Path: utzoo!attcan!uunet!husc6!bloom-beacon!tut.cis.ohio-state.edu!cs.utexas.edu!sm.unisys.com!ism780c!haddock!ima!johnl
From: johnl@ima.ima.isc.com (John R. Levine)
Newsgroups: comp.arch
Subject: Re: Why is the RT slow?
Keywords: risc RT IBM
Message-ID: <2921@ima.ima.isc.com>
Date: 16 Nov 88 15:47:29 GMT
References: <5046@polya.Stanford.EDU>
Reply-To: johnl@ima.UUCP (John R. Levine)
Organization: Not much
Lines: 27

There are several reasons why the RT is so slow.  The worst is that it is
lashed up to 16-bit PC/AT peripherals, which make I/O a real lose.  I hear
that the new versions use a 32 bit microchannel which should be a major
improvement; the RT's native bus is quite fast.

The CPU is another issue. The original version that we developed AIX on ran at
8MHz (pretty respectable at the time) and could start an instruction every
cycle, and overlapped up to two memory references with execution. Except that
they realized that they hadn't allowed for taking page faults with multiple
transactions in progress, so whenever virtual memory was on, i.e. almost all
the time, they turned off overlap, which meant that every load or store took a
full 5 cycles. The second version of the chip fixed that, but it took forever
to get it released. Also they decided to design the CPU to work without a
cache, basically because they guessed wrong about where memory technology was
going.

There's no reason you couldn't reimplement the chip to take advantage of
a cache, but IBM seems to move very slowly, so it's hard to tell if or when
they will do so.

If you're running AIX, you have the VRM virtual machine manager sitting between
Unix and the hardware, which is also a performance hit, but that's a story all
of its own.
-- 
John R. Levine, Segue Software, POB 349, Cambridge MA 02238, +1 617 492 3869
{ bbn | spdcc | decvax | harvard | yale }!ima!johnl, Levine@YALE.something
Kids spend more time with their parents than parents spend with their kids.