Date: Tue, 21 Aug 2018 23:44:19 -0700 From: Matthew Macy <mmacy@freebsd.org> To: sebastian.huber@embedded-brains.de Cc: freebsd-hackers@freebsd.org Subject: Re: epoch(9) background information? Message-ID: <CAPrugNp8NM5BbEzqf3pY5hGvfyrO7MnXXLiCfCyRxC3YMWzoWw@mail.gmail.com> 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 Tue, Aug 21, 2018 at 11:34 PM Sebastian Huber < sebastian.huber@embedded-brains.de> 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=3DDornierstr.+4,+D-82178+Puchheim,+Germany&ent= ry=3Dgmail&source=3Dg > > > > 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 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 : > > 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? > > -hackers is probably as good as any. Your questions are at a high enough level that they might be appropriate for -arch. 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. > EPOCH_LOCKED is something that one would only want to use in a fairly narrow set of circumstances. The only place it's being discussed currently is in pmap: https://reviews.freebsd.org/D15983 There it would conceivably replace a global mutex that currently serializes all munmap operations. -M
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