Re: Boost.Threads - Open Questions from the Review

From: williamkempf_at_[hidden]
Date: 2001-09-17 10:54:50

--- In boost_at_y..., Jens Maurer <Jens.Maurer_at_g...> wrote:
> I think this patch (nearly) does what you want.  It uses a similar
> trick than the Windows variant, but doesn't spread the mutex 
contention.

After looking closer at the proposed patch this simply won't work.  
The Win32 implementation uses the DCL pattern, which for IA32 
implementations is safe, so I'm not as concerned there.  But for 
POSIX this implementation is going to run on architectures that use 
aggressive reordering and the DCL simply isn't thread safe in that 
case.  To make it thread safe would require memory barriers, which 
are non-portable (creating a portable interface may be nice, but this 
is beyond my own ability and will complicate the usability of the 
library when ported to new architectures).  The only portable way to 
use this technique is to not use DCL but instead lock the mutex every 
time.  Since pthread_once() can use an implementation that properly 
applies memory barriers this means there may be a significant 
performance penalty for using boost::call_once() that doesn't exist 
for pthread_once().  (Even worse is the fact that the patch uses a 
global mutex for all calls, which would add significantly more 
overhead still, even on platforms that don't require memory barriers, 
but there is a way to eliminate this problem.)

I'm still left wanting a better solution.

Bill Kempf