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MCF Gthread is a threading support library for Windows 7 and above that implements the gthread interface set, which is used internally both by GCC to provide synchronization of initialization of local static objects, and by libstdc++ to provide C++11 threading facilities.
Warning
This project uses some undocumented NT system calls and is not guaranteed to work on some Windows versions. The author gives no warranty for this project. Use it at your own risk.
Compiling natively can be done in MSYS2. We take the UCRT64 shell as an example. Others are similar. Clang shells are also supported.
Caution
If you are using GCC with the MCF thread model, you must build libmcfgthread-1.dll before anything else. Meson creates the DLL in the working directory, which might get picked up by the compiler when it is only half-baked. Microsoft documentation says that DLLs in the working directory take precedence over those in PATH. There is no immediate plan on fixing this.
pacman -S --noconfirm mingw-w64-ucrt-x86_64-{{headers,crt,tools}-git,gcc,binutils,meson}
meson setup build_debug
cd build_debug
ninja libmcfgthread-1.dll # see CAUTION above
ninja testCross-compiling from Debian, Ubuntu or Linux Mint is supported. In order to run tests, Wine is required.
sudo apt-get install -y --no-install-recommends mingw-w64-{x86-64-dev,tools} \
{gcc,g++,binutils}-mingw-w64-x86-64 meson wine wine-binfmt
meson setup --cross-file meson.cross.x86_64-w64-mingw32 build_debug
cd build_debug
ninja testTip
In order for __cxa_atexit() (and the non-standard __cxa_at_quick_exit()) to
conform to the Itanium C++ ABI, it is required 1) for a process to call
__cxa_finalize(NULL) when exiting, and 2) for a DLL to call
__cxa_finalize(&__dso_handle) when it is unloaded dynamically. This requires
hacking the CRT. If you don't
have the modified CRT, you may still get standard compliance by 1) calling
__MCF_exit() instead of exit() from your program, and 2) calling
__cxa_finalize(&__dso_handle) followed by fflush(NULL) upon receipt of
DLL_PROCESS_DETACH in your DllMain().
- #THREADS: number of threads
- #ITERATIONS: number of iterations per thread
-
SRWLOCK: Windows
SRWLOCK -
CRITICAL_SECTION: Windows
CRITICAL_SECTION -
WINPTHREAD: winpthread
pthread_mutex_t -
MCFGTHREAD: mcfgthread
_MCF_mutexwith-fno-inline
These are results of the test program on an x86-64 Windows 10 machine with a 10-core Intel i9 10900K processor:
| #THREADS | #ITERATIONS | SRWLOCK | CRITICAL_SECTION | WINPTHREAD | MCFGTHREAD |
|---|---|---|---|---|---|
| 1 | 20,000,000 | 1541.035 ms | 1684.556 ms | 1537.788 ms | 1539.504 ms |
| 2 | 10,000,000 | 1410.687 ms | 1916.520 ms | 2135.853 ms | 1377.103 ms |
| 4 | 5,000,000 | 2070.238 ms | 4613.832 ms | 2979.166 ms | 1553.278 ms |
| 6 | 3,000,000 | 2500.003 ms | 5016.650 ms | 3159.182 ms | 1409.130 ms |
| 10 | 1,500,000 | 2416.953 ms | 6239.123 ms | 3004.653 ms | 1177.269 ms |
| 20 | 600,000 | 2266.024 ms | 8687.350 ms | 2559.691 ms | 1001.314 ms |
| 60 | 200,000 | 2831.348 ms | 10164.012 ms | 3814.880 ms | 3299.509 ms |
| 200 | 60,000 | 2849.850 ms | 10544.007 ms | 3825.518 ms | 3579.925 ms |
And these are results of the same program on Wine 6.0.3 on an x86-64 Ubuntu 22.04 virtual machine with a 16-core AMD EPYC2 processor:
| #THREADS | #ITERATIONS | SRWLOCK | CRITICAL_SECTION | WINPTHREAD | MCFGTHREAD |
