epoch(9) background information?

[Matthew Macy]

On Tue, Aug 21, 2018 at 11:34 PM Sebastian Huber <
sebastian.huber at 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 at embedded-brains.de
> > <mailto:sebastian.huber at 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
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> >     E-Mail  : sebastian.huber at embedded-brains.de
> >     <mailto:sebastian.huber at embedded-brains.de>
> >     PGP     : Public key available on request.
> >
> >     Diese Nachricht ist keine geschäftliche Mitteilung im Sinne des EHUG.
> >
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> >
> > 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