boost::pool_allocator 比 std::allocator 慢很多
boost::pool_allocator significantly slower than std::allocator
我正在学习内存池，并尝试在我的项目中使用 boost::pool_allocator。根据documment，我做了一个关于时间成本的小测试：
template <typename Alloc>
void test()
{
using namespace std::chrono;
auto t0 = high_resolution_clock::now();
for (int i = 0; i < 1000; ++i) {
std::vector<int, Alloc> vec;
for (int j = 0; j < 10000; ++j)
vec.push_back(i + j);
}
auto t1 = high_resolution_clock::now();
auto time_ms = duration<double>(t1 - t0).count() * 1e3;
cout << "time cost: " << time_ms << " ms" << endl;
}
int main()
{
test<std::allocator<int>>();
test<boost::pool_allocator<int>>();
}
结果是：
time cost: 3.97602 ms
time cost: 91.3943 ms
Boost doc 说：
Pools are generally used when there is a lot of allocation and deallocation of small objects.
所以我预计 boost::pool_allocator 比上面代码中的 std::allocator 花费更少的时间，但测试结果显示它更差。
我使用boost::pool_allocator错了吗？在什么情况下我可以通过使用内存池（或只是 Boost pool/pool_allocator）获得加速？
Pools are generally used when there is a lot of allocation and deallocation of small objects.
vector 不分配“小对象”。 vector<T> 分配 个数组 。或者更具体地说，分配在一块内存中的单个数组足以容纳（至少）sizeof(T) * size 字节。
池分配器在这种分配模式下非常糟糕。
供参考：
template <typename T,
typename UserAllocator = default_user_allocator_new_delete,
typename Mutex = details::pool::default_mutex,
unsigned NextSize = 32,
unsigned MaxSize = 0>
class pool_allocator;
我想也许是锁定造成的。另外，可以有更好的提示。
让我们测试一下！ Live On Compiler Explorer
#include <boost/core/demangle.hpp>
#include <boost/pool/pool_alloc.hpp>
#include <chrono>
#include <iomanip>
#include <iostream>
#include <vector>
using namespace std::chrono_literals;
auto static now = std::chrono::high_resolution_clock::now;
template <typename Alloc> void test(int run, Alloc alloc = {}) {
auto load = [=](bool RESERVE, unsigned ITERATIONS = 1'000, unsigned SIZE = 10'000) {
for (unsigned i = 0; i < ITERATIONS; ++i) {
std::vector<int, Alloc> vec(alloc);
if (RESERVE)
vec.reserve(SIZE);
for (unsigned j = 0; j < SIZE; ++j)
vec.push_back(i + j);
}
};
auto lap_time = [t0 = now()]() mutable {
return now() - std::exchange(t0, now());
};
load(false); auto without_reserve = lap_time() / 1.0ms;
load(true); auto with_reserve = lap_time() / 1.0ms;
std::cout << "run " << run //
<< " naive: " << std::setw(7) << without_reserve << "ms" //
<< " reserved: " << std::setw(7) << with_reserve << "ms" //
<< "(" << boost::core::demangle(typeid(Alloc).name()) << ")" //
<< std::endl;
}
void run_tests(int run) {
test<std::allocator<int>>(run);
using NullMx = boost::details::pool::null_mutex;
using Mx = boost::details::pool::default_mutex;
using Malloc = boost::default_user_allocator_malloc_free;
using NewDelete = boost::default_user_allocator_new_delete;
//
// no hints
//
test<boost::pool_allocator<int, Malloc, NullMx>>(run);
test<boost::pool_allocator<int, NewDelete, NullMx>>(run);
test<boost::pool_allocator<int, Malloc, Mx>>(run);
test<boost::pool_allocator<int, NewDelete, Mx>>(run);
//
// hinted
//
test<boost::pool_allocator<int, Malloc, NullMx, 1'000, 0>>(run);
test<boost::pool_allocator<int, NewDelete, NullMx, 1'000, 0>>(run);
test<boost::pool_allocator<int, Malloc, Mx, 1'000, 0>>(run);
test<boost::pool_allocator<int, NewDelete, Mx, 1'000, 0>>(run);
}
int main()
{
std::cout << std::fixed << std::setprecision(3);
for (int run : {1,2,3}) {
auto t0 = now();
run_tests(run);
std::cout << " -- Done (" << (now() - t0) / 1.ms << "ms)" << std::endl;
}
}
Compiler Explorer is showing some real inconsistent spikes; my own machine doesn't:
run 1 naive: 8.025ms reserved: 5.412ms(std::allocator<int>)
run 1 naive: 92.212ms reserved: 31.166ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 32u, 0u>)
run 1 naive: 93.466ms reserved: 29.901ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 32u, 0u>)
