[boost] boost::thread try_mutex, Win32 performance enhancement suggestion...

From: Robert Saccone (rsaccone_at_[hidden])
Date: 2002-04-12 09:19:27

Two Suggestions:
 
I was looking over the implementation of the try_mutex class and noticed
that internally it uses a Win32 Mutex and does a WaitForSingleObject
with a timeout of 0 in the do_trylock method.  The same thing can be
achieved by using a CriticalSection and the TryEnterCriticalSection API
which is light weight compared to using a Mutex with a
WaitForSingleObject call.  
 
TryEnterCriticalSection works as follows:
 
If there is no contention on the lock, the lock will be acquired in user
mode by using a few lock prefixed assembly instructions. 
 
If there is contention on the lock the TryEnterCriticalSection will
return immediately.  
 
The EnterCriticalSection API works similarly with the exception that
when there is contention on the lock it will follow the same code path
as the WaitForSingleObject call and not return until the lock is
acquired.  
 
 WaitForSingleObject is heavy weight in that the work is always done in
kernel mode with a much longer code path.  Since TryEnterCriticalSection
is only available on WinNT 4.0, Win 2K, and XP a possible a way to
implement this is to restructure try_mutex to use the pimpl idiom and
check the platform at the time of construction of the object.  The
results of the platform check can be cached to speed construction of
future try_mutex objects.
 
Also, I found that it is more efficient to use WaitForSingleObjectEx and
WaitForMultipleObjectsEx over the non-Ex versions.  Tracing in the
debugger reveals that the non-Ex versions call the Ex versions with the
Alertable parameter set to FALSE.  You'll avoid the overhead of an extra
function call by using the Ex versions directly.
 
Regards,
Robert Saccone
rsaccone_at_[hidden]