Path: utzoo!mnetor!uunet!husc6!cmcl2!brl-adm!adm!mike@BRL.ARPA
From: mike@BRL.ARPA (Mike Muuss)
Newsgroups: comp.unix.wizards
Subject: Re:  RAM disk as /dev/swap
Message-ID: <10796@brl-adm.ARPA>
Date: 12 Dec 87 03:43:32 GMT
Sender: news@brl-adm.ARPA
Lines: 44

BRL gave a lot of business to the "BULK MOS" RF-11 emulator companies
back in the PDP-11 days.  It was indeed true that the best choice
for a bulk memory system was /tmp.  The second best choice was the
root itself.

This is due to the high frequency of access, and the fact that all
filesystem accesses were single block transfers (512 bytes).  The
zero rotational delay on the bulk memory, plus the very high DMA rates,
helped enormously.  We could sustain more than 200 1-block DMAs through
the filesystem per second off such a "drive".

Using the bulk memory for swapping helped very little.  Recall that
PDP-11 UNIX allocated a contiguous area on the swap device for each
process (two areas, if pure text), so that a swap in could be
accomplished in one (or two) DMAs.  Seek time didn't make much
difference.  Being thorough and compiling all system programs to have
pure text helped even more. Our strategy was to use an old RK-11 on a
dedicated controller as our swap device.

As an example, the machine BRL-BMD.ARPA (now defunct) was a PDP-11/70
with 2 Mbytes of main memory, and roughly the following disk configuration:

Root:	Dataram BULK CORE (non-volatile)
Swap:	RK-11 drive on a dedicated controller (well, with one other drive
	used occasionally for private user packs)
/tmp:	Dataram BULK MOS
/usr:	CDC 9762 (80 Mbyte) drive on Cachebus ("MASSBUS") via SI 9400
users:	CDC 9766 (300 Mbyte) drive on Cachebus ("MASSBUS") via SI 9400

Note that this obeyed the "5 drives minimum" rule for configuring PDP-11
UNIX systems, taking it even further by having 5 controllers.

This system, running BRL/JHU UNIX, had full TCP/IP support, and could
handle 30+ users with sub-second response time for text editing in EMACS
during peak load, due to the "high performance scheduler" work (which I
reported on at the 1979 Summer USENIX Conference).

The processor, a trusty old friend purchased in 1977, is still with us,
now running BRL's LOS GATEWAY software (written by Ron Natalie, with
some assistance from the rest of ACST), and functioning as one of BRL's
primary IP gateways (as are our other 2 PDP-11/70 processors).

Ah, memories.  (pun intended).
	-Mike