Re: [Boost-users] Boost Threading Multiple Reader / Single Writer Usage Questions

Subject: Re: [Boost-users] Boost Threading Multiple Reader / Single Writer Usage Questions
From: Anthony Williams (anthony.ajw_at_[hidden])
Date: 2010-02-19 05:16:49

Brian Nantz <brian_at_[hidden]> writes:

> 1.        There could be nested locks and the RWL should handle unwinding them
> correctly.  I see that recursive_mutex seems to handle that.  Does shared_mutex
> handle that as well?

shared_mutex does not support nested locks by the same thread.

> 2.        Is there a way to give a higher priority to writes?  I am worried
> about the readers starving the writers, so I would like any pending writers to
> be given preference over pending readers.

If there are pending writers then new readers are queued. When the
current lock is released (whether reader or writer) then writers +
readers compete for the lock. 

> How can I get this functionality from the Boost thread library?  If I need to
> build my own, what class do you recommend using as a base to build from?

You may actually better NOT using a read/write lock. The lock itself is
a point of contention, and for many short-lived readers (the natural use
of a read/write lock) the time locking/unlocking may be a significant
portion of the overall execution time, especially on multi-core and
multi-processor systems. Joe Duffy posted some timings on his blog:

http://www.bluebytesoftware.com/blog/2009/02/21/AMoreScalableReaderwriterLockAndABitLessHarshConsiderationOfTheIdea.aspx

Anthony
-- 
Author of C++ Concurrency in Action     http://www.stdthread.co.uk/book/
just::thread C++0x thread library             http://www.stdthread.co.uk
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