run 1 naive: 92.488ms reserved: 29.883ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 32u, 0u>)
run 1 naive: 92.450ms reserved: 29.824ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 32u, 0u>)
run 1 naive: 82.879ms reserved: 27.478ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 1000u, 0u>)
run 1 naive: 82.775ms reserved: 28.187ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 1000u, 0u>)
run 1 naive: 83.189ms reserved: 27.404ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 1000u, 0u>)
run 1 naive: 83.159ms reserved: 27.468ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 1000u, 0u>)
-- Done (947.595ms)
run 2 naive: 8.007ms reserved: 5.543ms(std::allocator<int>)
run 2 naive: 92.225ms reserved: 29.882ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 32u, 0u>)
run 2 naive: 92.311ms reserved: 29.805ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 32u, 0u>)
run 2 naive: 92.601ms reserved: 29.873ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 32u, 0u>)
run 2 naive: 92.421ms reserved: 30.028ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 32u, 0u>)
run 2 naive: 83.028ms reserved: 27.493ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 1000u, 0u>)
run 2 naive: 82.822ms reserved: 27.427ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 1000u, 0u>)
run 2 naive: 83.230ms reserved: 27.493ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 1000u, 0u>)
run 2 naive: 83.104ms reserved: 27.466ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 1000u, 0u>)
-- Done (944.958ms)
run 3 naive: 8.068ms reserved: 5.422ms(std::allocator<int>)
run 3 naive: 92.282ms reserved: 29.880ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 32u, 0u>)
run 3 naive: 92.064ms reserved: 29.960ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 32u, 0u>)
run 3 naive: 92.339ms reserved: 29.928ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 32u, 0u>)
run 3 naive: 92.977ms reserved: 29.890ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 32u, 0u>)
run 3 naive: 82.906ms reserved: 27.388ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 1000u, 0u>)
run 3 naive: 82.784ms reserved: 27.585ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 1000u, 0u>)
run 3 naive: 83.157ms reserved: 28.233ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 1000u, 0u>)
run 3 naive: 83.098ms reserved: 27.466ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 1000u, 0u>)
-- Done (945.629ms)
仍然，很明显，总是更慢。让我们来介绍一下
探查器
仅将标准分配器与 boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 1000u, 0u> 进行比较：
到目前为止一切顺利！什么事要花这么多时间？
其中 unordered_malloc 内联到来自各种 Boost Pool headers 的大量行。从 boost/pool/simple_segregated_storage.hpp 内联了最严重的违规者：（第二列是相对于 parent 的成本百分比）：
这些行在 try_malloc_n
template <typename SizeType>
void * simple_segregated_storage<SizeType>::try_malloc_n(
void * & start, size_type n, const size_type partition_size)
{
void * iter = nextof(start);
while (--n != 0)
{
void * next = nextof(iter);
if (next != static_cast<char *>(iter) + partition_size)
{
// next == 0 (end-of-list) or non-contiguous chunk found
start = iter;
return 0;
}
iter = next;
}
return iter;
}
其中 self-describes 作为：
The function attempts to find n contiguous chunks of size partition_size in the free list, starting at start. If it succeds, it returns the last chunk in that contiguous sequence, so that the sequence is known by [start, {retval}] If it fails, it does do either because it's at the end of the free list or hits a non-contiguous chunk. In either case, it will return 0, and set start to the last considered chunk. You are at the end of the free list if nextof(start) == 0. Otherwise, start points to the last chunk in the contiguous sequence, and nextof(start) points to the first chunk in the next contiguous sequence (assuming an ordered free list).
