Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]
Groups > comp.programming.threads > #2186
| From | nmm@needham.csi.cam.ac.uk (Nick Maclaren) |
|---|---|
| Newsgroups | comp.programming.threads, comp.programming, comp.arch |
| Subject | Re: About lockfree and waitfree... (2nd try) |
| Date | 2014-04-18 11:56 +0100 |
| Organization | Department of Deniable Assertions |
| Message-ID | <lir0dp$qnh$1@needham.csi.cam.ac.uk> (permalink) |
| References | <lif821$inj$1@news.albasani.net> <lipq6r$l97$1@dont-email.me> <liqp7o$1g2$1@needham.csi.cam.ac.uk> <liqrdp$13i$1@dont-email.me> |
Cross-posted to 3 groups.
In article <liqrdp$13i$1@dont-email.me>, Ivan Godard <ivan@ootbcomp.com> wrote: >> >> Do you mean the same thing by sequential consistency and sequential >> concurrency? > >"Sequential concurrency" is not a familiar term to me, nor, it appears, >to Google, which has only one cite and that for some slides that appear >to have invented the term. So no, I don't :-) Then could you explain what you mean by it? I am lost. >> However, I find it odd that there is no global clock, because it >> is easy to provide if you already have sequential consistency. >> The 'implementation' is just that one thread updates the clock >> and others read it. While that does not provide 'physical' >> consistency of the time, it's all any sane algorithm needs. > >I'm describing hardware; there is none in hardware, although a software >clock is straightforward if you can accept the drawbacks. Global time is >very difficult in hardware. There may be a single global clock, but >non-uniform delays in reading the clock mean that two clients of the >same clock will not agree on the time. That is true but, if you have sequential consistency, they all get consistent times. That is all that almost all algorithms need. >It is certainly possible to have a single global take-a-number that >gives a unique number to each client on request, and that can be used to >provide a global ordering, but such systems do not scale - tghe >take-a-number is a single-point bottleneck, and the round-trip latency >to the TAN and back is often non-trivial. Now, there I disagree. The point is that providing a global time and providing a set of globally ordered tokens are entirely separate requirements, and the former is fairly easy to scale. The main requirement for a global clock is for debugging and tracing - I had to write a mechanism for MPI when I needed it, which delivers consistency comparable to the round-trip latency. I could do better in hardware. Absolute consistency with memory accesses is not needed, but microsecond-level precision is. The global take-a-number requirements DON'T need a timestamp, but only something that is unique and increases monotonically. I agree that those mechanisms don't scale. There are two approaches that I know of. One is that a single 'thread' updates a location, and let the sequentially consistent memory access handle its distribution. This means that, if other threads start hammering on the timestamp, it runs like a drain, but most genuine requirements for times don't do that. The other is to have a hierarchical distribution, and not attempt to maintain consistency with memory accesses. This can still provide a guaranteed, small error bound. I prefer this one, as it is both highly scalable even under heavy load and gives all that is needed for almost all clock requirements. Regards, Nick Maclaren.
Back to comp.programming.threads | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
About lockfree and waitfree... aminer <aminer@toto.net> - 2014-04-13 19:54 -0700
Re: About lockfree and waitfree... "Chris M. Thomasson" <no@spam.invalid> - 2014-04-13 19:38 -0700
Re: About lockfree and waitfree... "Chris M. Thomasson" <no@spam.invalid> - 2014-04-13 19:47 -0700
Re: About lockfree and waitfree... aminer <aminer@toto.net> - 2014-04-13 22:56 -0700
Re: About lockfree and waitfree... "Chris M. Thomasson" <no@spam.invalid> - 2014-04-14 14:08 -0700
Re: About lockfree and waitfree... "Chris M. Thomasson" <no@spam.invalid> - 2014-04-14 14:23 -0700
Re: About lockfree and waitfree... Ivan Godard <ivan@ootbcomp.com> - 2014-04-14 16:27 -0700
