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Re: big-little

From MitchAlsup <MitchAlsup@aol.com>
Newsgroups comp.arch
Subject Re: big-little
Date 2011-11-04 10:06 -0700
Organization http://groups.google.com
Message-ID <11596169.387.1320426416698.JavaMail.geo-discussion-forums@yqce21> (permalink)
References <j7pu4q$ndv$1@USTR-NEWS.TR.UNISYS.COM> <22374797.1522.1319825720498.JavaMail.geo-discussion-forums@yqp37> <j8eu6n$e95$1@gosset.csi.cam.ac.uk> <12858059.253.1319918966244.JavaMail.geo-discussion-forums@yqiu15> <j8j140$q9e$1@gosset.csi.cam.ac.uk>

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On Sunday, October 30, 2011 3:16:00 AM UTC-5, nm...@cam.ac.uk wrote:
> In article <12858059.253.1319918966244.JavaMail.geo-discussion-forums@yqiu15>,
> MitchAlsup  <comp...@googlegroups.com> wrote:
> >>
> >> Well, the first problem is that either the amount of snoop traffic
> >> goes up quadratically with the number of cores, or the amount of
> >> logic does (or some combination).  And a second is that the latency
> >> is a serious problem, because of RC delay, and even the speed of
> >> light limits the performance.
> 	>
> >Enter the snoop filter: A section of some large multi-way cache is
> >sacrificed to contain snoop information and this storage is allocated to
> >the memory controller. Say 1 way from a 8MB 16-way cache. ...
> 
> I know almost nothing about the details of snoop filters, but my remarks
> applied both to basic snoop systems and to directories, and I should be
> flabberghasted if snoop filters weren't in some sense an intermediate
> form.  I.e. they may well help, or even help a lot, but they will be
> subject to the same basic limitations of scalability, perhaps with
> a different constant.

Think of the snoop filter like this:
You get a swath of memory to keep track of things (groups of lines).
The things you end up keeping track of don't need to get snooped (for example, you KNOW that the line is not in any processor cache) but for a few cases (You know that the line is in a cache on a node, but not whether is has been written or not. That is whether it made the transition from exclusive to modified).
The things you did not keep track of need snooped as normal. (No info means revert back to snoop based cache coherence).
The real question is how do you compress the directory so that it fits into the swath of memory you might have available.
 
> Do you know of a paper that describes the principles of snoop filters
> in enough mathematical detail to do realistic scalability calculations?

Sorry, no.
<snip>
> And, as you know, the problem about shrinking large directory-based
> designs onto a single chip is that you need an extra generation of
> process just to cater for all the extra logic and wiring!  It's
> that sort of thing that makes me doubt large-scale, sequentially
> or Intel causally consistent chips, even if the bandwidth could be
> resolved.  And, as you know, that's not needed, because explicitly
> synchronised memory is good enough for almost all purposes.

Couple of points:
A) the snoop filter remembers which NODE contains the cache line, but not which processor. THis means that as the core count increases the directory pretty much stays the same.
B) as the core count increases, the on-die largest cache increases in size--giving more room for the snoop filtering tables.
C) Feeding large core count processors a standard amount of memory bandwidth is going to get the medium node systems (4-8) about the same time all the snoop filtering tricks run out of steam.
D) one COULD put the directory in DRAM by using a more robust ECC which contains a compressed version of the directory--in the same number of bit as are currently used by data+ECC.

