, linear search is the best thing caches know and for reasonable sizes, the search will run faster than a std::map lookup.
We could sort the entries and do a binary search, but since the number of entries will be small, it is probably not even worth bothering.
3: Rewrite using a structure-of-arrays approach
One thing that does slow the search for a particular key down is all the additional data we need to load into the cache to perform the search. As we scan the vector for a particular key, the cache will also be filled by the type and value data.
We can solve this with the standard techinque of rewriting our "array of structures" as a "structure of arrays". I.e., we break out each field of the structure into its own array:
enum class DataType {BOOL, FLOAT, STRING, OBJECT, ARRAY};
struct Value
{
union {
bool b;
float f;
std::string *s;
std::vector<float> *array;
}
};
std::vector<unsigned> keys;
std::vector<DataType> types;
std::vector<Value> values;Now when searching for a particular key, we only need to load the keys table which means the cache only contains useful data.
4: Co-allocate the arrays
Not too shabby, but we still have three separate allocations for the three vectors. To get rid of that we have to stop using these fancy std::vectors and go back to the basics, and in terms of basics, a std::vector<unsigned> is really just:
struct {
int capacity;
int size;
unsigned *data;
};So we can represent our three vectors as:
int capacity;
int size;
unsigned *keys;
DataType *types;
Value *values;Note that as a bonus, we can now share the capacity and the size fields between the vectors, since they all have the same values and change together.
The arrays are also all reallocated at the same time. We can make use of this and instead of doing three separate allocations, we just allocate one big buffer and lay them out sequentially in that buffer:
-----------------------------------
buffer | keys | types | values |
-----------------------------------Which in code would be something like:
char *buffer = allocate(capacity * (sizeof(unsigned) + sizeof(DataType) + sizeof(Value));
keys = (unsigned *)buffer;
types = (DataType *)(keys + capacity);
values = (Value *)(types + capacity);Presto, now we have all the header data stored in a single buffer.
5: Get rid of STL
We still have these pointers to deal with in the values union:
std::string *s;
std::vector<float> *array;Let's start by getting rid of those STL types. At this point they are no longer helping. STL is based around individual allocations which we don't want. We also have two levels of indirection there. First, the pointer to the std::vector, and then the pointer to the actual data inside the std::vector type.
So, we just replace them with their non-STL equivalents:
struct {
char *data;
} s;
struct {
int capacity;
int size;
float *data;
} array;This actually bloats the size of the Value union from 64 to 128 bits, which is less than stellar, but have faith, we are going in the right direction.
6: Put the value data in a buffer
We can now repeat the same trick that we did with the header data and put all the strings and float arrays in a big single buffer:
-------------------------------------------------------------
| "hello" | [0 1 3] | "mana" | [0 2] | ... unused space ... |
-------------------------------------------------------------To allocate some data we just keep track of this buffer and how much of it is currently in use:
struct value_buffer
{
char *p;
unsigned capacity;
unsigned size;
};
void *allocate_memory(value_buffer &vb, unsigned size)
{
if (vb.size + size > vb.capacity)
return nullptr;
auto res = vb.p + vb.size;
vb.size += size;
return res;
}There are two things we need to take care of here. First, we may run out of space in this buffer as we add more data. Second, the individual values we store may change size and no longer fit in their designated place.
The first issue is no big problem. If we run out of space, we can just allocate a bigger buffer as std::vector does.
The second issue is no biggie either. If an item needs to grow, we just allocate a new bigger slot for it from the buffer. This will leave a hole where the item was originally located:
-------------------------------------------------------------
| ....... | [0 1 3] | "mana" | [0 2] | "hello more" | ..... |
-------------------------------------------------------------These "holes" will cause fragmentation of the buffer and eventually make it run out of space. If this was a general purpose memory allocator that would be a serious issue that we would have to be seriously worried about.
But in our case it doesn't really matter. The number of items is small and we control the pointers to them, so we can easily move them around and defragment the buffer when necessary. But in fact, we don't even have to do that. If we get fragmentation we will eventually run out of space. This will reallocate the buffer which will also defragment it. This is enough for our purposes.
7: Slim down the Value struct
Since all our value data are now allocated in a single buffer we can use the buffer offset rather than the pointer to refer to the data. This saves us memory (from 64 bits to 32 bits) and as an added bonus, it also allows us to relocate the buffer without having to do pointer patching.
For these small snippets of data 64 K is plenty, which means we can fit both the offset and the size in 32 bits:
struct Data {
uint16_t offset;
uint16_t size;
};(If you need more memory, you could use a full uint32_t for the offset and store the size in the buffer, together with the data.)
So know we have:
struct Value
{
union {
bool b;
float f;
Data data;
}
};where the data field is used for both strings and arrays. This means our Value structure is now just 32 bits, half the size of the STL version.
8: Merge the final two allocations
Now we are down to just two buffers: one for the header (keys and types), and one for the values (strings and arrays). Wouldn't it be great if we could merge them to use just a single allocation?
Let's try our old trick and putting them together in the same buffer:
--------------------------------------------------------
| Header | Values | ....... free space ...... |
--------------------------------------------------------This doesn't really work, because now the Header part can't grow unless we move the values out of the way.
We could give them each a little bit of free space:
----------------------------------------------------------
| Header | ... free ... | Values | ... free ... |
----------------------------------------------------------Now they both have some room to grow, but we are not utilizing the free space to its maximum potential. The Values section may run out of free space before the Header, forcing us to reallocate the buffer even though we actually have some free space left in it. That's a bummer.
But what if we do this:
----------------------------------------------------------
| Header | ........ free space ........ | Values |
----------------------------------------------------------Instead of allocating the values bottom up, we allocate them top down in the buffer. Now the headers and the values can share the same free space and we don't have to reallocate the buffer until they meet in the middle. Nice!
Benefits of single buffer structures
Stripping everything down to a single buffer gives us many of the same benefits that the blob approach gives us for static data.
Since the data is stored in a single buffer and doesn't contain any pointers it is fully relocatable. We can move it around in memory as we please and make copies of it with a simple memcpy().
If we want to save the data to disk, as a static resource or as part of a saved game, we don't need any special serialization code. We can just write and read the data directly.
The pointer arithmetic that is needed to manipulate data in this way may seem complex at first, if you are not used to think about pointer operations on raw memory. But I would argue that once you get used to that, this kind of representation is in many ways simpler than the multi-level std::vector, std::map based approach that we started with. And in that simplicity lies a lot of power.
But wait, I have been cheating!
Yes, to be honest I have been cheating all this time.
I have ignored the old adage of data-oriented programming:
Where there is one, there are many.
All this time I have been talking about how one DataComponent can store its data in a single buffer. But in our real system we will have many DataComponents. We might have 1000 entities, each with a DataComponent that stores a single health float. Even if each component uses a contiguous buffer, that's still a lot of individual memory allocations.
Wouldn't it be nice if we could somehow bundle them together too?
But that represents a trickier challenge. We need to find a way to allocate a lot of individual objects in a single buffer in such a way that each individual object can grow and shrink independently.
And that will be the topic for another post.