|---|---|---|---|---|---|
| 1 | 10,000,000 | 2466.983 ms | 2574.892 ms | 2444.599 ms | 3167.704 ms |
| 2 | 5,000,000 | 1940.147 ms | 1918.091 ms | 2078.076 ms | 2213.607 ms |
| 4 | 2,000,000 | 3717.442 ms | 5356.369 ms | 3859.484 ms | 1974.007 ms |
| 6 | 1,000,000 | 3517.333 ms | 4519.209 ms | 2474.208 ms | 1582.614 ms |
| 10 | 500,000 | 3105.191 ms | 4706.027 ms | 2388.662 ms | 1363.926 ms |
| 20 | 200,000 | 2721.077 ms | 4262.151 ms | 1966.195 ms | 1340.997 ms |
| 60 | 60,000 | 2397.048 ms | 3807.141 ms | 1530.147 ms | 1511.931 ms |
| 200 | 20,000 | 2632.933 ms | 4148.604 ms | 1615.904 ms | 1784.553 ms |
And these are results of the same program on an ARM Windows 11 machine with an 8-core Qualcomm Snapdragon 8cx Gen 3 processor, compiled with Clang:
| #THREADS | #ITERATIONS | SRWLOCK | CRITICAL_SECTION | WINPTHREAD | MCFGTHREAD |
|---|---|---|---|---|---|
| 1 | 10,000,000 | 2105.027 ms | 2164.209 ms | 2122.998 ms | 2033.915 ms |
| 2 | 5,000,000 | 1701.007 ms | 1620.484 ms | 1547.963 ms | 1496.309 ms |
| 4 | 2,000,000 | 1395.439 ms | 3067.075 ms | 2583.215 ms | 1525.453 ms |
| 6 | 1,000,000 | 1181.352 ms | 4334.280 ms | 2167.916 ms | 1354.046 ms |
| 10 | 500,000 | 2738.153 ms | 2799.624 ms | 2687.904 ms | 2739.022 ms |
| 20 | 100,000 | 3259.999 ms | 3220.732 ms | 3287.581 ms | 3291.146 ms |
| 60 | 30,000 | 2931.157 ms | 2934.896 ms | 2938.784 ms | 2922.015 ms |
| 200 | 10,000 | 3197.414 ms | 3216.323 ms | 3221.090 ms | 3229.249 ms |
A condition variable is implemented as an atomic counter of threads that are currently waiting on it. Initially the counter is zero, which means no thread is waiting.
When a thread is about to start waiting on a condition variable, it increments the counter and suspends itself using the global keyed event, passing the address of the condition variable as the key. Another thread may read the counter to tell how many threads that it will have to wake up (note this has to be atomic), and release them from the global keyed event, also passing the address of the condition variable as the key.
A primitive mutex is just a condition variable with a boolean bit, which designates whether the mutex is LOCKED. A mutex is initialized to all-bit zeroes which means it is unlocked and no thread is waiting.
When a thread wishes to lock a mutex, it checks whether the LOCKED bit is clear. If so, it sets the LOCKED bit and returns, having taken ownership of the mutex. If the LOCKED bit has been set by another thread, it goes to wait on the condition variable. If the thread wishes to unlock this mutex, it clears the LOCKED bit and wakes up at most one waiting thread on the condition variable, if any.
In reality, critical sections are fairly small. If a thread fails to lock a mutex, it might be able to do so soon, and we don't want it to give up its time slice as a syscall is an overkill. Therefore, it is reasonable for a thread to perform some spinning (busy waiting), before it actually decides to sleep.
This could however lead to severe problems in case of heavy contention. When there are hundreds of thread attempting to lock the same mutex, the system scheduler has no idea whether they are spinning or not. As it is likely that a lot of threads will eventually give up spinning and make a syscall to sleep, we are wasting a lot of CPU time and aggravating the situation.
