What is the difference between mutex and critical

2019-01-03 20:05发布

Please explain from Linux, Windows perspectives?

I am programming in C#, would these two terms make a difference. Please post as much as you can, with examples and such....

Thanks

9条回答
我欲成王,谁敢阻挡
2楼-- · 2019-01-03 20:46

The 'fast' Windows equal of critical selection in Linux would be a futex, which stands for fast user space mutex. The difference between a futex and a mutex is that with a futex, the kernel only becomes involved when arbitration is required, so you save the overhead of talking to the kernel each time the atomic counter is modified. That .. can save a significant amount of time negotiating locks in some applications.

A futex can also be shared amongst processes, using the means you would employ to share a mutex.

Unfortunately, futexes can be very tricky to implement (PDF). (2018 update, they aren't nearly as scary as they were in 2009).

Beyond that, its pretty much the same across both platforms. You're making atomic, token driven updates to a shared structure in a manner that (hopefully) does not cause starvation. What remains is simply the method of accomplishing that.

查看更多
迷人小祖宗
3楼-- · 2019-01-03 20:46

Just to add my 2 cents, critical Sections are defined as a structure and operations on them are performed in user-mode context.

ntdll!_RTL_CRITICAL_SECTION
   +0x000 DebugInfo        : Ptr32 _RTL_CRITICAL_SECTION_DEBUG
   +0x004 LockCount        : Int4B
   +0x008 RecursionCount   : Int4B
   +0x00c OwningThread     : Ptr32 Void
   +0x010 LockSemaphore    : Ptr32 Void
   +0x014 SpinCount        : Uint4B

Whereas mutex are kernel objects (ExMutantObjectType) created in the Windows object directory. Mutex operations are mostly implemented in kernel-mode. For instance, when creating a Mutex, you end up calling nt!NtCreateMutant in kernel.

查看更多
啃猪蹄的小仙女
4楼-- · 2019-01-03 20:48

In addition to the other answers, the following details are specific to critical sections on windows:

  • in the absence of contention, acquiring a critical section is as simple as an InterlockedCompareExchange operation
  • the critical section structure holds room for a mutex. It is initially unallocated
  • if there is contention between threads for a critical section, the mutex will be allocated and used. The performance of the critical section will degrade to that of the mutex
  • if you anticipate high contention, you can allocate the critical section specifying a spin count.
  • if there is contention on a critical section with a spin count, the thread attempting to acquire the critical section will spin (busy-wait) for that many processor cycles. This can result in better performance than sleeping, as the number of cycles to perform a context switch to another thread can be much higher than the number of cycles taken by the owning thread to release the mutex
  • if the spin count expires, the mutex will be allocated
  • when the owning thread releases the critical section, it is required to check if the mutex is allocated, if it is then it will set the mutex to release a waiting thread

In linux, I think that they have a "spin lock" that serves a similar purpose to the critical section with a spin count.

查看更多
孤傲高冷的网名
5楼-- · 2019-01-03 20:54

Great answer from Michael. I've added a third test for the mutex class introduced in C++11. The result is somewhat interesting, and still supports his original endorsement of CRITICAL_SECTION objects for single processes.

mutex m;
HANDLE mutex = CreateMutex(NULL, FALSE, NULL);
CRITICAL_SECTION critSec;
InitializeCriticalSection(&critSec);

LARGE_INTEGER freq;
QueryPerformanceFrequency(&freq);
LARGE_INTEGER start, end;

// Force code into memory, so we don't see any effects of paging.
EnterCriticalSection(&critSec);
LeaveCriticalSection(&critSec);
QueryPerformanceCounter(&start);
for (int i = 0; i < 1000000; i++)
{
    EnterCriticalSection(&critSec);
    LeaveCriticalSection(&critSec);
}

QueryPerformanceCounter(&end);

int totalTimeCS = (int)((end.QuadPart - start.QuadPart) * 1000 / freq.QuadPart);

// Force code into memory, so we don't see any effects of paging.
WaitForSingleObject(mutex, INFINITE);
ReleaseMutex(mutex);

QueryPerformanceCounter(&start);
for (int i = 0; i < 1000000; i++)
{
    WaitForSingleObject(mutex, INFINITE);
    ReleaseMutex(mutex);
}

QueryPerformanceCounter(&end);

int totalTime = (int)((end.QuadPart - start.QuadPart) * 1000 / freq.QuadPart);

// Force code into memory, so we don't see any effects of paging.
m.lock();
m.unlock();

QueryPerformanceCounter(&start);
for (int i = 0; i < 1000000; i++)
{
    m.lock();
    m.unlock();
}

QueryPerformanceCounter(&end);

int totalTimeM = (int)((end.QuadPart - start.QuadPart) * 1000 / freq.QuadPart);


printf("C++ Mutex: %d Mutex: %d CritSec: %d\n", totalTimeM, totalTime, totalTimeCS);

My results were 217, 473, and 19 (note that my ratio of times for the last two is roughly comparable to Michael's, but my machine is at least four years younger than his, so you can see evidence of increased speed between 2009 and 2013, when the XPS-8700 came out). The new mutex class is twice as fast as the Windows mutex, but still less than a tenth the speed of the Windows CRITICAL_SECTION object. Note that I only tested the non-recursive mutex. CRITICAL_SECTION objects are recursive (one thread can enter them repeatedly, provided it leaves the same number of times).

查看更多
放我归山
6楼-- · 2019-01-03 20:59

From a theoretical perspective, a critical section is a piece of code that must not be run by multiple threads at once because the code accesses shared resources.

A mutex is an algorithm (and sometimes the name of a data structure) that is used to protect critical sections.

Semaphores and Monitors are common implementations of a mutex.

In practice there are many mutex implementation availiable in windows. They mainly differ as consequence of their implementation by their level of locking, their scopes, their costs, and their performance under different levels of contention. See CLR Inside Out - Using concurrency for scalability for an chart of the costs of different mutex implementations.

Availiable synchronization primitives.

The lock(object) statement is implemented using a Monitor - see MSDN for reference.

In the last years much research is done on non-blocking synchronization. The goal is to implement algorithms in a lock-free or wait-free way. In such algorithms a process helps other processes to finish their work so that the process can finally finish its work. In consequence a process can finish its work even when other processes, that tried to perform some work, hang. Usinig locks, they would not release their locks and prevent other processes from continuing.

查看更多
萌系小妹纸
7楼-- · 2019-01-03 20:59

Critical Section and Mutex are not Operating system specific, their concepts of multithreading/multiprocessing.

Critical Section Is a piece of code that must only run by it self at any given time (for example, there are 5 threads running simultaneously and a function called "critical_section_function" which updates a array... you don't want all 5 threads updating the array at once. So when the program is running critical_section_function(), none of the other threads must run their critical_section_function.

mutex* Mutex is a way of implementing the critical section code (think of it like a token... the thread must have possession of it to run the critical_section_code)

查看更多
登录 后发表回答