Mitch

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big-little timcaffrey@aol.com (Tim McCaffrey) - 2011-10-20 19:51 +0000
  Re: big-little Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-20 22:35 +0200
    Re: big-little Mark Thorson <nospam@sonic.net> - 2011-10-20 14:18 -0800
      Re: big-little nmm1@cam.ac.uk - 2011-10-21 07:54 +0100
        Re: big-little David Kanter <dkanter@gmail.com> - 2011-11-09 01:06 -0800
          Re: big-little nmm1@cam.ac.uk - 2011-11-09 11:07 +0000
      Re: big-little Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-21 09:25 +0200
      Re: big-little EricP <ThatWouldBeTelling@thevillage.com> - 2011-10-21 11:07 -0400
        Re: big-little timcaffrey@aol.com (Tim McCaffrey) - 2011-10-21 16:28 +0000
          Re: big-little nmm1@cam.ac.uk - 2011-10-21 17:07 +0100
          Re: big-little EricP <ThatWouldBeTelling@thevillage.com> - 2011-10-21 13:25 -0400
            Re: big-little timcaffrey@aol.com (Tim McCaffrey) - 2011-10-21 19:45 +0000
              Re: big-little EricP <ThatWouldBeTelling@thevillage.com> - 2011-10-22 14:44 -0400
          Re: big-little ChrisQ <meru@devnull.com> - 2011-10-22 16:57 +0000
            Re: big-little nmm1@cam.ac.uk - 2011-10-22 17:40 +0100
              Re: big-little ChrisQ <meru@devnull.com> - 2011-10-23 14:39 +0000
                Re: big-little nmm1@cam.ac.uk - 2011-10-23 16:11 +0100
                Re: big-little ChrisQ <meru@devnull.com> - 2011-10-23 17:44 +0000
                Re: big-little nmm1@cam.ac.uk - 2011-10-23 18:46 +0100
                Re: big-little Thomas Womack <twomack@chiark.greenend.org.uk> - 2011-10-23 19:30 +0100
                Re: big-little nmm1@cam.ac.uk - 2011-10-23 19:59 +0100
                Re: big-little BGB <cr88192@hotmail.com> - 2011-10-23 13:21 -0700
                Re: big-little Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-23 20:38 +0200
                Re: big-little nmm1@cam.ac.uk - 2011-10-23 22:38 +0100
                Re: big-little BGB <cr88192@hotmail.com> - 2011-10-23 17:56 -0700
                Re: big-little nmm1@cam.ac.uk - 2011-10-24 07:58 +0100
                Re: big-little BGB <cr88192@hotmail.com> - 2011-10-24 16:34 -0700
                Re: big-little nmm1@cam.ac.uk - 2011-10-25 08:09 +0100
                Re: big-little Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-25 14:52 +0200
                Re: big-little nmm1@cam.ac.uk - 2011-10-25 17:28 +0100
                Re: big-little BGB <cr88192@hotmail.com> - 2011-10-25 13:06 -0700
                Re: big-little BGB <cr88192@hotmail.com> - 2011-10-25 15:22 -0700
                Re: big-little "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-10-25 19:47 -0700
                Re: big-little nmm1@cam.ac.uk - 2011-10-26 08:09 +0100
                Re: big-little Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-26 12:06 +0200
                Re: big-little nmm1@cam.ac.uk - 2011-10-26 10:47 +0100
                Re: big-little Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-26 11:58 +0200
                Re: big-little nmm1@cam.ac.uk - 2011-10-26 10:44 +0100
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-26 03:45 -0700
                Re: big-little nmm1@cam.ac.uk - 2011-10-26 11:45 +0100
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-26 09:15 -0700
                Re: big-little "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-10-26 12:51 -0700
                Re: big-little Andrew Reilly <areilly---@bigpond.net.au> - 2011-10-26 11:45 +0000
                Re: big-little nmm1@cam.ac.uk - 2011-10-26 15:42 +0100
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-26 09:07 -0700
                Re: big-little "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-10-26 12:53 -0700
                Re: big-little nmm1@cam.ac.uk - 2011-10-26 21:05 +0100
