This is the mail archive of the gcc@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Re: Adding a new thread model to GCC


Yes I learnt those *ByAddress* functions a few months ago and was shocked that Microsoft had plagiarized futex from Linux.
As you have pointed out already, those APIs are only available on  Windows 8 and later.

If we need fuex semantics on Windows 7 and earlier, we must simulate it.
However, since futexes and keyed events work differently, simulation of futexes using keyed events would:
  0) require a lot of work, and
  1) suffer from performance penalty just like if we simulate keyed events on Windows 2000.

The major difference is that, the FUTEX_WAIT syscall is an atomic compare-and-sleep operation, while the NtWaitForKeyedEvent syscall provides no comparison.
Instead, the NtReleasedKeyedEvent syscall blocks the current thread until one thread is woken up, while FUTEX_WAKE does nothing and returns immediately.

So here is my condition: in no event shall portability harm either efficiency or maintainability.
Linux and Windows work differently. I consider it 'harmful' to make one look like the other.


------------------				 
Best regards,
lh_mouse
2016-04-14

-------------------------------------------------------------
åääïTorvald Riegel <triegel@redhat.com>
åéææï2016-04-14 01:40
æääïlh_mouse
æéïgcc,mingw-w64-public
äéïRe: Adding a new thread model to GCC

On Wed, 2016-04-13 at 17:17 +0800, lh_mouse wrote:
> Hi all,
> 
> The 'win32' thread model of gcc has been there since long long ago, being compatible with very old Windows versions, also having a number of drawbacks:
>   0) its implementation is very inefficient, and
>   1) its mutexes and condition variables require dynamic initialization and are unusable in C++11 as std::mutex requires a `constexpr` constructor, and
>   2) allocating a number of rarely used mutexes or condition variables would eventually make the system run out of kernel objects.
> 
> As a solution for 1) and 2), Microsoft introduced keyed events, details of which can be found here:
> http://joeduffyblog.com/2006/11/28/windows-keyed-events-critical-sections-and-new-vista-synchronization-features/

Have you looked at WaitOnAddress and WakeByAddressSingle /
WakeByAddressAll too?  AFAIK this is new in Windows 8, and seems similar
to futexes.

I think it might be better to get a std::synchronic (or similar)
implementation into GCC, and then use these to implement at least the
mutexes:
https://github.com/ogiroux/synchronic/blob/master/include/synchronic

One benefit would be that we then have one place where we have optimized
spinning/blocking in libstdc++ on Windows, and less platform-specific
code.




Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]