Re: commit: ldap/libraries/libldap_r rmutex.c threads.c

[Howard Chu <hyc@symas.com>]

Hallvard B Furuseth wrote:
> Howard Chu writes:
>   
> > Hallvard B Furuseth wrote:
> >     
> > > Opheader.oh_tid (alias Operation.o_tid) is never set, yet
> > > overlays/accesslog.c uses the value as the owner thread ID.
> > > (...)
> > >       
> > Hm, I don't know what happened with oh_tid, but I think it would be
> > better to actually set it, in case others need the thread ID in the
> > future.  On some systems pthread_self() is an expensive call, best to
> > avoid calling it needlessly, especially since we have only a small
> > pool of threads; we can get the thread IDs once and never need to ask
> > again.
> >     
>
> Actually, I don't see any code in OpenLDAP which needs the thread ID
> from ldap_pvt_thread_self().  What it it needs is _some_ value which is
> unique to the thread.  In slapd, the operation's thread context will do
> nicely for that - including in your rmutex, since the caller passes it
> an ID.
>   

Yes, we could just pass the thread context and change the rmutex owner 
to a void *, that would be fine.

> Though when I googled for recursive mutexes, the first seemingly
> relevant hit was a long rant from some Posix personality about why
> recursive mutexes are a bad idea, "you should fix your data structure
> instead".
>   

I presume you're talking about this rant 
http://zaval.org/resources/library/butenhof1.html
His primary point is that big locks kill concurrency. In fact, that is 
exactly what the accesslog overlay must do - it fully serializes writes 
in order to insure that they complete in CSN order, and that they can be 
logged in the same order. In this particular case, we cannot tolerate 
concurrency, and the recursive mutex is precisely the type of lock 
needed for the design.

Yes, it's a bottleneck. That's life. In practice, it usually improves 
write throughput, because it eliminates BDB deadlocks, which avoids a 
lot of wasted effort and I/O in rollbacks and retries.

--
 -- Howard Chu
 Chief Architect, Symas Corp.  http://www.symas.com
 Director, Highland Sun        http://highlandsun.com/hyc
 OpenLDAP Core Team            http://www.openldap.org/project/