From: "Theodore Ts'o" <tytso@mit.edu>
To: Noel Chiappa <jnc@mercury.lcs.mit.edu>
Cc: tuhs@minnie.tuhs.org
Subject: Re: [TUHS] CMU Mach sources?
Date: Sun, 23 Jun 2019 19:54:00 -0400 [thread overview]
Message-ID: <20190623235400.GA1805@mit.edu> (raw)
In-Reply-To: <20190623220856.AADD018C0CF@mercury.lcs.mit.edu>
On Sun, Jun 23, 2019 at 06:08:56PM -0400, Noel Chiappa wrote:
>
> Hammer-nail syndrome.
>
> When the only tool you have for creating separate subsystems is processes, you
> wind up with a lot of processes. Who'd a thunk it.
>
> A system with a segmented memory which allows subroutine calls from one subsystem
> to another will have a lot less overhead. It does take hardware support to be
> really efficient, though. The x86 processors had that support, until Intel dropped
> it from the latest ones because nobody used it.
One of the real problems with how x86 implemented segments was that
the segments were layered on top of the 32-bit virtual address space
implemented by the page tables. So if you wanted to use a pure
segmented architecture, ala Multics, you run into the same problem as
32-bit IP addresses. If you need to allow for segments to grow, you
very quickly fragment the 32-bit address space.
If I recall correctly, this wasn't an issue with Multics because the
DPS-8 had page tables for each segment.
Maybe if Intel/AMD had kept segmentation support when x86_64 was
developed, trying to do something more novel with segments could have
worked when we went to 64-bits (or maybe, like IPv6, what's really
needed is 128-bits of address space :-P). But, Itanic was supposed to
be the dominant 64-bit architecture that was going to take over the
whole world, and when that turned out not to be the case, AMD threw
together x86_64 as the "just good enough" architectural extension.
- Ted
next prev parent reply other threads:[~2019-06-23 23:59 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-23 22:08 Noel Chiappa
2019-06-23 23:54 ` Theodore Ts'o [this message]
2019-06-24 17:04 ` Jason Stevens
-- strict thread matches above, loose matches on Subject: below --
2019-07-01 13:20 Jason Stevens
2019-06-23 4:38 Chris Hanson
2019-06-23 5:15 ` Larry McVoy
2019-06-23 8:52 ` Andrew Warkentin
2019-06-23 13:39 ` Jon Forrest
2019-06-23 13:59 ` arnold
2019-06-23 14:03 ` Jason Stevens
2019-06-23 8:04 ` Jason Stevens
2019-06-23 14:54 ` Henry Bent
2019-06-23 21:52 ` Clem Cole
2019-06-25 0:06 ` Larry McVoy
2019-06-25 0:31 ` Theodore Ts'o
2019-06-25 0:45 ` Larry McVoy
2019-06-25 0:55 ` Kurt H Maier
2019-06-25 4:18 ` Larry McVoy
2019-06-25 1:00 ` Richard Salz
2019-06-25 8:00 ` Kevin Bowling
2019-06-25 12:11 ` Arthur Krewat
2019-06-25 12:17 ` Arthur Krewat
2019-06-26 2:45 ` Kurt H Maier
2019-06-26 2:56 ` Larry McVoy
2019-06-26 15:11 ` Theodore Ts'o
2019-06-26 17:44 ` Larry McVoy
2019-06-26 18:01 ` arnold
2019-06-26 18:18 ` Warner Losh
2019-06-26 19:22 ` Chris Hanson
2019-06-26 19:32 ` Ben Greenfield via TUHS
2019-06-26 20:21 ` Larry McVoy
2019-06-27 0:22 ` Chris Hanson
2019-06-27 1:02 ` Larry McVoy
2019-06-27 1:26 ` Chris Hanson
2019-06-27 4:01 ` Lyndon Nerenberg
2019-06-27 10:34 ` Ben Greenfield via TUHS
2019-06-27 10:59 ` arnold
2019-06-27 11:13 ` Ben Greenfield via TUHS
2019-06-27 11:39 ` arnold
2019-06-27 14:58 ` Warner Losh
2019-06-27 17:25 ` Larry McVoy
2019-06-26 19:30 ` Dennis Boone
2019-06-26 19:25 ` Adam Thornton
2019-06-23 8:27 ` Kevin Bowling
2019-06-25 3:07 ` Gregg Levine
2019-06-25 8:15 ` Kevin Bowling
2019-06-25 18:18 ` Chris Hanson
2019-06-25 20:23 ` Gregg Levine
2019-06-26 1:04 ` Jason Stevens
2019-06-26 0:53 ` Jason Stevens
2019-06-25 7:49 ` Jason Stevens
2019-06-25 7:59 ` Andreas Grapentin
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