确实出现了，那么在这种情况下追逐隔离堆上的空闲块毕竟代价太大了。餐巾纸计算表明 try_malloc_n 占用了我们之前看到的 higher-level unordered_malloc 调用的 99.75%。
SHOCK：替代实施？
在我的调查过程中，我发现了一些可用于获得更多洞察力的定义，例如：
#define NDEBUG
//#define BOOST_POOL_INSTRUMENT 1
//#define BOOST_POOL_VALIDATE 1
//#define BOOST_POOL_VALGRIND 1
现在，我使用 VALIDATE/INSTRUMENT 观察预期效果（非常冗长的输出和轻微的性能下降）。
从阅读 name/code 我预计 BOOST_POOL_VALGRIND 会类似地降低性能（毕竟，当 运行ning Valgrind 时，它可能应该做额外的工作来避免误报内存错误， 正确的？）。令我惊讶的是，定义它使整个事情 运行 快如闪电：Live On Compiler Explorer
run 1 naive: 8.166ms reserved: 5.267ms(std::allocator<int>)
run 1 naive: 9.713ms reserved: 5.267ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 32u, 0u>)
run 1 naive: 8.853ms reserved: 5.226ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 32u, 0u>)
run 1 naive: 8.990ms reserved: 5.282ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 32u, 0u>)
run 1 naive: 8.899ms reserved: 5.246ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 32u, 0u>)
run 1 naive: 8.620ms reserved: 5.237ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 1000u, 0u>)
run 1 naive: 8.622ms reserved: 5.247ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 1000u, 0u>)
run 1 naive: 8.963ms reserved: 5.257ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 1000u, 0u>)
run 1 naive: 8.990ms reserved: 5.271ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 1000u, 0u>)
-- Done (127.276ms)
run 2 naive: 7.965ms reserved: 5.208ms(std::allocator<int>)
run 2 naive: 8.503ms reserved: 5.236ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 32u, 0u>)
run 2 naive: 8.809ms reserved: 5.254ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 32u, 0u>)
run 2 naive: 8.954ms reserved: 5.278ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 32u, 0u>)
run 2 naive: 8.878ms reserved: 5.279ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 32u, 0u>)
run 2 naive: 8.694ms reserved: 5.243ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 1000u, 0u>)
run 2 naive: 8.661ms reserved: 5.249ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 1000u, 0u>)
run 2 naive: 8.920ms reserved: 5.248ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 1000u, 0u>)
run 2 naive: 8.952ms reserved: 5.261ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 1000u, 0u>)
-- Done (125.680ms)
run 3 naive: 7.949ms reserved: 5.221ms(std::allocator<int>)
run 3 naive: 8.498ms reserved: 5.238ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 32u, 0u>)
run 3 naive: 8.813ms reserved: 5.230ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 32u, 0u>)
run 3 naive: 9.033ms reserved: 5.279ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 32u, 0u>)
run 3 naive: 8.909ms reserved: 5.252ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 32u, 0u>)
run 3 naive: 8.605ms reserved: 5.244ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 1000u, 0u>)
run 3 naive: 8.623ms reserved: 5.246ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 1000u, 0u>)
run 3 naive: 8.918ms reserved: 5.247ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 1000u, 0u>)
run 3 naive: 8.969ms reserved: 5.268ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 1000u, 0u>)
可悲的是，查看细节证实它通过直接委托给标准库来作弊（同时增加了一些 free_list/used_list 地址集的开销）。
总结
是的，标准 pool/simple_segregated_storage 实现在此负载下表现不佳。这是否真的是一个错误，我无法确定，但根据文档（你 mentioned 也是如此）看起来确实如此。
我正在学习内存池，并尝试在我的项目中使用 boost::pool_allocator。根据documment，我做了一个关于时间成本的小测试：
template <typename Alloc>
void test()
{
using namespace std::chrono;
auto t0 = high_resolution_clock::now();
for (int i = 0; i < 1000; ++i) {
std::vector<int, Alloc> vec;
for (int j = 0; j < 10000; ++j)
vec.push_back(i + j);
}
auto t1 = high_resolution_clock::now();
auto time_ms = duration<double>(t1 - t0).count() * 1e3;
cout << "time cost: " << time_ms << " ms" << endl;
}
int main()
{
test<std::allocator<int>>();
test<boost::pool_allocator<int>>();
}
结果是：
time cost: 3.97602 ms
time cost: 91.3943 ms
Boost doc 说：
Pools are generally used when there is a lot of allocation and deallocation of small objects.