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-26 13:20 -0700
                Re: big-little "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-10-26 17:55 -0700
                Re: big-little nmm1@cam.ac.uk - 2011-10-27 09:10 +0100
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-27 01:34 -0700
                Re: big-little nmm1@cam.ac.uk - 2011-10-27 10:29 +0100
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-27 04:28 -0700
                Re: big-little EricP <ThatWouldBeTelling@thevillage.com> - 2011-10-27 12:22 -0400
                Re: big-little timcaffrey@aol.com (Tim McCaffrey) - 2011-10-27 18:08 +0000
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-27 11:50 -0700
                Re: big-little Jean-Marc Bourguet <jm@bourguet.org> - 2011-10-27 14:10 +0200
                Re: big-little nmm1@cam.ac.uk - 2011-10-27 17:58 +0100
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-26 13:15 -0700
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-27 04:10 -0700
                Re: big-little MitchAlsup <MitchAlsup@aol.com> - 2011-10-27 14:04 -0700
                Re: big-little nmm1@cam.ac.uk - 2011-10-27 23:07 +0100
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-27 15:58 -0700
                Re: big-little Rick Jones <rick.jones2@hp.com> - 2011-10-27 22:57 +0000
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-27 16:39 -0700
                Re: big-little Rick Jones <rick.jones2@hp.com> - 2011-10-28 00:16 +0000
                Re: big-little nmm1@cam.ac.uk - 2011-10-28 09:52 +0100
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-28 07:01 -0700
                Re: big-little Rick Jones <rick.jones2@hp.com> - 2011-10-28 16:45 +0000
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-29 13:13 -0700
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-30 03:36 -0700
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-30 03:55 -0700
                Re: big-little MitchAlsup <MitchAlsup@aol.com> - 2011-10-28 11:15 -0700
                Re: big-little nmm1@cam.ac.uk - 2011-10-28 20:01 +0100
                Re: big-little timcaffrey@aol.com (Tim McCaffrey) - 2011-10-29 17:46 +0000
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-29 11:30 -0700
                Re: big-little nmm1@cam.ac.uk - 2011-10-29 20:02 +0100
                Re: big-little MitchAlsup <MitchAlsup@aol.com> - 2011-10-29 13:09 -0700
                Re: big-little "Paul A. Clayton" <paaronclayton@gmail.com> - 2011-10-29 17:42 -0700
                Re: big-little nmm1@cam.ac.uk - 2011-10-30 08:16 +0000
                Re: big-little MitchAlsup <MitchAlsup@aol.com> - 2011-11-04 10:06 -0700
                Re: big-little "Paul A. Clayton" <paaronclayton@gmail.com> - 2011-11-04 15:19 -0700
                Re: big-little nmm1@cam.ac.uk - 2011-11-05 09:13 +0000
                Re: big-little "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-11-05 09:03 -0700
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-26 13:38 -0700
                Re: big-little Robert Wessel <robertwessel2@yahoo.com> - 2011-10-26 18:44 -0500
                Re: big-little Michael S <already5chosen@yahoo.com> - 2011-10-26 17:05 -0700
                Re: big-little Robert Wessel <robertwessel2@yahoo.com> - 2011-10-26 21:03 -0500
                Re: big-little EricP <ThatWouldBeTelling@thevillage.com> - 2011-10-26 14:37 -0400
                Re: big-little EricP <ThatWouldBeTelling@thevillage.com> - 2011-10-26 15:38 -0400
                Re: big-little EricP <ThatWouldBeTelling@thevillage.com> - 2011-10-26 16:06 -0400
                Re: big-little BGB <cr88192@hotmail.com> - 2011-10-27 00:15 -0700
                Re: big-little Morten Reistad <first@last.name> - 2011-11-03 17:30 +0100
                Re: big-little "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-11-03 20:45 -0700
