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XZ/XL Performance Jump on Indigo2

R4000/100MHz or R4400/150MHz
Compared to R4400/200MHz

A colleague pointed out to me in email that there are odd jumps in performance for XZ and XL when going from R4000 100MHz or R4400 150MHz to R4400 200MHz on Indigo2 systems. The data comes from spec sheets published by SGI in 1993 and 1994. I'd asked if anyone could explain the oddities, and a Mr. Dan Meyer (dankmeyer@tref.nl) of The Netherlands kindly supplied a sound explanation.

First, a description of the original query. Here is a comparison of the data from the two spec sheets:

                                     Aug 1993        Sept 1994

   3D Lines                            640K             1M
   3D Lines GouraudZ Depth Cued        275K            452K
   Tmesh, Flat No Z                    250K            408K
   Tmesh, Gouraud Z, Lit               113K            181K
   Quads, FlatZ                        84K             129K
   Quads, GouraudZ, Lit                43K             67K

Note that there were probably OS and graphics library improvements during the year which spans these two data sheets.

Comparison for XZ:

About 50% better. The difference between old XL and new XL is even more striking, more than 100% better (eg. 3D Lines jumps from 490K for R4000 to 1M for R4400/200. The other figures have similar increases).

Actually, I suspect that the older XZ figures were probably measured with an R4000/100. A rather odd thing to do when they could have used an R4400/150... but I notice that all the chip performance figures on the older sheet are 'estimated' (MFLOPS, MIPS, SPEC89, SPEC92); perhaps an R4400/150 wasn't actually available to run the XZ tests with.

Also, the figures will be underestimates now since I have no data for R4400/250 or R10000 in use with XZ. I know R10000 can be used with Extreme.

The Extreme figures don't change that much between data sheets (10% max, 3% in most cases).

Note that the Quads figures are from the original IMPACT home page (I keep copies :), so actually they ought to be higher (they were measured with an R4400/250, not an R10K).

So that's the mystery. Now for Dan's explanation; he gave me a copy of a post made by an SGI employee which should shed some light on what's going on. Here is that post, written by Brad Reger of SGI's ISD (Interactive Systems Division):

From: Brad Reger (reger@esd.sgi.com)
22 Dec 1994 16:29:19 GMT

> > I have some some new Indigo2 boxes which, I am told, have
> > the XZ graphics package. However, the hinv tells me that
> > the graphics card is "GR3-Elan". How do I determine whether
> > I have Elan or XZ graphics?
> GR3 is the XZ board. GR2 is Elan. GU1 is Extreme.

While this may be true, I suspect it isn't very interesting to
most people. What our marketing folks want to call things doesn't
usually matter that much either except that it's the customer-
visible name. To confuse matters, we in engineering (along with
marketing) sometimes change things in hardware or software, but
leave the marketing name the same. We'll never (consciously)
remove a feature, but there have been times where we've added
things and left the name the same. XZ is an example.

Purple Indigo:

  Entry graphics has no hardware GE (it's done in s/w)
  XS graphics has 1 GE
  XZ graphics has 2 GE's
  Elan graphics has 4 GE's

Original Green Indigo2:

  XL has no GE's (done in s/w again)
  XZ had 2 GE's
  EX has 8 GE's

Later Green Indigo2:

  XZ has 4 GE's

Some history (my interpretation):

Purple Indigo came out with Entry graphics. Great machine.
Great price. Graphics just okay. Most people wanted more
bitplanes and the option of putting in a hardware Z-buffer.

Elan graphics comes out in purple Indigo. Excellent graphics.
Completely blew away anyone else doing desktop 3D graphics.
Along with Elan, we came out with XS (1 GE) with optional
hardware z-buffer and optional 24-bit. Now have high, medium
and low graphics configs.

Turns out the market really wanted 2-GE performance. One
wasn't quite enough and 4 was too expensive. So we came out
with the XZ. Now have the performance of 2 GE's, 24-bit,
and standard hardware z-buffer. Great product. Sold lots
of 'em. [this may have happened after the Indigo2 intro]

Green Indigo2 comes out. Intro with high-end EX graphics
(8 GE's and double the raster performance). Best graphics
on the desktop. A quarter later we intro the XZ (2 GE) config
and one quarter after that we intro the XL (0 GE) config.
All three sell well. As competition heats up (and some things
become possible in hardware) we realize the XZ needs a boost.
We bump the Indigo2/XZ from a 2 GE config to a 4 GE config.
Nothing else changes, including the name. We figure people
will be happy to get more for the same price. Problem is,
people get confused and ask lots of questions.

Now on to 'hinv'. As far as identifying graphics configs, it
uses a pretty simple algorithm. It first looks at the base
graphics board (LG*, GR*) and then just counts the number of
GE's it sees. Then it goes through a fixed mapping of #GE's
to marketing name. GR2 is purple (or brown). GR3 is green.
So you get GR2-XZ if it sees a GR2 board and 2 GE's. You get
GR3-XZ if it sees a (green) GR3 board and 2 GE's. When it sees
4 GE's it wants to call that an Elan. So you'll get either
GR2-Elan (purple) or GR3-Elan (green). Since we never had an
Elan product in green, you now have to interpret this as the
4 GE version of XZ.

Long-winded. Did I explain enough?

Brad Reger                            reger@sgi.com
Hardware Systems Engineering
Silicon Graphics -
Interactive Systems Division (green & purple boxes)

The above explanation certainly clears up the mystery performance jumps as far as XZ is concerned, but that still leaves XL. My guess is that the simple use of a faster CPU and perhaps better graphics libraries is responsible for those improvements, though I'd be happy to hear from anyone who has more accurate information.

Thanks Dan and Brad!

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