From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/8242 Path: news.gmane.org!not-for-mail From: Jens Gustedt Newsgroups: gmane.linux.lib.musl.general Subject: Re: New optimized normal-type mutex? 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Cj15cu23XZI4/826Px/xqB37z547sPH374oOvnKTsOb+1zuCoImZARM8ZiLlKkXcWOcoFRJ+Do/CwFO4e2DEe+9Ti+ZXA8ny/cU+X4bMKRZxZIVEiqXI6/+NKuUzPHb7z40ovnX3zx+Ctuq38HyuqWG7Tu+A0AAAAASUVORK5CYII= X-Mailer: Evolution 3.12.9-1+b1 Xref: news.gmane.org gmane.linux.lib.musl.general:8242 Archived-At: --=-wtHbfhsPwYZkyf9vPeaU Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Am Mittwoch, den 29.07.2015, 20:10 -0400 schrieb Rich Felker: > On Thu, Jul 30, 2015 at 01:49:20AM +0200, Jens Gustedt wrote: > > Hm, could you be more specific about where this hurts? > >=20 > > In the code I have there is > >=20 > > for (;val & lockbit;) { > > __syscall(SYS_futex, loc, FUTEX_WAIT, val, 0); > > val =3D atomic_load_explicit(loc, memory_order_consume); > > } > >=20 > > so this should be robust against spurious wakeups, no? >=20 > The problem is that futex_wait returns immediately with EAGAIN if > *loc!=3Dval, which happens very often if *loc is incremented or > otherwise changed on each arriving waiter. Yes, sure, it may change. Whether or not this is often may depend, I don't think we can easily make a quantitative statement, here. In the case of atomics the critical section is extremely short, and the count, if it varies so much, should have a bit stabilized during the spinlock phase before coming to the futex part. That futex part is really only a last resort for the rare case that the thread that is holding the lock has been descheduled in the middle. My current test case is having X threads hammer on one single location, X being up to some hundred. On my 2x2 hyperthreaded CPU for a reasonable number of threads (X =3D 16 or 32) I have an overall performance improvement of 30%, say, when using my version of the lock instead of the original musl one. The point of inversion where the original musl lock is better is at about 200 threads. I'll see how I can get hold on occurrence statistics of the different phases without being too intrusive (which would change the scheduling). Jens --=20 :: INRIA Nancy Grand Est ::: Camus ::::::: ICube/ICPS ::: :: ::::::::::::::: office Strasbourg : +33 368854536 :: :: :::::::::::::::::::::: gsm France : +33 651400183 :: :: ::::::::::::::: gsm international : +49 15737185122 :: :: http://icube-icps.unistra.fr/index.php/Jens_Gustedt :: --=-wtHbfhsPwYZkyf9vPeaU 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 v2 iEYEABECAAYFAlW520cACgkQD9PoadrVN+L17gCZASZYskDE6u87AWsvM/aixIgF ljQAnRc1clAumkSHB72gOV3G7cRGNsVr =dYfa -----END PGP SIGNATURE----- --=-wtHbfhsPwYZkyf9vPeaU--