Date: Wed, 22 Aug 2018 08:42:49 +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: <15e3f080-2f82-a243-80e9-f0a916445828@embedded-brains.de> In-Reply-To: <fc088eb4-f306-674c-7404-ebe17a60a5f8@embedded-brains.de> References: <db397431-2c4c-64de-634a-20f38ce6a60e@embedded-brains.de> <CALX0vxBAN6nckuAnYR3_mOfwbCjJCjHGuuOFh9njpxO%2BGUzo3w@mail.gmail.com> <fc088eb4-f306-674c-7404-ebe17a60a5f8@embedded-brains.de>
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On 22/08/18 08:34, Sebastian Huber wrote: > On 21/08/18 15:38, Jacques Fourie wrote: >> >> >> On Tue, Aug 21, 2018 at 8:33 AM, Sebastian Huber >> <sebastian.huber@embedded-brains.de >> <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)? >> >> -- Sebastian Huber, embedded brains GmbH >> >> Address : Dornierstr. 4, D-82178 Puchheim, Germany >> <https://maps.google.com/?q=Dornierstr.+4,+D-82178+Puchheim,+Germany&entry=gmail&source=g>; >> Phone : +49 89 189 47 41-16 >> Fax : +49 89 189 47 41-09 >> E-Mail : sebastian.huber@embedded-brains.de >> <mailto:sebastian.huber@embedded-brains.de> >> PGP : Public key available on request. >> >> Diese Nachricht ist keine geschäftliche Mitteilung im Sinne des >> 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 : >> http://concurrencykit.org/presentations/ebr.pdf >> <http://concurrencykit.org/presentations/ebr.pdf>. The way I >> understand it is that it is mostly used in place of read locks to >> provide liveness guarantees without using atomics. Additional detail >> is available in the commit messages. As an example see r333813 for >> some performance data. >> > > Thanks, for the reference. The "epoch reclamation" are good keywords > to find more information. > > What is the right mailing list to ask questions about the epoch > implementation of the FreeBSD kernel? > > To support this machinery in RTEMS is a bit difficult (in particular > EPOCH_LOCKED). Since RTEMS is supposed to be a real-time operating > system it supports only fixed-priority and job-level fixed priority > (EDF) schedulers. To allow some scaling to larger SMP systems it > supports clustered scheduling together with the mutual exclusion > locking protocols MrsP > (http://www-users.cs.york.ac.uk/~burns/MRSPpaper.pdf) and OMIP > (http://www.mpi-sws.org/~bbb/papers/pdf/ecrts13b.pdf). This makes the > thread pinning hard to implement (which is very easy to support in > FreeBSD). The locking protocols may temporarily move a thread which > owns a mutex to a foreign scheduler instance, e.g. a thread which > wants to obtain the mutex helps the owner to make progress if it was > pre-empted in its home scheduler instance. Due to a timeout of the > helper the owner may loose the right to execute in the foreign > scheduler instance. This would make it impossible to fulfil the > processor pinning constraint (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 an exotic feature which is unlikely to get used in FreeBSD. > Another question, is it a common use case to call epoch_enter_preempt() and epoch_exit_preempt() while owning a mutex? -- 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 geschäftliche Mitteilung im Sinne des EHUG.
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