Date: Wed, 22 Aug 2018 08:34:35 +0200 From: Sebastian Huber <sebastian.huber@embedded-brains.de> To: FreeBSD <freebsd-hackers@freebsd.org> Cc: Matthew Macy <mmacy@freebsd.org> Subject: Re: epoch(9) background information? Message-ID: <fc088eb4-f306-674c-7404-ebe17a60a5f8@embedded-brains.de> In-Reply-To: <CALX0vxBAN6nckuAnYR3_mOfwbCjJCjHGuuOFh9njpxO%2BGUzo3w@mail.gmail.com> References: <db397431-2c4c-64de-634a-20f38ce6a60e@embedded-brains.de> <CALX0vxBAN6nckuAnYR3_mOfwbCjJCjHGuuOFh9njpxO%2BGUzo3w@mail.gmail.com>
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On 21/08/18 15:38, Jacques Fourie wrote: > > > On Tue, Aug 21, 2018 at 8:33 AM, Sebastian Huber=20 > <sebastian.huber@embedded-brains.de=20 > <mailto:sebastian.huber@embedded-brains.de>> wrote: > > Hello, > > I update currently a port of the FreeBSD network stack, etc. to > the real-time operating system RTEMS from the head version at > 2017-04-04 to the head version of today. I noticed that some > read-write locks are replaced by a relatively new stuff called > EPOCH(9). Is there some background information available for this? > The man page is a bit vague and searching for something named > epoch on the internet is not really great. For example, what is > the motivation for this change? How is this related to > read-copy-update (RCU)? > > --=20 > Sebastian Huber, embedded brains GmbH > > Address : Dornierstr. 4, D-82178 Puchheim, Germany > <https://maps.google.com/?q=3DDornierstr.+4,+D-82178+Puchheim,+Germ= any&entry=3Dgmail&source=3Dg> > Phone=C2=A0 =C2=A0: +49 89 189 47 41-16 > Fax=C2=A0 =C2=A0 =C2=A0: +49 89 189 47 41-09 > E-Mail=C2=A0 : sebastian.huber@embedded-brains.de > <mailto:sebastian.huber@embedded-brains.de> > PGP=C2=A0 =C2=A0 =C2=A0: Public key available on request. > > Diese Nachricht ist keine gesch=C3=A4ftliche Mitteilung im Sinne de= s EHUG. > > _______________________________________________ > freebsd-hackers@freebsd.org <mailto:freebsd-hackers@freebsd.org> > mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-hackers > <https://lists.freebsd.org/mailman/listinfo/freebsd-hackers>; > To unsubscribe, send any mail to > "freebsd-hackers-unsubscribe@freebsd.org > <mailto:freebsd-hackers-unsubscribe@freebsd.org>" > > > Additional information is available here :=20 > http://concurrencykit.org/presentations/ebr.pdf=20 > <http://concurrencykit.org/presentations/ebr.pdf>. The way I=20 > understand it is that it is mostly used in place of read locks to=20 > provide liveness guarantees without using atomics. Additional detail=20 > is available in the commit messages. As an example see r333813 for=20 > some performance data. > Thanks, for the reference. The "epoch reclamation" are good keywords to=20 find more information. What is the right mailing list to ask questions about the epoch=20 implementation of the FreeBSD kernel? To support this machinery in RTEMS is a bit difficult (in particular=20 EPOCH_LOCKED). Since RTEMS is supposed to be a real-time operating=20 system it supports only fixed-priority and job-level fixed priority=20 (EDF) schedulers. To allow some scaling to larger SMP systems it=20 supports clustered scheduling together with the mutual exclusion locking=20 protocols MrsP (http://www-users.cs.york.ac.uk/~burns/MRSPpaper.pdf) and=20 OMIP (http://www.mpi-sws.org/~bbb/papers/pdf/ecrts13b.pdf). This makes=20 the thread pinning hard to implement (which is very easy to support in=20 FreeBSD). The locking protocols may temporarily move a thread which owns=20 a mutex to a foreign scheduler instance, e.g. a thread which wants to=20 obtain the mutex helps the owner to make progress if it was pre-empted=20 in its home scheduler instance. Due to a timeout of the helper the owner=20 may loose the right to execute in the foreign scheduler instance. This=20 would make it impossible to fulfil the processor pinning constraint=20 (e.g. the thread priority in the foreign scheduler instance is undefined)= . It would save me a lot of trouble if I could assume that EPOCH_LOCKED is=20 an exotic feature which is unlikely to get used in FreeBSD. --=20 Sebastian Huber, embedded brains GmbH Address : Dornierstr. 4, D-82178 Puchheim, Germany Phone : +49 89 189 47 41-16 Fax : +49 89 189 47 41-09 E-Mail : sebastian.huber@embedded-brains.de PGP : Public key available on request. Diese Nachricht ist keine gesch=C3=A4ftliche Mitteilung im Sinne des EHUG= .
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