前几天我正在玩,试图看看我可以在多大程度上优化某些东西.我决定从一个简单的地图开始,只是进行线性搜索,找出是否存在元素,然后尝试优化其中的大部分.另外,为了比较,我使用std :: find对std :: map和std :: vector做同样的事情.
地图的结果是预期的,比我的地图更慢的创建和破坏,但更快的速度(实际上,我无法测量它,它总是返回0).
问题出在std :: vector上.我希望它比我的实现慢,但不是,我真的不明白它是如何相同或更快,因为我的实现正在跳过最坏的情况(值不在向量中)并且是使用结果缓存.
谁能在这里解决一些问题？我知道stl背后的人是半神,但仍然没有意义.
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,Third : Not Found
Release : 1425.08
--------------------
Linear Map V0:
Build : 194.012 ms
Loop : 49.0052 ms
Find : 1.88915 ms -> First : Found,Third : Not Found
Release : 109.004
这是地图的代码：
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(),seed[1000] / 2) != v.end();
// Shouldn't exist
found3 = find(v.begin(),-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") << ",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") << ",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;
}
解决方法
您的容器是链表,而std :: vector是动态大小的数组.
链表方法具有诸多优点,例如能够在不重新分配的情况下插入元素.
然而阵列方法有一些显着的优点：
>线性搜索只扫描内存,这正是构建缓存和预取程序的内容.链接列表的扫描效率较低,因为每次跳转到未缓存的内存意味着昂贵的缓存未命中.>线性阵列扫描易于矢量化.如果使用-O3进行编译,则编译器可能会使用std :: find的矢量化版本.由于内存依赖性,无法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下一个指针,这有效地使您的元素更大.而且,每个非预分配的节点必须支付分配器的开销(即新的和删除的计费数据).这意味着您可以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.