所以我预计 boost::pool_allocator 比上面代码中的 std::allocator 花费更少的时间，但测试结果显示它更差。
我使用boost::pool_allocator错了吗？在什么情况下我可以通过使用内存池（或只是 Boost pool/pool_allocator）获得加速？
Pools are generally used when there is a lot of allocation and deallocation of small objects.
vector 不分配“小对象”。 vector<T> 分配 个数组 。或者更具体地说，分配在一块内存中的单个数组足以容纳（至少）sizeof(T) * size 字节。
池分配器在这种分配模式下非常糟糕。
供参考：
template <typename T,
typename UserAllocator = default_user_allocator_new_delete,
typename Mutex = details::pool::default_mutex,
unsigned NextSize = 32,
unsigned MaxSize = 0>
class pool_allocator;
我想也许是锁定造成的。另外，可以有更好的提示。
让我们测试一下！ Live On Compiler Explorer
#include <boost/core/demangle.hpp>
#include <boost/pool/pool_alloc.hpp>
#include <chrono>
#include <iomanip>
#include <iostream>
#include <vector>
using namespace std::chrono_literals;
auto static now = std::chrono::high_resolution_clock::now;
template <typename Alloc> void test(int run, Alloc alloc = {}) {
auto load = [=](bool RESERVE, unsigned ITERATIONS = 1'000, unsigned SIZE = 10'000) {
for (unsigned i = 0; i < ITERATIONS; ++i) {
std::vector<int, Alloc> vec(alloc);
if (RESERVE)
vec.reserve(SIZE);
for (unsigned j = 0; j < SIZE; ++j)
vec.push_back(i + j);
}
};
auto lap_time = [t0 = now()]() mutable {
return now() - std::exchange(t0, now());
};
load(false); auto without_reserve = lap_time() / 1.0ms;
load(true); auto with_reserve = lap_time() / 1.0ms;
std::cout << "run " << run //
<< " naive: " << std::setw(7) << without_reserve << "ms" //
<< " reserved: " << std::setw(7) << with_reserve << "ms" //
<< "(" << boost::core::demangle(typeid(Alloc).name()) << ")" //
<< std::endl;
}
void run_tests(int run) {
test<std::allocator<int>>(run);
using NullMx = boost::details::pool::null_mutex;
using Mx = boost::details::pool::default_mutex;
using Malloc = boost::default_user_allocator_malloc_free;
using NewDelete = boost::default_user_allocator_new_delete;
//
// no hints
//
test<boost::pool_allocator<int, Malloc, NullMx>>(run);
test<boost::pool_allocator<int, NewDelete, NullMx>>(run);
test<boost::pool_allocator<int, Malloc, Mx>>(run);
test<boost::pool_allocator<int, NewDelete, Mx>>(run);
//
// hinted
//
test<boost::pool_allocator<int, Malloc, NullMx, 1'000, 0>>(run);
test<boost::pool_allocator<int, NewDelete, NullMx, 1'000, 0>>(run);
test<boost::pool_allocator<int, Malloc, Mx, 1'000, 0>>(run);
test<boost::pool_allocator<int, NewDelete, Mx, 1'000, 0>>(run);
}
int main()
{
std::cout << std::fixed << std::setprecision(3);
for (int run : {1,2,3}) {
auto t0 = now();
run_tests(run);
std::cout << " -- Done (" << (now() - t0) / 1.ms << "ms)" << std::endl;
}
}
Compiler Explorer is showing some real inconsistent spikes; my own machine doesn't:
run 1 naive: 8.025ms reserved: 5.412ms(std::allocator<int>)
run 1 naive: 92.212ms reserved: 31.166ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 32u, 0u>)
run 1 naive: 93.466ms reserved: 29.901ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 32u, 0u>)
run 1 naive: 92.488ms reserved: 29.883ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 32u, 0u>)
run 1 naive: 92.450ms reserved: 29.824ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 32u, 0u>)
run 1 naive: 82.879ms reserved: 27.478ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 1000u, 0u>)
run 1 naive: 82.775ms reserved: 28.187ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 1000u, 0u>)