This issue is ultimately solved by mcfgthread by encoding a spin failure counter in each mutex. If a thread gives up spinning because it couldn't lock the mutex within a given number of iterations, the spin failure counter is incremented. If a thread locks a mutex successfully while it is spinning, the spin failure counter is decremented. This counter provides a heuristic way to determine how heavily a mutex is seized. If there have been many spin failures, newcomers will not attempt to spin, but will make a syscall to sleep on the mutex directly.
A once-initialization flag contains a READY byte (this is the first one according to Itanium ABI) which indicates whether initialization has completed. The other bytes are used as a primitive mutex.
A thread that sees the READY byte set to non-zero knows initialization has been done, so it will return immediately. A thread that sees the READY byte set to zero will lock the bundled primitive mutex, and shall perform initialization thereafter. If initialization fails, it unlocks the primitive mutex without setting the READY byte, so the next thread that locks the primitive mutex will perform initialization. If initialization is successful, it sets the READY byte and unlocks the primitive mutex, releasing all threads that are waiting on it. (Do you remember that a primitive mutex actually contains a condition variable?)
| Function | DLL | Category |
|---|---|---|
BaseGetNamedObjectDirectory |
KERNEL32 | Undocumented |
CreateThread |
KERNEL32 | Windows API |
DecodePointer |
KERNEL32, NTDLL | Windows API |
EncodePointer |
KERNEL32, NTDLL | Windows API |
ExitThread |
KERNEL32 | Windows API |
FormatMessageW |
KERNEL32 | Windows API |
GetCurrentProcessId |
KERNEL32 | Windows API |
GetLastError |
KERNEL32 | Windows API |
GetModuleFileNameW |
KERNEL32 | Windows API |
GetModuleHandleExW |
KERNEL32 | Windows API |
GetProcAddress |
KERNEL32 | Windows API |
GetProcessHeap |
KERNEL32 | Windows API |
GetSystemInfo |
KERNEL32 | Windows API |
GetSystemTimeAsFileTime |
KERNEL32 | Windows API |
GetThreadPriority |
KERNEL32 | Windows API |
GetTickCount64 |
KERNEL32 | Windows API |
HeapAlloc |
KERNEL32 | Windows API |
HeapFree |
KERNEL32 | Windows API |
HeapReAlloc |
KERNEL32 | Windows API |
HeapSize |
KERNEL32 | Windows API |
NtClose |
NTDLL | Windows Driver API |
NtCreateSection |
NTDLL | Windows Driver API |
NtDelayExecution |
NTDLL | Undocumented |
NtDuplicateObject |
NTDLL | Windows Driver API |
NtMapViewOfSection |
NTDLL | Windows Driver API |
NtRaiseHardError |
NTDLL | Undocumented |
NtReleaseKeyedEvent |
NTDLL | Undocumented |
NtUnmapViewOfSection |
NTDLL | Windows Driver API |
NtWaitForKeyedEvent |
NTDLL | Undocumented |
NtWaitForSingleObject |
NTDLL | Windows Driver API |
QueryPerformanceCounter |
KERNEL32 | Windows API |
QueryPerformanceFrequency |
KERNEL32 | Windows API |
QueryUnbiasedInterruptTime |
KERNEL32 | Windows API |
RtlCompareMemory |
KERNEL32, NTDLL | Windows Driver API |
RtlDllShutdownInProgress |
NTDLL | Windows API |
RtlFillMemory |
NTDLL | Windows Driver API |
RtlMoveMemory |
NTDLL | Windows Driver API |
RtlZeroMemory |
NTDLL | Windows Driver API |
SetConsoleCtrlHandler |
KERNEL32 | Windows API |
SetLastError |
KERNEL32 | Windows API |
SetThreadPriority |
KERNEL32 | Windows API |
SwitchToThread |
KERNEL32 | Windows API |
TerminateProcess |
KERNEL32 | Windows API |
TlsAlloc |
KERNEL32 | Windows API |
TlsGetValue |
KERNEL32 | Windows API |
TlsSetValue |
KERNEL32 | Windows API |
VirtualProtect |
KERNEL32 | Windows API |
