如何突破Windows系統單進程最大線程數2000的限制

 

     這個問題的產生是因為windows32位系統，一個進程所能使用的最在虛擬記憶體為2G，而一個線程的預設StackSize為1024K（1M），這樣當線程數量逼近2000時，2000*1024K=2G(大約），記憶體資源就相當於耗盡。
     

     第一種方式：要求有windows2003SDK
     Does Windows have a limit of 2000 threads per process?
     Often I see people asking why they can't create more than around 2000 threads in a process. The reason is not that there is any particular limit inherent in Windows. Rather, the programmer failed to take into account the amount of address space each thread uses.

     A thread consists of some memory in kernel mode (kernel stacks and object management), some memory in user mode (the thread environment block, thread-local storage, that sort of thing), plus its stack. (Or stacks if you're on an Itanium system.)

     Usually, the limiting factor is the stack size.

 

#include <stdio.h>
#include <windows.h>
DWORD CALLBACK ThreadProc(void*)
{
     Sleep(INFINITE);
     return 0;
}
int __cdecl main(int argc, const char* argv[])
{
     int i;
     for (i = 0; i < 100000; i++) 
     {
          DWORD id;
          HANDLE h = CreateThread(NULL, 0, ThreadProc, NULL, 0, &id);
          if (!h)
               break;
          CloseHandle(h);
     }
     printf("Created %d threads\n", i); return 0;
}

     This program will typically print a value around 2000 for the number of threads.

     Why does it give up at around 2000?

     Because the default stack size assigned by the linker is 1MB, and 2000 stacks times 1MB per stack equals around 2GB, which is how much address space is available to user-mode programs.

     You can try to squeeze more threads into your process by reducing your stack size, which can be done either by tweaking linker options or manually overriding the stack size passed to the CreateThread functions as described in MSDN.

      HANDLE h = CreateThread(NULL, 4096, ThreadProc, NULL, STACK_SIZE_PARAM_IS_A_RESERVATION, &id);

     With this change, I was able to squeak in around 13000 threads. While that's certainly better than 2000, it's short of the naive expectation of 500,000 threads. (A thread is using 4KB of stack in 2GB address space.) But you're forgetting the other overhead. Address space allocation granularity is 64KB, so each thread's stack occupies 64KB of address space even if only 4KB of it is used. Plus of course you don't have free reign over all 2GB of the address space; there are system DLLs and other things occupying it.

     But the real question that is raised whenever somebody asks, "What's the maximum number of threads that a process can create?" is "Why are you creating so many threads that this even becomes an issue?"

     The "one thread per client" model is well-known not to scale beyond a dozen clients or so. If you're going to be handling more than that many clients simultaneously, you should move to a model where instead of dedicating a thread to a client, you instead allocate an object. (Someday I'll muse on the duality between threads and objects.) Windows provides I/O completion ports and a thread pool to help you convert from a thread-based model to a work-item-based model.

     Note that fibers do not help much here, because a fiber has a stack, and it is the address space required by the stack that is the limiting factor nearly all of the time.

     

     第二種方式：增加串連參數

     /Stack:reverse[,commit]

     注意區別committed   size和reserved   size。   
     預設情況下(除非dwCreationFlags設為STACK_SIZE_PARAM_IS_A_RESERVATION)，dwStackSize這個參數用來調整一開始commit給棧的空間，即initially   committed   size。   
    
     那麼調整了committed   size後，reserved   size應該怎麼有什麼相應的調整呢？   
     如果dwStackSize小於預設reserve大小，則reserve   size使用預設reserve大小；   
     如果dwStackSize大於預設reserve   size，則reserve   size將會向上取整變成1MB的整數倍

     如果要求預設的StackSize為64K，則將設定/Stack:65536

     

     第三種方式：用工具editbin.exe(VC工具裡）

     editbin /StackSize:reverse[,commit]