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From: Larry McVoy <lm@mcvoy.com>
To: Arthur Krewat <krewat@kilonet.net>
Cc: TUHS main list <tuhs@minnie.tuhs.org>
Subject: Re: [TUHS] If not Linux, then what?
Date: Tue, 27 Aug 2019 07:55:56 -0700
Message-ID: <20190827145556.GD13570@mcvoy.com> (raw)
In-Reply-To: <faaebe25-e5e5-0f6e-6eda-fc2fc6a0dc95@kilonet.net>

On Mon, Aug 26, 2019 at 11:14:45PM -0400, Arthur Krewat wrote:
> On 8/26/2019 10:45 PM, Larry McVoy wrote:
> >  Which was that the page cache is
> >*the*  cache.  There is nothing else.
> Yeah, I re-read what you wrote a few times after I replied, and realized
> what you meant ... eventually ;)

I might be making too big of a deal about it.  mmap semantics mattered
a lot when SMPs first showed up and main memory was small.  It meant
that you could have multiple CPUs seeing and working on the same chunk
of data at the same time.

It's very similar to way that IOMMUs are exposed to user space these
days, enabling virtual machines direct access to the I/O devices.

ZFS breaks that model, the data is all in the ARC and if you mmap
it they have to bcopy the data out of the ARC, into the page cache
and now they have a consistency problem, you could modify stuff 
via mmap or write and they have to manage that.

That consistency problem is the main reason that Sun almost completely
killed the buffer cache (it still was used for inodes and directories
but that was it).  That consistency problem is a pain in the rear,
all sorts of race conditions and it tended to bit rot.

Jeff and Bill are smart people so I suspect they got it right but I'm
still stunned that they took such an architecturally bad approach.
And even more stunned that the oversight people approved it.  There
is zero chance that the Sun I worked at would have allowed that.

--lm

  reply index

Thread overview: 96+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-26 23:13 Arthur Krewat
2019-08-26 23:27 ` Warner Losh
2019-08-26 23:37   ` Larry McVoy
2019-08-26 23:56   ` William Pechter
2019-08-27  0:19     ` Arthur Krewat
2019-08-27  0:30       ` Larry McVoy
2019-08-27  0:58         ` Rob Pike
2019-08-27  1:06           ` Clem Cole
2019-08-27  2:53           ` Larry McVoy
2019-08-27  9:47             ` Rob Pike
2019-08-27  7:47           ` arnold
2019-08-27 16:05           ` [TUHS] Running v10 Angelo Papenhoff
2019-08-27 16:27             ` Henry Bent
2019-08-28  4:22               ` Jason Stevens
2019-08-28  7:34                 ` Angelo Papenhoff
2019-08-28 16:46                   ` Henry Bent
2019-08-27  0:59         ` [TUHS] If not Linux, then what? Arthur Krewat
2019-08-27  1:26           ` Dan Cross
2019-08-27  2:45             ` Larry McVoy
2019-08-27  3:14               ` Arthur Krewat
2019-08-27 14:55                 ` Larry McVoy [this message]
2019-08-27 22:30                   ` George Michaelson
2019-08-27 22:40                     ` Larry McVoy
2019-08-27 22:46                       ` George Michaelson
2019-08-27 22:59                         ` [TUHS] [SPAM] " Larry McVoy
2019-08-27 23:10                           ` [TUHS] " Clem Cole
2019-08-28  0:07                             ` George Michaelson
2019-08-28  3:22                           ` [TUHS] [SPAM] " Rob Pike
2019-08-28  3:25                             ` Rob Pike
2019-08-28  4:05                             ` Larry McVoy
2019-08-28 13:52                               ` Clem Cole
2019-08-28 14:31                                 ` [TUHS] " Larry McVoy
2019-08-28 14:57                                   ` Clem Cole
2019-08-28  6:19                         ` Wesley Parish
2019-08-28  6:30                           ` Peter Jeremy
2019-08-28 11:05                             ` Jason Stevens
2019-08-28 11:11                               ` Arrigo Triulzi
2019-08-28 14:04                               ` Clem Cole
2019-08-28 16:34                                 ` Henry Bent
2019-08-28 17:32                                   ` Larry McVoy
2019-08-28 17:51                                     ` Jon Forrest
2019-08-28 18:56                                     ` Clem Cole
2019-08-28 20:23                                       ` Arrigo Triulzi
2019-08-29  3:24                                       ` Lawrence Stewart
2019-08-29 10:55                                         ` Tony Finch
2019-08-28 13:57                             ` Clem Cole
2019-08-28 12:46                           ` Warner Losh
2019-08-27 23:16                       ` Bakul Shah
2019-08-27 23:33                         ` Larry McVoy
2019-08-28  0:21                           ` Bakul Shah
2019-08-28  1:21                             ` Arthur Krewat
2019-08-28  1:46                               ` Larry McVoy
2019-08-27  0:48   ` Clem Cole
2019-08-27  1:25     ` Gregg Levine
2019-08-27  2:16   ` Theodore Y. Ts'o
2019-08-27  2:39     ` Larry McVoy
2019-08-27  5:54       ` Adam Thornton
2019-08-27  6:05         ` Gregg Levine
2019-08-27  1:17 ` Dan Cross
2019-08-28  3:53 ` Charles H. Sauer
2019-08-28  4:30 ` Jason Stevens
2019-08-28  9:36 ` Angus Robinson
2019-08-28  9:50   ` Michael Kjörling
2019-08-28 10:48     ` arnold
2019-08-28 14:10   ` Earl Baugh
2019-08-28 14:55     ` Clem Cole
2019-08-28 14:22   ` Charles H Sauer
2019-08-28 15:00     ` Steve Nickolas
2019-08-28 15:37       ` Richard Salz
2019-08-28 19:54         ` Peter Jeremy
2019-08-28 20:05           ` Christopher Browne
2019-08-28 20:07 ` Christopher Browne
2019-08-28 20:27   ` Adam Thornton
2019-08-28 20:56     ` William Pechter
2019-08-28 22:24       ` Clem cole
2019-08-28 22:27     ` William Pechter
2019-08-28 22:53       ` Arthur Krewat
2019-08-29 18:40       ` Nemo Nusquam
2019-08-29 19:18         ` Steffen Nurpmeso
2019-08-28 22:28     ` Clem cole
2019-08-28 22:48       ` Adam Thornton
2019-08-28 23:01         ` William Pechter
2019-08-28 23:09           ` Adam Thornton
2019-08-29  6:37           ` Wesley Parish
2019-08-28 23:04       ` Gregg Levine
2019-08-29 11:12     ` Tony Finch
2019-08-28 23:19   ` Theodore Y. Ts'o
2019-08-29 13:31     ` A. P. Garcia
2019-08-29 13:55       ` Arthur Krewat
2019-08-29 15:54         ` Thomas Paulsen
2019-08-29 19:19           ` Steffen Nurpmeso
2019-08-31  1:35             ` Dave Horsfall
2019-08-31 15:14               ` Steffen Nurpmeso
2019-08-31 16:58     ` Christopher Browne
2019-08-31 21:20       ` Theodore Y. Ts'o
2019-08-28 21:02 ` Thomas Paulsen

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