C++ std:：shared_ptr与std:：make_shared：意外的缓存未命中和分支预测
我试图测量std:：shared_ptr和std:：make_shared创建的指针的效率 我有下一个测试代码：C++ std:：shared_ptr与std:：make_shared：意外的缓存未命中和分支预测,c++,caching,valgrind,branch-prediction,C++,Caching,Valgrind,Branch Prediction,我试图测量std:：shared_ptr和std:：make_shared创建的指针的效率 我有下一个测试代码： #include <iostream> #include <memory> #include <vector> struct TestClass { TestClass(int _i) : i(_i) {} int i = 1; }; void sum(const std::vector<std::shared_ptr&
#include <iostream>
#include <memory>
#include <vector>
struct TestClass {
TestClass(int _i) : i(_i) {}
int i = 1;
};
void sum(const std::vector<std::shared_ptr<TestClass>>& v) {
unsigned long long s = 0u;
for(size_t i = 0; i < v.size() - 1; ++i) {
s += v[i]->i * v[i + 1]->i;
}
std::cout << s << '\n';
}
void test_shared_ptr(size_t n) {
std::cout << __FUNCTION__ << "\n";
std::vector<std::shared_ptr<TestClass>> v;
v.reserve(n);
for(size_t i = 0u; i < n; ++i) {
v.push_back(std::shared_ptr<TestClass>(new TestClass(i)));
}
sum(v);
}
void test_make_shared(size_t n) {
std::cout << __FUNCTION__ << "\n";
std::vector<std::shared_ptr<TestClass>> v;
v.reserve(n);
for(size_t i = 0u; i < n; ++i) {
v.push_back(std::make_shared<TestClass>(i));
}
sum(v);
}
int main(int argc, char *argv[]) {
size_t n = (argc == 3 ) ? atoi(argv[2]) : 100;
if(atoi(argv[1]) == 1) {
test_shared_ptr(n);
} else {
test_make_shared(n);
}
return 0;
}
$ g++ --version
g++ (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
实际上，我通常忽略分支未命中，因为我经常调整没有不可预测分支的SIMD循环，并且可以编程计数不同事件的硬件计数器数量有限。如果多次运行测试，是否会得到一致的结果？这取决于实现。试着用其他编译器进行测试。@FrançoisAndrieux，是的，我不只是模拟，而不是测量？表示使用16384个2位饱和计数器阵列预测条件分支。它应该代表2004年的典型桌面/服务器。按照现代标准，这种过于简单的分支预测是一个笑话，即使在2004年，对于高性能x86也是过于简单的；看见Intel因为Haswell使用IT-TAGE。对于实际的性能测量，您可能想看看这个：如果您多次运行测试，是否会得到一致的结果？这取决于实现。试着用其他编译器进行测试。@FrançoisAndrieux，是的，我不只是模拟，而不是测量？表示使用16384个2位饱和计数器阵列预测条件分支。那就是苏
PPOS代表了2004年的典型桌面/服务器。按照现代标准，这种过于简单的分支预测是一个笑话，即使在2004年，对于高性能x86也是过于简单的；看见英特尔，因为Haswell使用IT-TAGE。对于真实的性能测量，您可能想看看以下内容：
valgrind --tool=cachegrind --branch-sim=yes ./cache_misses.bin 2 100000
==2014== Cachegrind, a cache and branch-prediction profiler
==2014== Copyright (C) 2002-2017, and GNU GPL'd, by Nicholas Nethercote et al.
==2014== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==2014== Command: ./cache_misses.bin 2 100000
==2014==
--2014-- warning: L3 cache found, using its data for the LL simulation.
--2014-- warning: specified LL cache: line_size 64 assoc 12 total_size 9,437,184
--2014-- warning: simulated LL cache: line_size 64 assoc 18 total_size 9,437,184
test_make_shared
18107093611968
==2014==
==2014== I refs: 41,283,983
==2014== I1 misses: 1,805
==2014== LLi misses: 1,696
==2014== I1 miss rate: 0.00%
==2014== LLi miss rate: 0.00%
==2014==
==2014== D refs: 14,997,474 (8,834,690 rd + 6,162,784 wr)
==2014== D1 misses: 241,781 ( 164,368 rd + 77,413 wr)
==2014== LLd misses: 84,413 ( 7,943 rd + 76,470 wr)
==2014== D1 miss rate: 1.6% ( 1.9% + 1.3% )
==2014== LLd miss rate: 0.6% ( 0.1% + 1.2% )
==2014==
==2014== LL refs: 243,586 ( 166,173 rd + 77,413 wr)
==2014== LL misses: 86,109 ( 9,639 rd + 76,470 wr)
==2014== LL miss rate: 0.2% ( 0.0% + 1.2% )
==2014==
==2014== Branches: 7,031,695 (6,426,222 cond + 605,473 ind)
==2014== Mispredicts: 216,010 ( 15,442 cond + 200,568 ind)
==2014== Mispred rate: 3.1% ( 0.2% + 33.1% )
$ g++ --version
g++ (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.