From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/5808 Path: news.gmane.org!not-for-mail From: Jens Gustedt Newsgroups: gmane.linux.lib.musl.general Subject: Re: Explaining cond var destroy [Re: [musl] C threads, v3.0] Date: Tue, 12 Aug 2014 21:09:21 +0200 Message-ID: <1407870561.15134.152.camel@eris.loria.fr> References: <1407397851.24324.354.camel@eris.loria.fr> <20140807161342.GH1674@brightrain.aerifal.cx> <1407430024.24324.387.camel@eris.loria.fr> <20140807172514.GI1674@brightrain.aerifal.cx> <1407489627.24324.419.camel@eris.loria.fr> <20140808191405.GP1674@brightrain.aerifal.cx> <20140808204855.GQ1674@brightrain.aerifal.cx> <1407566854.4988.231.camel@eris.loria.fr> <20140812025013.GY1674@brightrain.aerifal.cx> <1407827079.15134.109.camel@eris.loria.fr> <20140812160103.GB1674@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"; 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C3d0WolfKhTp6clRyhcg04xtGCHt1Dh/rExRamYWGyxXa7xn1yYuvkAwP7fClg0Zhx+qr/rM7d+Yesc73nHD5OW7Zh478tRzPxACH/iRa245duzkAdsJfMuy8aXm/FUFlpeWWOm2WZidI9WapaUlFIaTlQ4jfR6/vO9D2NE32LTeYnZOUmnA6PAohx9+qnTy1NnrP/ 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:5808 Archived-At: --=-6Sb9Boi8VbvQsAGR8GBj Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: quoted-printable Rich, thanks a lot for looking into the code. Am Dienstag, den 12.08.2014, 12:01 -0400 schrieb Rich Felker: > As far as I can tell, the only thing that's saving you from sending > futex wakes after free is that you're just using spinlocks. No, I don't think so. These protect critical sections at the beginning of the cnd_t calls. The cnd_*wait calls hold the mutex at that time anyhow, so even if these would be implemented with mutexes (an extra one per cnd_t to protect the critical section) this wouldn't cause late wakes, I think. > This is an > extremely expensive solution: While contention is rare, as soon as you > do hit contention, if there are many threads they all pile on and > start spinning, and the time to obtain a lock (and cpu time/energy > spent waiting) grows extremely high. And of course it becomes infinite > if you have any threads of differing priorities and the low-priority > thread has the lock... I think you dramatize a bit :) First you overlooked that all the cnd_*wait calls are done while in addition the mutex in question is held. So no two waiters will fight for the critical section at the same time. In addition this protects against the signal and broadcast calls. I don't think that a thundering herd of "wakers" (and not waiters) is a very common scenario. Second, the idea here is that inside these critical sections there is only a constant amount of actions that is to be performed, and in particular there are no waits or other system calls. E.g the malloc call is done outside, the data is prepared, and then only linked inside the critical section. I checked the assembler and this produces only some assembler lines inside the critical sections. They may only last at most some memory cycles. The worst case scenario is that the data that is used is not cached. So such a "regular" critical section last only a very small fraction of a scheduling interval. It is very unlikely that a thread that reaches the critical section is unscheduled *during* that critical section. If it is unscheduled, you are right, the wait can be long. But that event is very unlikely, so the average time inside the critical section is still short, with a probability distribution that is a bit skewed because of the outliers. (And then there is no concept of different scheduling priorities for C threads, all of them are equal.) 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 :: --=-6Sb9Boi8VbvQsAGR8GBj 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) iEYEABECAAYFAlPqZmEACgkQD9PoadrVN+KVEwCePAb4hZ/p0143f2Hp4g5TGIkY 1VUAn305HVkz+/Tyun76zWnXT6UQSFT9 =QZ7V -----END PGP SIGNATURE----- --=-6Sb9Boi8VbvQsAGR8GBj--