                Re: big-little Thomas Womack <twomack@chiark.greenend.org.uk> - 2011-11-04 11:13 +0000
                Re: big-little MitchAlsup <MitchAlsup@aol.com> - 2011-11-04 10:12 -0700
                Re: big-little BGB <cr88192@hotmail.com> - 2011-11-04 09:26 -0700
                Re: big-little EricP <ThatWouldBeTelling@thevillage.com> - 2011-11-05 11:32 -0400
                Re: big-little EricP <ThatWouldBeTelling@thevillage.com> - 2011-11-05 11:55 -0400
                Re: big-little BGB <cr88192@hotmail.com> - 2011-11-05 09:58 -0700
                Re: big-little Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-11-05 18:39 +0100
                Re: big-little BGB <cr88192@hotmail.com> - 2011-11-05 15:00 -0700
                Re: big-little wclodius@lost-alamos.pet (William Clodius) - 2011-11-05 18:43 -0600
                Re: big-little BGB <cr88192@hotmail.com> - 2011-11-05 19:46 -0700
                Re: big-little ChrisQ <meru@devnull.com> - 2011-11-11 12:31 +0000
                Re: big-little BGB <cr88192@hotmail.com> - 2011-11-11 12:39 -0700
                Re: big-little Chris Gray <cg@GraySage.com> - 2011-11-11 13:28 -0700
                Re: big-little BGB <cr88192@hotmail.com> - 2011-11-11 18:03 -0700
                Re: big-little Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-11-12 18:25 +0100
                Re: big-little BGB <cr88192@hotmail.com> - 2011-11-12 12:02 -0700
                cost of prolog/epilog (was: big-little) Kalle Olavi Niemitalo <kon@iki.fi> - 2011-11-12 22:57 +0200
                Re: cost of prolog/epilog Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-11-12 22:47 +0100
                Re: cost of prolog/epilog anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2011-11-14 09:45 +0000
                Re: cost of prolog/epilog Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-11-14 12:28 +0100
                Re: cost of prolog/epilog anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2011-11-15 16:48 +0000
                Re: cost of prolog/epilog Stefan Monnier <monnier@iro.umontreal.ca> - 2011-11-15 21:35 -0500
                Re: cost of prolog/epilog Bakul Shah <usenet@bitblocks.com> - 2011-11-16 02:41 -0800
                Re: cost of prolog/epilog nmm1@cam.ac.uk - 2011-11-16 18:14 +0000
                Re: cost of prolog/epilog BGB <cr88192@hotmail.com> - 2011-11-16 16:50 -0700
                Re: cost of prolog/epilog BGB <cr88192@hotmail.com> - 2011-11-14 17:00 -0700
                Re: cost of prolog/epilog torbenm@diku.dk (Torben Ægidius Mogensen) - 2011-11-15 10:26 +0100
                Re: cost of prolog/epilog BGB <cr88192@hotmail.com> - 2011-11-15 09:41 -0700
                Re: cost of prolog/epilog anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2011-11-15 16:40 +0000
                Re: cost of prolog/epilog Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-11-15 12:44 +0100
                Re: cost of prolog/epilog BGB <cr88192@hotmail.com> - 2011-11-15 09:01 -0700
                Re: cost of prolog/epilog Chris Gray <cg@GraySage.com> - 2011-11-15 15:18 -0700
                Re: cost of prolog/epilog BGB <cr88192@hotmail.com> - 2011-11-15 21:56 -0700
                Re: cost of prolog/epilog Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-11-16 09:21 +0100
                Re: cost of prolog/epilog anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2011-11-16 09:30 +0000
                Re: cost of prolog/epilog John Levine <johnl@iecc.com> - 2011-11-16 10:05 +0000
                Re: cost of prolog/epilog Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-11-16 11:08 +0100
                Re: cost of prolog/epilog Chris Gray <cg@GraySage.com> - 2011-11-16 16:29 -0700
                Re: cost of prolog/epilog BGB <cr88192@hotmail.com> - 2011-11-16 11:22 -0700
                Re: cost of prolog/epilog anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2011-11-17 13:27 +0000
