Fun with Threads

This is a Windows port of the code from Julia Evans's wonderful post about diving into concurrency. All the functionality that was provided by pthreads in the original code is supported directly in the Windows kernel, so these programs have no other dependencies besides the C runtime.

Build Instructions

• MingW: run make
• Visual Studio: run the VS2013 x86 Native Tools Command Prompt (or similar) from the Start Menu, then run build_msvc.bat.

Porting Notes

counter_race.c

pthread_create has been replaced with CreateThread, and the multiple calls to pthread_join have been replaced with a single call to WaitForMultipleObjects. Just like with pthreads, the final value is way off, and it runs pretty quickly.

$ time ./counter_race.exe
Final value of counter is: 5588731

real    0m0.172s
user    0m0.000s
sys     0m0.062s

counter_with_mutex.c

Here we naively use CreateMutex, and replace the pthread_mutex_{lock,unlock} calls with WaitForSingleObject and ReleaseMutex. The original pthreads code was pretty lethargic, but this is absurdly slow!

$ time ./counter_with_mutex.exe
Final value of counter is: 20000000

real    0m37.225s
user    0m0.031s
sys     0m0.015s

counter_with_critical_sections.c

It turns out Win32 mutexes are overkill for our purposes -- for one thing, they can be shared across processes -- but critical sections will provide us with the same synchronization and much lower overheads. We call {Enter,Leave}CriticalSection instead. This is way faster than using either Win32 or pthread mutexes.

$ time ./counter_with_critical_sections.exe
Final value of counter is: 20000000

real    0m0.858s
user    0m0.015s
sys     0m0.031s

counter_with_atomics.c

Julia's version used the GCC atomic builtin __sync_add_and_fetch, and we can use that with MinGW, since it's just GCC.

$ time ./counter_with_atomics.exe
Final value of counter is: 20000000

real    0m0.577s
user    0m0.031s
sys     0m0.031s

Win32 has a similar API called InterlockedIncrement. Visual Studio replaces this with a lock xadd instruction -- pretty similar to the GCC builtin. If we tried to use this with MinGW, it wouldn't be so clever, and would simply make a library call.