Re: About lockfree and waitfree... (2nd try) Ivan Godard <ivan@ootbcomp.com> - 2014-04-14 16:29 -0700
Re: About lockfree and waitfree... (2nd try) "Chris M. Thomasson" <no@spam.invalid> - 2014-04-15 10:04 -0700
Re: About lockfree and waitfree... (2nd try) Ivan Godard <ivan@ootbcomp.com> - 2014-04-15 10:34 -0700
Re: About lockfree and waitfree... (2nd try) "Chris M. Thomasson" <no@spam.invalid> - 2014-04-17 16:35 -0700
Re: About lockfree and waitfree... (2nd try) Ivan Godard <ivan@ootbcomp.com> - 2014-04-17 17:04 -0700
Re: About lockfree and waitfree... (2nd try) nmm@needham.csi.cam.ac.uk (Nick Maclaren) - 2014-04-18 09:54 +0100
Re: About lockfree and waitfree... (2nd try) Ivan Godard <ivan@ootbcomp.com> - 2014-04-18 02:31 -0700
Re: About lockfree and waitfree... (2nd try) nmm@needham.csi.cam.ac.uk (Nick Maclaren) - 2014-04-18 11:56 +0100
Re: About lockfree and waitfree... (2nd try) rpw3@rpw3.org (Rob Warnock) - 2014-04-18 11:26 +0000
Re: About lockfree and waitfree... (2nd try) Ivan Godard <ivan@ootbcomp.com> - 2014-04-18 13:30 -0700
Re: About lockfree and waitfree... (2nd try) "Chris M. Thomasson" <no@spam.invalid> - 2014-04-21 14:25 -0700
Re: About lockfree and waitfree... (2nd try) "Chris M. Thomasson" <no@spam.invalid> - 2014-04-21 14:31 -0700
Re: About lockfree and waitfree... (2nd try) Ivan Godard <ivan@ootbcomp.com> - 2014-04-21 14:53 -0700
Re: About lockfree and waitfree... (2nd try) "Chris M. Thomasson" <no@spam.invalid> - 2014-04-22 14:21 -0700
Re: About lockfree and waitfree... (2nd try) "Chris M. Thomasson" <no@spam.invalid> - 2014-04-22 14:36 -0700
Re: About lockfree and waitfree... (2nd try) Ivan Godard <ivan@ootbcomp.com> - 2014-04-22 15:30 -0700
Re: About lockfree and waitfree... (2nd try) "Chris M. Thomasson" <no@spam.invalid> - 2014-05-01 00:48 -0700
Re: About lockfree and waitfree... (2nd try) "Chris M. Thomasson" <no@spam.invalid> - 2014-05-25 13:16 -0700
Re: About lockfree and waitfree... (2nd try) "Chris M. Thomasson" <no@spam.invalid> - 2014-04-21 14:34 -0700
Re: About lockfree and waitfree... (2nd try) Robert Wessel <robertwessel2@yahoo.com> - 2014-04-21 16:42 -0500
Re: About lockfree and waitfree... (2nd try) Ivan Godard <ivan@ootbcomp.com> - 2014-04-21 15:02 -0700
Re: About lockfree and waitfree... (2nd try) nmm@needham.csi.cam.ac.uk (Nick Maclaren) - 2014-04-22 09:05 +0100
Re: About lockfree and waitfree... (2nd try) Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2014-04-22 09:27 -0700
Re: About lockfree and waitfree... (2nd try) Ivan Godard <ivan@ootbcomp.com> - 2014-04-22 09:45 -0700
Re: About lockfree and waitfree... (2nd try) nmm@needham.csi.cam.ac.uk (Nick Maclaren) - 2014-04-22 18:08 +0100
Re: About lockfree and waitfree... (2nd try) Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2014-04-23 12:08 -0700
Re: About lockfree and waitfree... (2nd try) nmm@needham.csi.cam.ac.uk (Nick Maclaren) - 2014-04-23 20:32 +0100
Re: About lockfree and waitfree... (2nd try) Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2014-04-23 13:21 -0700
Re: About lockfree and waitfree... (2nd try) nmm@needham.csi.cam.ac.uk (Nick Maclaren) - 2014-04-23 21:51 +0100
Re: About lockfree and waitfree... (2nd try) Chris Gray <cg@GraySage.COM> - 2014-04-24 10:53 -0600
Re: About lockfree and waitfree... (2nd try) nmm@needham.csi.cam.ac.uk (Nick Maclaren) - 2014-04-24 18:02 +0100
Re: About lockfree and waitfree... (2nd try) Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2014-04-25 13:39 -0700
Re: About lockfree and waitfree... (2nd try) "Chris M. Thomasson" <no@spam.invalid> - 2014-04-22 22:08 -0700
Re: About lockfree and waitfree... (2nd try) George Neuner <gneuner2@comcast.net> - 2014-04-17 21:15 -0400
Re: About lockfree and waitfree... (2nd try) George Neuner <gneuner2@comcast.net> - 2014-04-17 21:25 -0400
Re: About lockfree and waitfree... (2nd try) Ivan Godard <ivan@ootbcomp.com> - 2014-04-17 19:17 -0700
Re: About lockfree and waitfree... (2nd try) George Neuner <gneuner2@comcast.net> - 2014-04-18 00:46 -0400
Re: About lockfree and waitfree... (2nd try) Ivan Godard <ivan@ootbcomp.com> - 2014-04-17 18:52 -0700
Re: About lockfree and waitfree... aminer <aminer@toto.net> - 2014-04-13 22:54 -0700
csiph-web