run 1 naive: 83.189ms reserved: 27.404ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 1000u, 0u>)
run 1 naive: 83.159ms reserved: 27.468ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 1000u, 0u>)
-- Done (947.595ms)
run 2 naive: 8.007ms reserved: 5.543ms(std::allocator<int>)
run 2 naive: 92.225ms reserved: 29.882ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 32u, 0u>)
run 2 naive: 92.311ms reserved: 29.805ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 32u, 0u>)
run 2 naive: 92.601ms reserved: 29.873ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 32u, 0u>)
run 2 naive: 92.421ms reserved: 30.028ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 32u, 0u>)
run 2 naive: 83.028ms reserved: 27.493ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 1000u, 0u>)
run 2 naive: 82.822ms reserved: 27.427ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 1000u, 0u>)
run 2 naive: 83.230ms reserved: 27.493ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 1000u, 0u>)
run 2 naive: 83.104ms reserved: 27.466ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 1000u, 0u>)
-- Done (944.958ms)
run 3 naive: 8.068ms reserved: 5.422ms(std::allocator<int>)
run 3 naive: 92.282ms reserved: 29.880ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 32u, 0u>)
run 3 naive: 92.064ms reserved: 29.960ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 32u, 0u>)
run 3 naive: 92.339ms reserved: 29.928ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 32u, 0u>)
run 3 naive: 92.977ms reserved: 29.890ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 32u, 0u>)
run 3 naive: 82.906ms reserved: 27.388ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 1000u, 0u>)
run 3 naive: 82.784ms reserved: 27.585ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 1000u, 0u>)
run 3 naive: 83.157ms reserved: 28.233ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 1000u, 0u>)
run 3 naive: 83.098ms reserved: 27.466ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 1000u, 0u>)
-- Done (945.629ms)
仍然，很明显，总是更慢。让我们来介绍一下
探查器
仅将标准分配器与 boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 1000u, 0u> 进行比较：
到目前为止一切顺利！什么事要花这么多时间？
其中 unordered_malloc 内联到来自各种 Boost Pool headers 的大量行。从 boost/pool/simple_segregated_storage.hpp 内联了最严重的违规者：（第二列是相对于 parent 的成本百分比）：
这些行在 try_malloc_n
template <typename SizeType>
void * simple_segregated_storage<SizeType>::try_malloc_n(
void * & start, size_type n, const size_type partition_size)
{
void * iter = nextof(start);
while (--n != 0)
{
void * next = nextof(iter);
if (next != static_cast<char *>(iter) + partition_size)
{
// next == 0 (end-of-list) or non-contiguous chunk found
start = iter;
return 0;
}
iter = next;
}
return iter;
}
其中 self-describes 作为：
The function attempts to find n contiguous chunks of size partition_size in the free list, starting at start. If it succeds, it returns the last chunk in that contiguous sequence, so that the sequence is known by [start, {retval}] If it fails, it does do either because it's at the end of the free list or hits a non-contiguous chunk. In either case, it will return 0, and set start to the last considered chunk. You are at the end of the free list if nextof(start) == 0. Otherwise, start points to the last chunk in the contiguous sequence, and nextof(start) points to the first chunk in the next contiguous sequence (assuming an ordered free list).
