我正在尝试创建一个简单的池对象,我希望或多或少公平地将对一组共享资源的访问权限分配给任何要求它的线程.在
Windows中,我通常会有一个Mutexes数组并执行WaitForMultipleObjects,bWaitAll = FALSE(参见下面的windows_pool_of_n_t).但我希望有一天能够将其移植到其他操作系统,所以我想坚持使用标准.一个资源的deque,大小()！= 0的condition_variable似乎是一个明显的解决方案(参见下面的pool_of_n_t).
但由于我不明白的原因,该代码序列化了线程访问.我并不期望严格公平,但这几乎是最糟糕的情况 – 上次锁定的线程总是在下次获得锁定.这并不是说std :: mutex不符合Windows或多或少的公平调度,因为只使用没有条件变量的互斥锁可以按预期工作,当然只有一个池,当然(参见下面的pool_of_one_t).
有谁能解释一下？有没有解决的办法？
结果：
C:\temp\stdpool>bin\stdpool.exe pool:pool_of_one_t thread 0:19826 ms thread 1:19846 ms thread 2:19866 ms thread 3:19886 ms thread 4:19906 ms thread 5:19926 ms thread 6:19946 ms thread 7:19965 ms thread 8:19985 ms thread 9:20004 ms pool:windows_pool_of_n_t(1) thread 0:19819 ms thread 1:19838 ms thread 2:19858 ms thread 3:19878 ms thread 4:19898 ms thread 5:19918 ms thread 6:19938 ms thread 7:19958 ms thread 8:19978 ms thread 9:19997 ms pool:pool_of_n_t(1) thread 9:3637 ms thread 0:4538 ms thread 6:7558 ms thread 4:9779 ms thread 8:9997 ms thread 2:13058 ms thread 1:13997 ms thread 3:17076 ms thread 5:17995 ms thread 7:19994 ms pool:windows_pool_of_n_t(2) thread 1:9919 ms thread 0:9919 ms thread 2:9939 ms thread 3:9939 ms thread 5:9958 ms thread 4:9959 ms thread 6:9978 ms thread 7:9978 ms thread 9:9997 ms thread 8:9997 ms pool:pool_of_n_t(2) thread 2:6019 ms thread 0:7882 ms thread 4:8102 ms thread 5:8182 ms thread 1:8382 ms thread 8:8742 ms thread 7:9162 ms thread 9:9641 ms thread 3:9802 ms thread 6:10201 ms pool:windows_pool_of_n_t(5) thread 4:3978 ms thread 3:3978 ms thread 2:3979 ms thread 0:3980 ms thread 1:3980 ms thread 9:3997 ms thread 7:3999 ms thread 6:3999 ms thread 5:4000 ms thread 8:4001 ms pool:pool_of_n_t(5) thread 2:3080 ms thread 0:3498 ms thread 8:3697 ms thread 3:3699 ms thread 6:3797 ms thread 7:3857 ms thread 1:3978 ms thread 4:4039 ms thread 9:4057 ms thread 5:4059 ms
代码：
#include <iostream>
#include <deque>
#include <vector>
#include <mutex>
#include <thread>
#include <sstream>
#include <chrono>
#include <iomanip>
#include <cassert>
#include <condition_variable>
#include <windows.h>
using namespace std;
class pool_t {
public:
virtual void check_in(int size) = 0;
virtual int check_out() = 0;
virtual string pool_name() = 0;
};
class pool_of_one_t : public pool_t {
mutex lock;
public:
virtual void check_in(int resource) {
lock.unlock();
}
virtual int check_out() {
lock.lock();
return 0;
}
virtual string pool_name() {
return "pool_of_one_t";
}
};
class windows_pool_of_n_t : public pool_t {
vector<HANDLE> resources;
public:
windows_pool_of_n_t(int size) {
for (int i=0; i < size; ++i)
resources.push_back(CreateMutex(NULL,FALSE,NULL));
}
~windows_pool_of_n_t() {
for (auto resource : resources)
CloseHandle(resource);
}
virtual void check_in(int resource) {
ReleaseMutex(resources[resource]);
}
virtual int check_out() {
DWORD result = WaitForMultipleObjects(resources.size(),resources.data(),INFINITE);
assert(result >= WAIT_OBJECT_0
&& result < WAIT_OBJECT_0+resources.size());
return result - WAIT_OBJECT_0;
}
virtual string pool_name() {
ostringstream name;
name << "windows_pool_of_n_t(" << resources.size() << ")";
return name.str();
}
};
class pool_of_n_t : public pool_t {
deque<int> resources;
mutex lock;
condition_variable not_empty;
public:
pool_of_n_t(int size) {
for (int i=0; i < size; ++i)
check_in(i);
}
virtual void check_in(int resource) {
unique_lock<mutex> resources_guard(lock);
resources.push_back(resource);
