On 2022-10-21 11:36, LIU Hao wrote:
在 2022/10/21 18:09, i.nix...@autistici.org 写道:
On 2022-10-21 09:58, Jonathan Wakely via Libstdc++ wrote:
How does this compare with Eric B's proposal at
https://gcc.gnu.org/legacy-ml/gcc-patches/2019-06/msg01840.html ?
It would be good if we can accept one of them for GCC 13, but I don't
know Windows well enough to determine which is better.
I had the same question...
I would like to understand what is the difference?
Moreover I would like to understand what is the difference with the
already added support for the winpthreads library?
@LIU Hao, could you explain please?
Thank you for your interest. I'm glad to make an introduction of it.
I have read this patch before. Let's take the mutex as an example:
There are a lot of ways to implement a mutex on Windows. Basically, a
non-recursive mutex can be implemented with an atomic counter + a
binary semaphore / auto-reset event. This proposed patch contains a
`__gthr_win32_CRITICAL_SECTION` definition that I think is a duplicate
of the internal `CRITICAL_SECTION` structure, so should also work the
same way as it.
The problem about this approach is that, semaphores are valuable
kernel objects, and the maximum number of HANDLEs that a process can
open concurrently has a limit (like FDs on Linux), while 'many
critical sections are used only occasionally (or never at all),
meaning the auto-reset event often isn’t even necessary' [1], the
semaphores are actually allocated on demand. This means that locking
can fail. There is a story in article [1] which also explains the
origination of keyed events; it's worth reading.
And, since Vista we also have native win32 condition variables, also
implemented basing on keyed events.
The keyed events are undocumented and are only exposed via syscalls.
However, as with other documented syscalls, available from Windows
Drivers Kit, there are several advantages:
* There is a global keyed event, which requires no initialization,
but
can be utilized by all processes. Basing on that, mcfgthread
provides
mutexs, condition variables, once flags, etc. that are all
one-pointer
size structs, consume absolutely no additional resource, allow
constexpr initialization, and require no cleanup, much like on
Linux.
* The wait syscalls take a 64-bit integer, whose positive value
denotes
the number of 10^-7 seconds since 1600-01-01 00:00:00 Z, and whose
negative value denotes a relative timeout. Hence it's much more
simpler
to implement `__gthread_mutex_timedlock()` and
`__gthread_cond_wait()`
which take absolute timeouts. On the other hand, Win32 APIs
generally
take a 32-bit relative timeout in milliseconds, which not only
requires
translation from an absolute timepoint argument, but can also
easily
get overflown.
* Building mutexes on top of syscalls allows a better designed
algorithm
[2], and sometimes it can even outperform native `SRWLOCK`s [3].
* mcfgthread also provides standard-conforming `__cxa_atexit()` and
`__cxa_thread_atexit()` functions, for working around some strange,
weird, and broken behaviors [4][5][6]. On Linux it's glibc that
provides them, so this as a whole requires a little modification in
mingw-w64. I am working on it however; hopefully we can land it
soon.
thank you LIU Hao for the explanation!
I have a questions:
1) wouldn't it be logical not to write yet another implementation of
pthreads-wor-windows, but to make changes to the winpthreads library
because it's already supported by GCC? (maybe I don’t know about some
reasons why it wasn’t done ...)
It seems to me the ideal and logical option is to make your
implementation part of GCC, as suggested by Eric B.
the advantages are as follows:
1) we will get a high-quality native implementation.
2) there is no need to add another thread model for GCC.
3) with dynamic linking there is no need to ship another dll with the
program. (Windows users really don't like this =))
best!
[1]
http://joeduffyblog.com/2006/11/28/windows-keyed-events-critical-sections-and-new-vista-synchronization-features/
[2] https://github.com/lhmouse/mcfgthread/blob/master/MUTEX.md
[3] https://github.com/lhmouse/mcfgthread#benchmarking
[4] https://sourceforge.net/p/mingw-w64/mailman/message/37268447/
[5] https://reviews.llvm.org/D102944
[6] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80816
