20 Dec
2022
20 Dec
'22
8:52 a.m.
On Dec 19, 2022, at 1:19 PM, Rob Pike <robpike(a)gmail.com> wrote:
Reiser and London's Unix, which I greatly admired, died on the vine
for a variety of political reasons, as well as because it had
slightly different semantics in some important cases, and because
of a broad antipathy to virtual memory across the company due to
various people having used VM on inadequate hardware, and of course
then there was Multics. Sandy Fraser was very nervous about
Research adopting the BSD kernel because of his experience with
Atlas. But let it be said: Reiser's VM system was seriously
impressive, cleanly integrated, structurally central, and
wonderfully fast. And Sandy relented but the general warmth of 1127
towards Berkeley led to Research adopting Berkeley Unix as its VAX
VM platform, despite some, including myself, feeling that was
inferior choice.
Is there a publicly available description of Reiser's VM system?
I found "A Unix operating system for the DEC VAX 11/780 Computer"
by London & Reiser which includes a long paragraph on VM (included
below) but that is about it.
And it would be interesting to hear why and what you found in
Reiser's VM system that was better than Berkeley's VM system.
Thanks!
From the London&Reiser paper:
The virtual address space of a process running on the
VAX-11/780 consists of 2**32 8-bit bytes. The two high-order
bits of a 32-bit address determine one of four segments. Two
of these segments are system segments common to the address
space of all processes. One of the system segments is
reserved for future use. The other two segments are
separately defined for each process and are automatically
managed by the context switching instructions. One of the
per-process segments is designed for a stack which grows
towards lower-numbered memory addresses. Segments are divided
into pages of 512 bytes. Memory mapping hardware translates
virtual addresses into physical addresses using page tables.
A page table contains one four-byte entry for each page
mapped; the entry contains a valid bit, a four-bit field
which encodes access privileges, a modify bit, and the
physical page-frame number where the page is mapped. (There
is no reference bit which is maintained by hardware!) A base
register and a limit register describe the page table of each
segment. The base register of a per-process segment contains
a virtual address within the system segment; the base
register for the system segment contains a physical memory
address. The VAX11/780 central processor contains a virtual
address translation buffer holding 128 virtual address-page
frame number pairs which eliminates the need for extra memory
references during address translation for (typically) 9896 of
all memory references. The memory is implemented using MOS
semiconductor RAMs with an error correcting code which
corrects all single-bit errors and detects all double-bit
errors and 70% of all greater-than-double bit errors. A
memory controller can handle 8 memory boards; using 4K chips
each board can hold 128K bytes. There can be two memory
controllers, thus the maximum amount of physical memory is
currently 2 megabytes. When 16K chips are used (forecasted
for late 1978), each board will hold 512K, and physical
memory can be 8 megabytes. There is a battery backup option
for maintaining data in the event of a power failure. Each
optional battery will maintain l megabyte for 10 minutes.