Re: [boost] Re: Lightweight Mutex Code

From: Aaron W. LaFramboise (aaronrabiddog51_at_[hidden])
Date: 2004-07-06 02:00:58

Mattias Flodin wrote:

> Quoting "Aaron W. LaFramboise" <aaronrabiddog51_at_[hidden]>:
> 
>>Well, even critical sections, Windows's fastest mutex primative, are
>>much slower in the noncontended case than a spinlock.  A two-stage
>>method is needed to match the performance of the present spinlock: a
>>lightweight atomic operation followed by a heavy-weight mutex if the
>>lock is contended.  This is why I was mentioned 8 bytes (one word for
>>the critical section, one word for the atomic operation) would be necessary.
> 
> I'm quite surprised by this claim. What you describe is precisely how WIN32
> critical sections work. If your measures show them to be slower, there must be
> some other reason for it. WIN32 also provides
> InitializeCriticalSectionAndSpinCount which will cause a busy-wait for a few
> cycles before resorting to waiting on the kernel lock.

In fact, you are quite right.  I just tested, and performance for the
two methods was identical.  I had a peice of apparently stale knowledge
in my brain telling me otherwise.  I don't remember why I thought that.

Well, I see that there is a critical section version of the lwm.  Is
there some reason the spinlock is being used by default instead?

Perhaps the solution here should just be to define
BOOST_LWM_USE_CRITICAL_SECTION and be done with it.

Aaron W. LaFramboise