What Computer Architects
Really Do
“Or should.”
Bob Colwell
E-M Talk ISCA ‘05
But First, I’d Like to Thank
Ron Hoelzeman (Pitt), Doug Jensen, Dan
Siewiorek (CMU), George Cox, Kevin Kahn
 Paul Rodman, Dave Papworth, Rich Lethin,
Josh Fisher & amazing Multiflow team
 Incredible P6 team, esp. Randy Steck, Glenn
Hinton, Mike Fetterman, Andy Glew, Dave
Papworth, Gurbir Singh
 My parents, Ellen, Kelly, Ken, Kristen
 Joe Malingowski
 Yale Patt, Wen-mei Hwu, Guri Sohi, Tom
Conte, Computer Arch community
2

2
Is there a trend here…
3
3
A Misconception About What
Computer Architects Do
EE Times May 23, 2005
“Is the day of the architect over?”
“Microprocessor architects managed to recreate almost the whole history of the
mainframe computer industry…they used
all the tricks, from microprogramming
and stripped-down pipelines with loadstore architectures to speculative
execution and branch prediction. Best of
all, hardly anyone was unkind enough to
comment that all this ground had been
covered already, just at a lesser level
of integration.”
4
4
What would we have done…
Had we been born 300 years ago
Same IQ’s but no computers, no electronics
Power source = waterwheels and oxen
Same brains as today but different challenges
Likewise with computer pioneers
Issue isn’t “why were they so innovative & why
aren’t we”
They did what we do: whatever is necessary
Those things change over time
5
5
Intellectual Giant Theory
Intellectual giants did walk the earth in
the ’60’s
– Eckert, Mauchly, von N, Conway, Cocke, Brooks,
Flynn, Tomasulo…
– we should honor pioneering contributions
– but today’s designers are not leeches living off
that legacy
Intellectual giants did walk the earth in
the ’60’s. They still do.
6
6
Design today: more complex
1960’s complexity
system
– Poor tools, interaction of
electronics, packaging & ISA
CPU
Today’s complexity
– Today’s complexity from speed,
hyper-aggressive uArch’s, power
limits, SW compatibility, number of
usage models, CPU-Mesmerization
CPU-Mesmerization. Root cause? Profits.
7
7
64-bit extensions to IA32
CPU architect
IA32+64-bit exts
IA32
8
8
Multicore
$100B industry
9
9
Design today: scarier
 No-recalls
much harder than designfor-minimal-field-service
– Pioneers designed for 1,000 users
Design errors? Charge ‘em for service calls
– Today we design for 1,000,000,000 users
Design errors? Pray…
10
10
What architects really do

Insidious error: thinking architects design
instruction sets & uArch mechanisms

We have, and do, but that misses the point
– Architects start out as generals, moonlight as Special
Forces
– Range freely, identify needs, apply appropriate force
– Ensure that biggest risks are attacked first
– Make sure project goals are clear & focussed
– Seek odd viewing angles to drive out problems
– Supply judgment calls where data is lacking
And judgment as to when data should be collected
11
11
Recent arch history
– For past two decades an architect’s point of
highest leverage has been microarchitecture
Re-use what works
– Pipelining, caches, shared buses, superscalar
Invent where necessary
– Microdataflow/OOO, trace caches, speculative branch
predictors, cache coherency
With some major ISA work on RISCs
– But this is changing. Right now.
CPU architects must evolve into system designers
12
12
System Designers todo list
“Whatever needs doing” has become…
1. Products, not CPUs
2. Power-constrained system design
3. Multicore (gotta pay the bills, too)
4. Reliable systems from unreliable
components
No longer “what I’d like to sell you” but
designing what buyers want
13
13
Products Not CPUs
$$$
2004
iPOD
Cell phones
Ray tracing
Portable computing
Time
“PC era”
“Speed at any price”
Ubiq. comp.
“What’s in it for me?” -buyer 14
14
Power-constrained design

“fast as possible at max power” will yield to
“fast enough, no faster”
– Lesson from the embedded space

Thermal variability vs. guaranteed real-time
– Throw in wireless links for good measure

Battery life, not just cooling cost

Global warming, energy crisis looms
– It ain’t just cars and oil prices
– Be synchronized to public taste or lose
15
15
Multicore
 “Here
I come, ready or not…”
 We can build ‘em. Can we…
– Compile to them?
– Feed them? (bandwidth)
– Cool them? (power)
– Write apps for them?
 Clear
and present challenge
– There are pots of gold associated w/ this
16
16
Reliable Systems from
Unreliable Components
 N-mod
redundancy too expensive
 Transient errors, manufacturing
defects, design errors
– Must survive them all
– Solution can’t drive power up
– Still want guaranteed performance for real-time
 Intel’s
Shekhar Borkar says we have at
most 10 years to figure this out
17
17
Adjurations
 Computer
revolution is only getting
started
 Role of architects is changing
– If we don’t do it who will?
 Your
grandchildren will thank you
– And wonder if they’re as smart as you were 
18
18
backups
19
19