Resource: LCE12 Name: big.LITTLE Mini-Summit (session two) Blue Sky Date: 01-11-2012 Speaker: Vincent Guittot

1. Vincent Guittot
Linaro Power Management Working Group
Linaro Connect Q4'12
big.LITTLE mini-summit
Session 2 : Bluesky
What would the ideal power-aware kernel do?

2. Vincent Guittot
Linaro Power Management Working Group
3 Topics
● Correlate scheduler with C-state & P-state
● Scale invariance of a load
● How to differentiate user-space tasks

3. Vincent Guittot
Linaro Power Management Working Group
Correlate with C-state and P-state
● C-state can make the difference
● The migration on a powered down cluster is costly
900µs
off
migrate and
wake up task
on CPU0
schedule
task on
CPU0
wake up

4. Vincent Guittot
Linaro Power Management Working Group
Correlation with C-state and P-state
● C-State can make the difference
● The migration on a shallow C-state is efficient
off
76µs
migrate &
wake up task
on CPU0
schedule
task on
CPU0
wfi

5. Vincent Guittot
Linaro Power Management Working Group
Correlate with C-state and P-state
● Scheduler can break cpufreq statistic
● Migration moves the load
CPU0
CPU1
OnDemand CPU Load statistic

6. Vincent Guittot
Linaro Power Management Working Group
Scaling invariance
● Sysbench tests on TC2
● Linearity with P-State
● CPU test results are linear
● Memory test results are not linear (8% better)
● Linearity between micro-architecture
● CPU test results : A15 ~ 2 x A7
● Memory test results : A15 <= A7

7. Vincent Guittot
Linaro Power Management Working Group
Scaling invariance
Freq = x
Freq = 2x
Scale invariant load trackingWall clock based load tracking
Freq = 2x
Freq = x
Work achieved
Load
[1]

8. Vincent Guittot
Linaro Power Management Working Group
Differentiate user-space tasks
● Cgroup
● Cpuset
● Nice priority
● Foreground / Background
● Others …
● Qualify a task with resource reservation
● … like deadline scheduler

9. Vincent Guittot
Linaro Power Management Working Group
Reference
● [1] : big.LITTLE MP Scheduler/OSPM update on TC2

10. Vincent Guittot
Linaro Power Management Working Group
Thank you