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Power Efficient Idle
Injection
Jacob Pan
Intel Open Source Technology Center
LinuxCon Japan 2015
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Agenda
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Introduction to idle injection
Techniques available in Linux
Experiment results
Future work
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Why Injecting Idle?
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Primary: Thermal/Power limiting
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Secondary:
• Performance management
• Pay per use
• Idle power efficiency
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Understanding Processor Idle States/C-States
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Motivation For Idle Injection: Increasingly lower
Idle power
Deep idle power is negligible!
Idle Power vs Running Power On Broadwell
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14
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Power (watt)
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10
8
6
4
1.9
2
0.32
*TDP=Thermal Design Power
0
95% pc7
95% pc2
TDP C0
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When to use idle injection?
Idle injection at LFM
(low frequency mode)
Idle injection at LFM
(low frequency mode)
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Idle Injection in Linux
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Intel PowerClamp driver
Scheduler throttling, RT or CFS bandwidth control
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Intel Power Clamp V1
(current design in mainline kernel)
The idea: play idle!
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PowerClamp v1 timeline of idle injection
sched tick
throttled
unthrottled
RT kthread
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Limitations of Intel PowerClamp V1
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CPU appears busy while playing idle
Scheduler ticks not stopped in NOHZ idle
• Removal of tick_nohz_idle_enter/exit() API
• RCU grace period
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Relies on timely jiffies updates
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Limitations of Intel PowerClamp V1
CPU appears busy while playing idle
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Limitations of Intel PowerClamp V1
Scheduler ticks not stopped in NOHZ idle
• Interrupted sleep is less efficient in power
• Removal of tick_nohz_idle_enter/exit() API
• RCU grace period
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Limitations of Intel PowerClamp V1
Relies on secondary timing source
• timely jiffy updates
• periodic timers
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Scheduler Based Throttling
Normal tasks under completely fair scheduling (CFS) class
› Bandwidth control via CPU control group/container
› Runqueue throttling by enqueue/dequeue tasks
Root CG
CG1
CG1.1
CG2
CG1.2
CG2.1
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Time chart of CFS Bandwidth Control
(two cgroups multithreaded workload)
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Pros: No fake idle task, Finer per cgroup controls
Cons: No synchronization loss of package C-state opportunities
unthrottle
cgroup1
throttle
throttle
unthrottle
cgroup2
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Power Clamp V2(work in progress)
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Runqueue throttling of CFS class
Synchronization around rounded Ktime instead of jiffies
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Time Chart Powerclamp v1 vs. v2
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Experiment Data
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Goals:
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Comparing Power Efficiency
Scalability
CPU HW design trend: old vs. new
Configurations:
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CPUs: Ivy Bridge/Haswell/Broadwell clients, Haswell EX server
Workload:fspin by Len Brown. CPU bound, floating
Test case: Inject idle from 0 to 50% at 5% increment
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Power and Performance Control V1 vs. V2
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Power Efficiency Comparison On A Client Platform
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Scalability Tests V1 vs. V2
(144 core 4 socket Haswell EX)
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Power Efficiency Comparison On A Server Platform
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Comparing Deep vs. Shallow Package C-States
(powerclamp v2)
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Conclusions
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Idle injection can effectively reduce power beyond energy efficient frequency
With deeper package C-states, can achieve near linear performance and power
reduction
Scheduler runqueue throttling results in cleaner and more efficient solution
Align activities results in significant power savings
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Future plan
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Better handling of interrupts
Integration with scheduler
Synchronize with devices with latency tolerance
Work with hardware duty cycling
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Backups
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Time Chart of Redesigned Power Clamp
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Entering Idle Injection Period
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Exiting Idle Injection
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