resources_guard.unlock();
not_empty.notify_one();
}
virtual int check_out() {
unique_lock<mutex> resources_guard(lock);
not_empty.wait(resources_guard,[this](){return resources.size() > 0;});
auto resource = resources.front();
resources.pop_front();
bool notify_others = resources.size() > 0;
resources_guard.unlock();
if (notify_others)
not_empty.notify_one();
return resource;
}
virtual string pool_name() {
ostringstream name;
name << "pool_of_n_t(" << resources.size() << ")";
return name.str();
}
};
void worker_thread(int id,pool_t& resource_pool)
{
auto start_time = chrono::system_clock::now();
for (int i=0; i < 100; ++i) {
auto resource = resource_pool.check_out();
this_thread::sleep_for(chrono::milliseconds(20));
resource_pool.check_in(resource);
this_thread::yield();
}
static mutex cout_lock;
{
unique_lock<mutex> cout_guard(cout_lock);
cout << "thread " << id << ":"
<< chrono::duration_cast<chrono::milliseconds>(
chrono::system_clock::now() - start_time).count()
<< " ms" << endl;
}
}
void test_it(pool_t& resource_pool)
{
cout << "pool:" << resource_pool.pool_name() << endl;
vector<thread> threads;
for (int i=0; i < 10; ++i)
threads.push_back(thread(worker_thread,i,ref(resource_pool)));
for (auto& thread : threads)
thread.join();
}
int main(int argc,char* argv[])
{
test_it(pool_of_one_t());
test_it(windows_pool_of_n_t(1));
test_it(pool_of_n_t(1));
test_it(windows_pool_of_n_t(2));
test_it(pool_of_n_t(2));
test_it(windows_pool_of_n_t(5));
test_it(pool_of_n_t(5));
return 0;
}
解决方法
我在Linux上做了你的测试池：pool_of_n_t(2)并查看了问题
this_thread::yield();
在我的comp上查看测试池的结果：pool_of_n_t(2)：
1)this_thread :: yield()：
$./a.out pool:pool_of_n_t(2) thread 0,run for:2053 ms thread 9,run for:3721 ms thread 5,run for:4830 ms thread 6,run for:6854 ms thread 3,run for:8229 ms thread 4,run for:8353 ms thread 7,run for:9441 ms thread 2,run for:9482 ms thread 1,run for:10127 ms thread 8,run for:10426 ms
它们与你的相似.
2)当我用pthread_yield()替换this_thread :: yield()时的相同测试：
$./a.out pool:pool_of_n_t(2) thread 0,run for:7922 ms thread 3,run for:8853 ms thread 4,run for:8854 ms thread 1,run for:9077 ms thread 5,run for:9364 ms thread 9,run for:9446 ms thread 7,run for:9594 ms thread 2,run for:9615 ms thread 8,run for:10170 ms thread 6,run for:10416 ms
这更公平.您假设this_thread :: yield()确实将cpu提供给另一个线程,但它没有给出它.
这是gcc 4.8的this_thread :: yield的disas：
(gdb) disassembl this_thread::yield Dump of assembler code for function std::this_thread::yield(): 0x0000000000401fb2 <+0>: push %rbp 0x0000000000401fb3 <+1>: mov %rsp,%rbp 0x0000000000401fb6 <+4>: pop %rbp 0x0000000000401fb7 <+5>: retq End of assembler dump.
我没有看到任何重新安排
这是pthread_yield的disas：
(gdb) disassemble pthread_yield Dump of assembler code for function pthread_yield: 0x0000003149c084c0 <+0>: jmpq 0x3149c05448 <sched_yield@plt> End of assembler dump. (gdb) disassemble sched_yield Dump of assembler code for function sched_yield: 0x00000031498cf520 <+0>: mov $0x18,%eax 0x00000031498cf525 <+5>: syscall 0x00000031498cf527 <+7>: cmp $0xfffffffffffff001,%rax 0x00000031498cf52d <+13>: jae 0x31498cf530 <sched_yield+16> 0x00000031498cf52f <+15>: retq 0x00000031498cf530 <+16>: mov 0x2bea71(%rip),%rcx # 0x3149b8dfa8 0x00000031498cf537 <+23>: xor %edx,%edx 0x00000031498cf539 <+25>: sub %rax,%rdx 0x00000031498cf53c <+28>: mov %edx,%fs:(%rcx) 0x00000031498cf53f <+31>: or $0xffffffffffffffff,%rax 0x00000031498cf543 <+35>: jmp 0x31498cf52f <sched_yield+15> End of assembler dump.
