前几天我在玩游戏，想看看我能优化到什么程度。我决定从一个简单的 map 开始，它只进行线性搜索以查找元素是否存在，然后尝试优化其中的大部分内容。此外，为了进行比较，我使用 std::find 对 std::map 和 std::vector 执行相同的操作。
map 的结果是预期的，创建和销毁比我的 map 慢，但速度快得多(实际上，我一直无法测量它，它总是返回0)。 问题在于 std::vector。我预计它会比我的实现慢，但事实并非如此，而且我真的不明白它怎么可能相同或更快，因为我的实现跳过了最坏的情况(值不在 vector 中)并且是使用结果缓存。
任何人都可以在这里阐明一些问题吗？我知道 STL 背后的家伙是半神，但这仍然没有意义。
基准测试结果(i3、Windows 8.1 Pro 64、Visual Studio 2013):
std::vector :
Build : 85.0042 ms
Loop : 37.0011 ms
Find : 1.82259 ms -> First : Found, Second : Found, Third : Not Found
Release : 0 ms
--------------------
std::map :
Build : 6929.41 ms
Loop : 570.032 ms
Find : 0 ms -> First : Found, Second : Found, Third : Not Found
Release : 1425.08
--------------------
Linear Map V0:
Build : 194.012 ms
Loop : 49.0052 ms
Find : 1.88915 ms -> First : Found, Second : Found, Third : Not Found
Release : 109.004
这是 map 的代码:
template<typename T>
class LinearMap0
{
public:
LinearMap0()
{
_end = _root = new Node;
_prebuffer = nullptr;
prebufferCapacity = 0;
_alive = true;
prebufferMarker = 0;
_cache = _mm_set1_epi32(-1);
for (auto& ptr : _cacheBuffer) ptr = nullptr;
MinID = INT32_MAX - 1;
MaxID = -1;
}
void PreAllocate(int Count)
{
prebufferCapacity = Count;
_prebuffer = new Node[Count];
}
~LinearMap0()
{
if (_alive)
{
Release();
}
}
void Release()
{
Node* marker = _end;
while (marker->Prev)
{
marker = marker->Prev;
if (!marker->Next->IsPreAllocated) delete marker->Next;
}
if (!_root->IsPreAllocated) delete _root;
delete[] _prebuffer;
_alive = false;
}
void AddElement(int ID,T element)
{
Node* tmp = nullptr;
if (prebufferMarker < prebufferCapacity)
{
// Use a pre-allocated object
tmp = &_prebuffer[prebufferMarker];
prebufferMarker++;
tmp->IsPreAllocated = true;
}
else
{
tmp = new Node;
}
tmp->ID = ID;
tmp->Data = element;
// Update list
_end->Next = tmp;
Node* prevEnd = _end;
_end = tmp;
_end->Prev = prevEnd;
bool isMin = ID < MinID; MinID = ID * isMin + (1 - isMin) * MinID;
bool isMax = ID > MaxID; MaxID = ID * isMax + (1 - isMax) * MaxID;
}
void DeleteLast()
{
Node* tmp = _end;
_end = _end->Prev;
_end->Next = nullptr;
delete tmp;
}
template<class Function>
void Loop(Function&& f, bool Forward = true)
{
if (Forward)
{
Node* marker = _root;
while (marker->Next)
{
marker = marker->Next;
f(marker->Data);
}
}
else
{
Node* marker = _end;
while (marker->Prev)
{
marker = marker->Prev;
f(marker->Data);
}
}
}
T* Find(int ID)
{
// Bounds check
if (ID < MinID || ID > MaxID) return nullptr;
// Check it it's in the cache
// Compare the value to every value in the cache
__m128i idxSSE = _mm_set1_epi32(ID);
__m128i C = _mm_cmpeq_epi32(_cache, idxSSE);
// To change form -1 to 1
C = _mm_mul_epi32(C, _mm_set1_epi32(-1));
// Now C holds 1 if true, or 0 if false (in each of its 4 members). It should only be ONE set at 1
__m128i tmp = _mm_set1_epi32(1);
__m128i S = _mm_sub_epi32(tmp, C);
// Now find the index
int i = S.m128i_i32[0] * (C.m128i_i32[1] + S.m128i_i32[1] * (2 * C.m128i_i32[2] + S.m128i_i32[2] * (3 * C.m128i_i32[3] + S.m128i_i32[3] * -1)));
if (i != -1)
return _cacheBuffer[i];
// Traverse the list
Node* marker0 = _root;
T* obj = nullptr;
while (true)
{
if (marker0->ID == ID)
{
obj = &marker0->Data;
}
if (marker0->Next) marker0 = marker0->Next; else break;
}
// Cache value and return
_cache.m128i_i32[cacheMarker] = ID;
_cacheBuffer[cacheMarker] = obj;
cacheMarker = (cacheMarker + 1) & 3; // x & 3 = x % 4
return obj;
}
private:
struct Node
{
Node()
{
Prev = nullptr;
Next = nullptr;
IsPreAllocated = false;
ID = -1;
}
T Data;
Node* Prev;
Node* Next;
bool IsPreAllocated;
int ID;
};
Node* _root;
Node* _end;
Node* _prebuffer;
int prebufferCapacity;
int prebufferMarker;
bool _alive;
__m128i _cache;
T* _cacheBuffer[4];
int cacheMarker;
int MinID, MaxID;
