From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/6102 Path: news.gmane.org!not-for-mail From: Jens Gustedt Newsgroups: gmane.linux.lib.musl.general Subject: Re: In-progress stuff, week of Sept 1 Date: Sat, 06 Sep 2014 09:04:14 +0200 Message-ID: <1409987054.4856.98.camel@eris.loria.fr> References: <20140905011442.GA26136@brightrain.aerifal.cx> <1409902902.4856.32.camel@eris.loria.fr> <20140905083218.GI23797@brightrain.aerifal.cx> <1409909272.4856.37.camel@eris.loria.fr> <20140905164510.GM23797@brightrain.aerifal.cx> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/signed; micalg="pgp-sha1"; protocol="application/pgp-signature"; boundary="=-JyihX7NCuDWuaNEoQTBj" X-Trace: ger.gmane.org 1409988018 3544 80.91.229.3 (6 Sep 2014 07:20:18 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sat, 6 Sep 2014 07:20:18 +0000 (UTC) To: musl@lists.openwall.com Original-X-From: musl-return-6115-gllmg-musl=m.gmane.org@lists.openwall.com Sat Sep 06 09:20:13 2014 Return-path: Envelope-to: gllmg-musl@plane.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by plane.gmane.org with smtp (Exim 4.69) (envelope-from ) id 1XQA3k-0004vX-90 for gllmg-musl@plane.gmane.org; Sat, 06 Sep 2014 09:05:20 +0200 Original-Received: (qmail 7527 invoked by uid 550); 6 Sep 2014 07:05:11 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: Original-Received: (qmail 7387 invoked from network); 6 Sep 2014 07:04:21 -0000 X-IronPort-AV: E=Sophos;i="5.04,478,1406584800"; d="scan'";a="93288207" In-Reply-To: <20140905164510.GM23797@brightrain.aerifal.cx> Face: 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Cj15cu23XZI4/826Px/xqB37z547sPH374oOvnKTsOb+1zuCoImZARM8ZiLlKkXcWOcoFRJ+Do/CwFO4e2DEe+9Ti+ZXA8ny/cU+X4bMKRZxZIVEiqXI6/+NKuUzPHb7z40ovnX3zx+Ctuq38HyuqWG7Tu+A0AAAAASUVORK5CYII= X-Mailer: Evolution 3.4.4-3 Xref: news.gmane.org gmane.linux.lib.musl.general:6102 Archived-At: --=-JyihX7NCuDWuaNEoQTBj Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: quoted-printable Am Freitag, den 05.09.2014, 12:45 -0400 schrieb Rich Felker: > On Fri, Sep 05, 2014 at 11:27:52AM +0200, Jens Gustedt wrote: > > again, out of the head, i remember two places that have "adhoc" > > locking conventions that we touched in the last weeks, cv and > > pthread_once_t. Both encode waiters in a non-optimal way: cv only has > > an estimate of "somebody might be waiting", pthread_once_t uses a > > static `waiters` variable for all instances. >=20 > Yes, this is bad, but pthread_once_t would be fine with a waiters flag > in the futex int. I'll look at improving that later. But since the > number of pthread_once_t objects in the whole program is > finite/bounded, it's really irrelevant to performance. >=20 > > > but it's a bad idea to have the waiters > > > count inside the futex int. This is because every time the futex int > > > changes, you risk FUTEX_WAIT returning with EAGAIN. Under high load o= f > > > waiters arriving/leaving, it might be impossible to _ever_ start a > > > wait; all threads may just spin. The futex int should be something > > > that doesn't change except for a very small finite number of changes > > > (like adding a waiter flag, not a waiter count) before you actually > > > have a chance of successfully getting the lock or whatever. > >=20 > > First, I think you overestimate the trouble that we can have with > > that. >=20 > Actually I don't. It follows from the massive performance problems I > had with the original cv implementation where wait was "write your own > tid then futex wait" and signal was "write your own tid then futex > wake". This is trivially correct and avoids sequence number wrapping > issues like the current pshared implementation, but it had a HUGE > number of spurious wakes: something like 3:1 (spurious:genuine) for > several threads hammering the cv.=20 The situation would be different here. A thread would only update the waiters part of the `int` once, and then attempt to do the futex wait in a loop (while EAGAIN) without changing it again. > On typical cpus, there's a window of 500+ cycles between the atomic > operation in userspace and rechecking it in kernelspace. This is > plenty time for another core to cause your futex wait to EAGAIN. >=20 > > In any case, for the two use cases I mention above this problem isn't > > much relevant, I think. For cv this is the internal lock, there will > > be at most one of "waiting" threads on that (he has to hold the mutex) > > and perhaps several signaling threads, but it should be rare that this > > is contended by more than two threads at a time. For the current > > version the "may-have-waiters" flag may trigger some superfluous wake > > up, I think. >=20 > This is possibly true, but would need some analysis, and thinking > about it now is distracting me from getting done other things that we > want to get done now. ;-) >=20 > > And pthread_once_t is of minor performance importance, anyhow. >=20 > I don't see how a may-have-waiters approach has any cost for > pthread_once_t unless some of the waiters are cancelled. And that's a > pathological case that's not worth optimizing. right, at least here such an approach would help to avoid the ugly static "waiters" variable. Jens --=20 :: INRIA Nancy Grand Est ::: AlGorille ::: ICube/ICPS ::: :: ::::::::::::::: office Strasbourg : +33 368854536 :: :: :::::::::::::::::::::: gsm France : +33 651400183 :: :: ::::::::::::::: gsm international : +49 15737185122 :: :: http://icube-icps.unistra.fr/index.php/Jens_Gustedt :: --=-JyihX7NCuDWuaNEoQTBj Content-Type: application/pgp-signature; name="signature.asc" Content-Description: This is a digitally signed message part Content-Transfer-Encoding: 7bit -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (GNU/Linux) iEYEABECAAYFAlQKse4ACgkQD9PoadrVN+J7fQCcCXXkDqMN61rWDbVSzpAQpi+j tO8AnjIERF01veWLaj9WJlWAlQIr50JI =xdyI -----END PGP SIGNATURE----- --=-JyihX7NCuDWuaNEoQTBj--