Re: [PATCH] Unify pthread_once (bug 15215)

[Rich Felker <dalias at aerifal dot cx>]

On Mon, Aug 26, 2013 at 02:49:55PM +0200, OndÅej BÃlka wrote:
> On Thu, May 23, 2013 at 12:15:32AM -0400, Carlos O'Donell wrote:
> > On 05/08/2013 10:43 AM, Torvald Riegel wrote:
> > > Note that this will make a call to pthread_once that doesn't need to
> > > actually run the init routine slightly slower due to the additional
> > > acquire barrier.  If you're really concerned about this overhead, speak
> > > up.  There are ways to avoid it, but it comes with additional complexity
> > > and bookkeeping.
> > 
> > We want correctness. This is a place where correctness is infinitely
> > more important than speed. We should be correct first and then we
> > should argue about how to make it fast.
> >
> As pthread_once calls tend to be called once per thread performance is
> not an issue. 

No, pthread_once _calls_ tend to be once per access to an interface
that requires static data to have been initialized, so possibly very
often. On the other hand, pthread_once only invokes the init function
once per program instance. I don't see anything that would typically
happen once per thread, although I suppose you could optimize out
calls to pthread_once with tls:

    static __thread int once_done = 0;
    static pthread_once_t once;
    if (!once_done) {
        pthread_once(&once, init);
        once_done = 1;
    }

This requires work at the application level, though, and whether it's
a net advantage depends a lot on whether multiple threads are likely
to be hammering pthread_once on the same once object, and whether the
arch has expensive acquire barriers and inexpensive TLS access.

Rich