};
这是基准:
// Initialize seeds
const __int64 ecount = 5 * 1000*1000;
vector<__int64> seed(ecount);
for (__int64 i = 0; i < ecount; i++)
{
seed[i] = i;
}
random_shuffle(seed.begin(), seed.end());
///////////// std::vector
vector<__int64> v;
cout << "--------------------" << endl;
cout << "std::vector :" << endl;
cout << " Build : " << time_call([&]()
{
v.resize(ecount/2);
for (__int64 i = 0; i < ecount; i++)
{
if (i < (ecount / 2))
v[i] = seed[i];
else
v.push_back(seed[i]);
}
}) << " ms" << endl;
cout << " Loop : " << time_call([&]()
{
for (auto& n : v)
n /= 2;
}) << " ms" << endl;
bool found1, found2, found3;
cout << " Find : " << (((float)time_call([&]()
{
for (int i = 0; i < 15; i++)
{
// Should exist
found1 = find(v.begin(), v.end(), seed[5] / 2) != v.end();//find(seed[5]) != m.end();
found2 = find(v.begin(), v.end(), seed[1000] / 2) != v.end();
// Shouldn't exist
found3 = find(v.begin(), v.end(), -1234) != v.end();
}
})) / 15.0) / 3.0;
cout << " ms " << " -> First : " << ((found1) ? "Found" : "Not Found") << ", Second : " << ((found2) ? "Found" : "Not Found") << ", Third : " << ((found3) ? "Found" : "Not Found") << endl;
cout << " Release : " << time_call([&]()
{
v.clear();
}) << " ms" << endl;
///////////// std::map
map<__int64, __int64> m;
cout << "--------------------" << endl;
cout << "std::map :" << endl;
cout << " Build : " << time_call([&]()
{
for (__int64 i = 0; i < ecount; i++)
{
m[seed[i]] = seed[i];
}
}) << " ms" << endl;
cout << " Loop : " << time_call([&]()
{
for (auto& n : m)
n.second /= 2;
}) << " ms" << endl;
cout << " Find : " << (((float)time_call([&]()
{
for (int i = 0; i < 15; i++)
{
// Should exist
found1 = m.find(seed[5]) != m.end();
found2 = m.find(seed[1000]) != m.end();
// Shouldn't exist
found3 = m.find(-1234) != m.end();
}
})) / 15.0) / 3.0;
cout << " ms " << " -> First : " << ((found1) ? "Found" : "Not Found") << ", Second : " << ((found2) ? "Found" : "Not Found") << ", Third : " << ((found3) ? "Found" : "Not Found") << endl;
cout << " Release : " << time_call([&]()
{
m.clear();
}) << endl;
///////////// Linear Map V0
LinearMap0<__int64> c;
cout << "--------------------" << endl;
cout << "Linear Map V0:" << endl;
cout << " Build : " << time_call([&]()
{
c.PreAllocate(ecount / 2);
for (__int64 i = 0; i < ecount; i++)
{
c.AddElement(seed[i],seed[i]);
}
}) << " ms" << endl;
cout << " Loop : " << time_call([&]()
{
c.Loop([](__int64& Data)
{
Data /= 2;
});
}) << " ms" << endl;
cout << " Find : " << (((float)time_call([&]()
{
for (int i = 0; i < 15; i++)
{
// Should exist
found1 = c.Find(seed[5]);
found2 = c.Find(seed[1000]);
// Shouldn't exist
found3 = c.Find(-1234);
}
})) / 15.0) / 3.0;
cout << " ms -> First : " << ((found1) ? "Found" : "Not Found") << ", Second : " << ((found2) ? "Found" : "Not Found") << ", Third : " << ((found3) ? "Found" : "Not Found") << endl;
cout << " Release : " << time_call([&]()
{
c.Release();
}) << endl;
编辑:time_call 是:
template <class Function>
double time_call(Function&& f)
{
chrono::time_point<chrono::high_resolution_clock> start, end;
start = chrono::high_resolution_clock::now();
f();
end = chrono::high_resolution_clock::now();
return ((double)(chrono::duration_cast<chrono::nanoseconds>(end - start).count())) / 1000000.0;
}
最佳答案
您的容器是一个链表，而 std::vector 是一个动态大小的数组。
链表方法有很多好处，例如无需重新分配即可插入元素。
但是数组方法有一些显着的优点:
- 线性搜索只是扫描内存，这正是缓存和预取器的目的所在。链表扫描的效率会降低，因为每次跳转到未缓存的内存都意味着代价高昂的缓存未命中。
- 线性阵列扫描很容易矢量化。如果您使用
-O3进行编译，那么编译器可能会使用std::find的矢量化版本。由于内存依赖性，不可能对链表扫描进行矢量化。 - 使用的内存量。您的链接列表必须维护一个
next指针，它有效地使您的元素更大。此外，每个非预分配的Node都必须支付分配器的开销(即new和delete的会计数据)。这意味着您会更快地达到内存带宽限制，并且可以在缓存中容纳更少的元素。
关于c++ - 为什么 std::vector 这么快(或者我的实现太慢了)，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/19985827/