From owner-freebsd-hackers@freebsd.org Wed Aug 22 06:34:41 2018 Return-Path: Delivered-To: freebsd-hackers@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 677A9108E63F for ; Wed, 22 Aug 2018 06:34:41 +0000 (UTC) (envelope-from sebastian.huber@embedded-brains.de) Received: from dedi548.your-server.de (dedi548.your-server.de [85.10.215.148]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id F21E184A24; Wed, 22 Aug 2018 06:34:40 +0000 (UTC) (envelope-from sebastian.huber@embedded-brains.de) Received: from [78.46.172.2] (helo=sslproxy05.your-server.de) by dedi548.your-server.de with esmtpsa (TLSv1.2:DHE-RSA-AES256-GCM-SHA384:256) (Exim 4.85_2) (envelope-from ) id 1fsMj1-00055O-A3; Wed, 22 Aug 2018 08:34:39 +0200 Received: from [82.135.62.35] (helo=mail.embedded-brains.de) by sslproxy05.your-server.de with esmtpsa (TLSv1.2:DHE-RSA-AES256-GCM-SHA384:256) (Exim 4.89) (envelope-from ) id 1fsMj0-000MsY-Ub; Wed, 22 Aug 2018 08:34:39 +0200 Received: from localhost (localhost.localhost [127.0.0.1]) by mail.embedded-brains.de (Postfix) with ESMTP id 9F9BC2A165C; Wed, 22 Aug 2018 08:34:40 +0200 (CEST) Received: from mail.embedded-brains.de ([127.0.0.1]) by localhost (zimbra.eb.localhost [127.0.0.1]) (amavisd-new, port 10032) with ESMTP id zwduocX7XopA; Wed, 22 Aug 2018 08:34:38 +0200 (CEST) Received: from localhost (localhost.localhost [127.0.0.1]) by mail.embedded-brains.de (Postfix) with ESMTP id 355BA2A167E; Wed, 22 Aug 2018 08:34:38 +0200 (CEST) X-Virus-Scanned: amavisd-new at zimbra.eb.localhost Received: from mail.embedded-brains.de ([127.0.0.1]) by localhost (zimbra.eb.localhost [127.0.0.1]) (amavisd-new, port 10026) with ESMTP id zfAf8mP6bkTu; Wed, 22 Aug 2018 08:34:38 +0200 (CEST) Received: from [192.168.96.149] (unknown [192.168.96.149]) by mail.embedded-brains.de (Postfix) with ESMTPSA id ED0922A165C; Wed, 22 Aug 2018 08:34:37 +0200 (CEST) Subject: Re: epoch(9) background information? To: FreeBSD Cc: Matthew Macy References: From: Sebastian Huber Message-ID: Date: Wed, 22 Aug 2018 08:34:35 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: quoted-printable X-Authenticated-Sender: smtp-embedded@poldinet.de X-Virus-Scanned: Clear (ClamAV 0.100.1/24863/Tue Aug 21 18:48:32 2018) X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.27 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 22 Aug 2018 06:34:41 -0000 On 21/08/18 15:38, Jacques Fourie wrote: > > > On Tue, Aug 21, 2018 at 8:33 AM, Sebastian Huber=20 > > 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 > > 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 > > 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 > mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-hackers > > To unsubscribe, send any mail to > "freebsd-hackers-unsubscribe@freebsd.org > " > > > Additional information is available here :=20 > http://concurrencykit.org/presentations/ebr.pdf=20 > . 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= .