Why running std :: thread with an empty function is wasting a lot of memory

Question

I wrote a simple program that should run two threads, sort a small array (~ 4096 bytes) and write to the output file. Input data is contained in one large file (~ 4 GB). The computer has 128 MB of memory. I found that running only an empty main function uses 14 MB of memory. If you run std :: thread with an empty function application, start using ~ 8 MB per thread. BUT, if I make only one dynamic memory allocation program, it will start using approximately 64 MB per stream. I do not understand what can spend so much memory. How can I control this size? And how to allocate dynamic memory to minimize system default allocation?

• System: Ubuntu 14.04.3
• Compiler: gcc 4.8.4

• Compiler option: '- std = C ++ 11 -O3 -pthread'

• This is sample code.

  void dummy(void)
  {
      std::vector<unsigned int> g(1);
      int i = 0;
      while( i<500000000)
      {
          ++i;
      }
  }
  
  int main(void)
  {
      std::thread t1(&dummy);
      std::thread t2(&dummy);
      std::thread t3(&dummy);
      t1.join();
      t2.join();
      t3.join();
      return 0;
  }
  

Answer 1

Each thread has its own stack. On Linux, the default stack size is 8 MB. When you start allocating memory for the first time, the heap memory allocator can actually reserve a large chunk in front. This may explain 64 MB per stream that you see.

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