mark mark - 13 days ago 9
C++ Question

Why does memory_order_relaxed and memory_order_seq_cst make no difference?

I was playing with one of the examples in C++ Concurrency in Action which uses

std::memory_order_relaxed
for reading and writing 3 atomic variables from 5 different threads. The example program is as follows:

#include <thread>
#include <atomic>
#include <iostream>

std::atomic<int> x(0);
std::atomic<int> y(0);
std::atomic<int> z(0);
std::atomic<bool> go(false);

const unsigned int loop_count = 10;

struct read_values
{
int x;
int y;
int z;
};

read_values values1[loop_count];
read_values values2[loop_count];
read_values values3[loop_count];
read_values values4[loop_count];
read_values values5[loop_count];

void increment( std::atomic<int>* v, read_values* values )
{
while (!go)
std::this_thread::yield();

for (unsigned i=0;i<loop_count;++i)
{
values[i].x=x.load( std::memory_order_relaxed );
values[i].y=y.load( std::memory_order_relaxed );
values[i].z=z.load( std::memory_order_relaxed );
v->store( i+1, std::memory_order_relaxed );
std::this_thread::yield();
}
}

void read_vals( read_values* values )
{

while (!go)
std::this_thread::yield();

for (unsigned i=0;i<loop_count;++i)
{
values[i].x=x.load( std::memory_order_relaxed );
values[i].y=y.load( std::memory_order_relaxed );
values[i].z=z.load( std::memory_order_relaxed );
std::this_thread::yield();
}
}

void print( read_values* values )
{
for (unsigned i=0;i<loop_count;++i)
{
if (i)
std::cout << ",";
std::cout << "(" << values[i].x <<","
<< values[i].y <<","
<< values[i].z <<")";
}
std::cout << std::endl;
}

int main()
{
std::thread t1( increment, &x, values1);
std::thread t2( increment, &y, values2);
std::thread t3( increment, &z, values3);
std::thread t4( read_vals, values4);
std::thread t5( read_vals, values5);

go = true;

t5.join();
t4.join();
t3.join();
t2.join();
t1.join();

print( values1 );
print( values2 );
print( values3 );
print( values4 );
print( values5 );

return 0;
}


Every time I run the program I get exactly the same output:

(0,10,10),(1,10,10),(2,10,10),(3,10,10),(4,10,10),(5,10,10),(6,10,10),(7,10,10),(8,10,10),(9,10,10)
(0,0,1),(0,1,2),(0,2,3),(0,3,4),(0,4,5),(0,5,6),(0,6,7),(0,7,8),(0,8,9),(0,9,10)
(0,0,0),(0,1,1),(0,2,2),(0,3,3),(0,4,4),(0,5,5),(0,6,6),(0,7,7),(0,8,8),(0,9,9)
(0,0,0),(0,0,0),(0,0,0),(0,0,0),(0,0,0),(0,0,0),(0,0,0),(0,0,0),(0,0,0),(0,0,0)
(0,0,0),(0,0,0),(0,0,0),(0,0,0),(0,0,0),(0,0,0),(0,0,0),(0,0,0),(0,0,0),(0,0,0)


If I change from
std::memory_order_relaxed
to
std::memory_order_seq_cst
the program gives exactly the same output!

I would have expected different output from the 2 versions of the program. Why is there no difference between the output for
std::memory_order_relaxed
and
std::memory_order_seq_cst
?

Why does
std::memory_order_relaxed
always produce exactly the same results for every run of the program?

I am using:
- 32bit Ubuntu installed as a virtual machine (under VMWare)
- An INtel Quad Core processor
- GCC 4.6.1-9

The code is compiled with:
g++ --std=c++0x -g mem-order-relaxed.cpp -o relaxed -pthread

Note the -pthread is necessary, otherwise the following error is reported:
terminate called after throwing an instance of 'std::system_error'
what(): Operation not permitted

Is the behaviour I am seeing due to lack of support with GCC, or as a result of running under VMWare?

Answer

How many processor cores do you have assigned to the VM? Assign multiple cores to the VM to let it take advantage of concurrency.