确实出现了，那么在这种情况下追逐隔离堆上的空闲块毕竟代价太大了。餐巾纸计算表明 try_malloc_n 占用了我们之前看到的 higher-level unordered_malloc 调用的 99.75%。
SHOCK：替代实施？
在我的调查过程中，我发现了一些可用于获得更多洞察力的定义，例如：
#define NDEBUG
//#define BOOST_POOL_INSTRUMENT 1
//#define BOOST_POOL_VALIDATE 1
//#define BOOST_POOL_VALGRIND 1
现在，我使用 VALIDATE/INSTRUMENT 观察预期效果（非常冗长的输出和轻微的性能下降）。
从阅读 name/code 我预计 BOOST_POOL_VALGRIND 会类似地降低性能（毕竟，当 运行ning Valgrind 时，它可能应该做额外的工作来避免误报内存错误， 正确的？）。令我惊讶的是，定义它使整个事情 运行 快如闪电：Live On Compiler Explorer
run 1 naive: 8.166ms reserved: 5.267ms(std::allocator<int>)
run 1 naive: 9.713ms reserved: 5.267ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 32u, 0u>)
run 1 naive: 8.853ms reserved: 5.226ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 32u, 0u>)
run 1 naive: 8.990ms reserved: 5.282ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 32u, 0u>)
run 1 naive: 8.899ms reserved: 5.246ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 32u, 0u>)
run 1 naive: 8.620ms reserved: 5.237ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 1000u, 0u>)
run 1 naive: 8.622ms reserved: 5.247ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 1000u, 0u>)
run 1 naive: 8.963ms reserved: 5.257ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 1000u, 0u>)
run 1 naive: 8.990ms reserved: 5.271ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 1000u, 0u>)
-- Done (127.276ms)
run 2 naive: 7.965ms reserved: 5.208ms(std::allocator<int>)
run 2 naive: 8.503ms reserved: 5.236ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 32u, 0u>)
run 2 naive: 8.809ms reserved: 5.254ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 32u, 0u>)
run 2 naive: 8.954ms reserved: 5.278ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 32u, 0u>)
run 2 naive: 8.878ms reserved: 5.279ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 32u, 0u>)
run 2 naive: 8.694ms reserved: 5.243ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 1000u, 0u>)
run 2 naive: 8.661ms reserved: 5.249ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 1000u, 0u>)
run 2 naive: 8.920ms reserved: 5.248ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 1000u, 0u>)
run 2 naive: 8.952ms reserved: 5.261ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 1000u, 0u>)
-- Done (125.680ms)
run 3 naive: 7.949ms reserved: 5.221ms(std::allocator<int>)
run 3 naive: 8.498ms reserved: 5.238ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 32u, 0u>)
run 3 naive: 8.813ms reserved: 5.230ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 32u, 0u>)
run 3 naive: 9.033ms reserved: 5.279ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 32u, 0u>)
run 3 naive: 8.909ms reserved: 5.252ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 32u, 0u>)
run 3 naive: 8.605ms reserved: 5.244ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, boost::details::pool::null_mutex, 1000u, 0u>)
run 3 naive: 8.623ms reserved: 5.246ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, boost::details::pool::null_mutex, 1000u, 0u>)
run 3 naive: 8.918ms reserved: 5.247ms(boost::pool_allocator<int, boost::default_user_allocator_malloc_free, std::mutex, 1000u, 0u>)
run 3 naive: 8.969ms reserved: 5.268ms(boost::pool_allocator<int, boost::default_user_allocator_new_delete, std::mutex, 1000u, 0u>)
可悲的是，查看细节证实它通过直接委托给标准库来作弊（同时增加了一些 free_list/used_list 地址集的开销）。
总结
是的，标准 pool/simple_segregated_storage 实现在此负载下表现不佳。这是否真的是一个错误，我无法确定，但根据文档（你 mentioned 也是如此）看起来确实如此。