                Re: cost of prolog/epilog BGB <cr88192@hotmail.com> - 2011-11-17 10:30 -0700
                Re: cost of prolog/epilog torbenm@diku.dk (Torben Ægidius Mogensen) - 2011-11-17 11:21 +0100
                Re: cost of prolog/epilog Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-11-17 11:56 +0100
                Re: cost of prolog/epilog timcaffrey@aol.com (Tim McCaffrey) - 2011-11-17 20:27 +0000
                Re: cost of prolog/epilog torbenm@diku.dk (Torben Ægidius Mogensen) - 2011-11-18 10:57 +0100
                Re: cost of prolog/epilog BGB <cr88192@hotmail.com> - 2011-11-17 10:42 -0700
                Re: cost of prolog/epilog (was: big-little) "Paul A. Clayton" <paaronclayton@gmail.com> - 2011-11-12 14:05 -0800
                Re: cost of prolog/epilog (was: big-little) Bernd Paysan <bernd.paysan@gmx.de> - 2011-11-18 02:39 +0100
                Re: cost of prolog/epilog torbenm@diku.dk (Torben Ægidius Mogensen) - 2011-11-18 10:59 +0100
                Re: cost of prolog/epilog Bernd Paysan <bernd.paysan@gmx.de> - 2011-11-18 22:06 +0100
                Re: cost of prolog/epilog (was: big-little) mac <acolvin@efunct.com> - 2011-11-20 23:01 +0000
                Re: cost of prolog/epilog Chris Gray <cg@GraySage.com> - 2011-11-13 13:35 -0700
                Re: big-little wclodius@lost-alamos.pet (William Clodius) - 2011-11-11 22:13 -0700
                Re: big-little BGB <cr88192@hotmail.com> - 2011-11-11 23:43 -0700
                Re: big-little Chris Gray <cg@GraySage.com> - 2011-11-12 14:08 -0700
                Re: big-little BGB <cr88192@hotmail.com> - 2011-11-12 19:10 -0700
                Re: big-little Chris Gray <cg@GraySage.com> - 2011-11-13 13:40 -0700
                Re: big-little Bakul Shah <usenet@bitblocks.com> - 2011-11-13 14:49 -0800
                Re: big-little BGB <cr88192@hotmail.com> - 2011-11-13 18:51 -0700
                Re: big-little Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-24 10:12 +0200
                Re: big-little Stefan Monnier <monnier@iro.umontreal.ca> - 2011-10-24 12:11 -0400
                Re: big-little "Paul A. Clayton" <paaronclayton@gmail.com> - 2011-10-24 10:28 -0700
                Re: big-little nospam@ab-katrinedal.dk (Niels Jørgen Kruse) - 2011-10-24 19:38 +0200
                Re: big-little Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-24 21:02 +0200
                Re: big-little "Paul A. Clayton" <paaronclayton@gmail.com> - 2011-10-23 12:02 -0700
        Re: big-little Morten Reistad <first@last.name> - 2011-11-03 00:51 +0100
    Re: big-little "Paul A. Clayton" <paaronclayton@gmail.com> - 2011-10-20 16:59 -0700
      Re: big-little Anne & Lynn Wheeler <lynn@garlic.com> - 2011-10-20 21:13 -0400
      Re: big-little nmm1@cam.ac.uk - 2011-10-21 08:01 +0100
        Re: big-little "Paul A. Clayton" <paaronclayton@gmail.com> - 2011-10-21 05:39 -0700
        Re: big-little torbenm@diku.dk (Torben Ægidius Mogensen) - 2011-10-24 10:18 +0200
      Re: big-little Pedro Pereira <ppereira@grupopie.com> - 2011-10-21 10:40 +0100
        Re: big-little Paul Gotch <paulg@at-cantab-dot.net> - 2011-10-21 12:42 +0100
    Re: big-little nik Simpson <nik_s@knology.net> - 2011-10-21 11:00 -0500
  Re: big-little Rick Jones <rick.jones2@hp.com> - 2011-10-20 21:13 +0000
    Re: big-little timcaffrey@aol.com (Tim McCaffrey) - 2011-10-20 21:43 +0000
    Re: big-little Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-21 09:23 +0200
      Re: big-little Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-21 14:35 +0200
        Re: big-little Rick Jones <rick.jones2@hp.com> - 2011-10-21 17:15 +0000
      Re: big-little BGB <cr88192@hotmail.com> - 2011-10-21 16:41 -0700
  Re: big-little jacko <jackokring@gmail.com> - 2011-10-26 14:12 -0700

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