From: Rich Felker <dalias@aerifal.cx>
To: "Ondřej Bílka" <neleai@seznam.cz>
Cc: Carlos O'Donell <carlos@redhat.com>,
Torvald Riegel <triegel@redhat.com>,
GLIBC Devel <libc-alpha@sourceware.org>,
libc-ports <libc-ports@sourceware.org>
Subject: Re: [PATCH] Unify pthread_once (bug 15215)
Date: Mon, 26 Aug 2013 16:45:00 -0000 [thread overview]
Message-ID: <20130826164507.GA20515@brightrain.aerifal.cx> (raw)
In-Reply-To: <20130826124955.GA6065@domone.kolej.mff.cuni.cz>
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
next prev parent reply other threads:[~2013-08-26 16:45 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-05-08 14:44 Torvald Riegel
2013-05-08 17:51 ` Rich Felker
2013-05-08 20:47 ` Torvald Riegel
2013-05-08 21:25 ` Rich Felker
2013-05-09 8:39 ` Torvald Riegel
2013-05-09 14:02 ` Rich Felker
2013-05-09 15:14 ` Torvald Riegel
2013-05-09 15:56 ` Rich Felker
2013-05-10 8:31 ` Torvald Riegel
2013-05-10 13:22 ` Rich Felker
2013-05-23 4:15 ` Carlos O'Donell
2013-08-26 12:50 ` Ondřej Bílka
2013-08-26 16:45 ` Rich Felker [this message]
2013-08-26 18:41 ` Ondřej Bílka
2013-08-27 2:29 ` Rich Felker
2013-10-06 0:20 ` Torvald Riegel
2013-10-06 21:41 ` Torvald Riegel
2013-10-07 16:04 ` Joseph S. Myers
2013-10-07 21:53 ` Torvald Riegel
2014-03-31 11:44 ` Will Newton
2014-03-31 20:09 ` Torvald Riegel
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