Question

I’ve been using volatile bool for years for thread execution control and it worked fine

// in my class declaration
volatile bool stop_;

-----------------

// In the thread function
while (!stop_)
{
     do_things();
}

Now, since C++11 added support for atomic operations, I decided to try that instead

// in my class declaration
std::atomic<bool> stop_;

-----------------

// In the thread function
while (!stop_)
{
     do_things();
}

But it’s several orders of magnitude slower than the volatile bool!

Simple test case I’ve written takes about 1 second to complete with volatile bool approach. With std::atomic<bool> however I’ve been waiting for about 10 minutes and gave up!

I tried to use memory_order_relaxed flag with load and store to the same effect.

My platform:

• Windows 7 64-bit
• MinGW gcc 4.6.x

What I’m doing wrong?

NB: I know that volatile does not make a variable thread-safe. My question is not about volatile, it’s about why atomic is ridiculously slow.

Answer 1

Code from “Olaf Dietsche”

 USE ATOMIC
 real   0m1.958s
 user   0m1.957s
 sys    0m0.000s

 USE VOLATILE
 real   0m1.966s
 user   0m1.953s
 sys    0m0.010s

IF YOU ARE USING GCC SMALLER 4.7

http://gcc.gnu.org/gcc-4.7/changes.html

Support for atomic operations specifying the C++11/C11 memory model has been added. These new __atomic routines replace the existing __sync built-in routines.

Atomic support is also available for memory blocks. Lock-free instructions will be used if a memory block is the same size and alignment as a supported integer type. Atomic operations which do not have lock-free support are left as function calls. A set of library functions is available on the GCC atomic wiki in the “External Atomics Library” section.

So yeah .. only solution is to upgrade to GCC 4.7