Optimal Fan Speed Control for Thermal Management of Servers

1
Optimal Fan Speed Control for Thermal Management
of Servers

• UMass-Amherst Green Computing Seminar
• September 21st, 2009

2
Problem

• Power consumption key challenge in data centers
• 1.5 U.S. consumption in 2006 (60b kwh)
• Every 1W server power ? 0.5-1W cooling power
• Increasing density Need more cooling power
• gt76 data centers use blades (and rising)
• Fan power can be 2000W or 23 in blade enclosures
• Is airflow overprovisioned? Can we decrease
  consumption by regulating fan speed better?

3
Blade Enclosure Overview
5 on top
5 on bottom

• 10 fans total 5 on top, 5 on bottom
• Fans pull air through blades and out

4
Basic Approach

• Model the cooling/fan system
• Need model relating blade temperatures to fan
  speed
• Complex interactions with multiple variables
• Develop smart fan speed controller
• Use model as input to MIMO controller
• Compare with existing integrated controller
• Problem Definition
• Minimize fan power consumption
• Ensure blade temperatures dont exceed threshold

5
The Models

• Power models are (kind of) easy
• Blade power consumption ? linear
• Fan power consumption ? cubic
• Temperature models are hard
• Complex zonal variations
• Lack of sensors
• Steady-state versus transient

6
Temperature Models

• Steady-state use thermal resistance
  co-efficient R
• CPU transfers heat to ambient air
• Relationship between R and temp. change and power
  consumption
• Relationship between R and fan speed
• Solve for CPU temp.
• Transient use energy balance equation
• Takes some time to transfer heat to air
• Model as a first-order discrete time system
• Validated the models though experiments
• Models have 167 parameters!!!
• Basically, 10 blades with 16 fans

7
Fan Controller

• Optimization problem is hard
• Fan speed to power consumption relationship is
  non-linear
• Transient model has non-linear relationship b/t
  CPU and fan speed
• May not have feasible solution
• Simplify the problem
• Find minimum airflow needed at each blade
  (locally)
• Solve the global problem using these airflows
• Compare with Integrated Fan Controller
• If temp goes up speed fan up
• If temp goes down lower fan speed

8
Results

• Reduced fan power from 213W to 172W or 20
  (Figure 9)
• On-demand cooling of high utilization CPUs
  (Figure 10)
• More stable temperatures (Figure 11)
• Both controllers operate below thresholds (Figure
  12)
• Results conservative b/c of low-powered CPUs and
  over-provisioned for cooling (i.e., large
  enclosure)