Subj : Re: recursive mutexes
To   : comp.programming.threads
From : David Schwartz
Date : Thu May 19 2005 11:14 pm


"Uenal Mutlu" <520001085531-0001@t-online.de> wrote in message 
news:d6gbdt$buc$00$1@news.t-online.com...

> "Casper H.S. Dik" wrote
>> "Uenal Mutlu" writes:

>> (The recursive mutex is only slightly more expensive
>> than the ordinary mutex as the unlock path is more
>> expensive)

> Actually it is just an increment and a comparision of the currentthreadid 
> vs.
> the lock holding threadid in Lock(), and a simple decrement in Unlock().
> The overhead for recursively locking is far less than the first aquisition
> (== non-recursive mutex) of the lock. Cf. code snippet of my other posting 
> to doug and > David.

    I'm afraid that's not true. A typical recursive unlock will need one 
interlocked operation to figure out whether it has to wake other threads or 
not followed by waking the threads if necessary. A typical non-recursive 
unlock will need one non-interlocked operation to release the lock followed 
by waking any threads.

    On a p4, it is typically about 70 cycles faster to release a 
non-recursive mutex than a recursive one assuming it's the last release and 
no threads are blocked.

    DS

.