scheduling in android
TRANSCRIPT
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About
● Author of:
● Introduced Linux Trace Toolkit in 1999● Originated Adeos and relayfs (kernel/relay.c)● Training, Custom Dev, Consulting, ...
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Agenda
1. Architecture
2. Linux scheduler history
3. Completely Fair Scheduler (CFS)
4. Sched Classes
5. CPU power management
6. Load tracking
7. Android problems w/ Linux scheduler
8. User-space vs. Linux scheduler
9. Framework
10. Summing up
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1. Architecture
● Hardware on which Android is based● Android stack● Startup● System services● Binder driver
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2. Linux scheduler history
● 1995 - Linux 1.2● Circular queue of tasks w/ round-robin
● 1999 - Linux 2.2● Scheduling classes: real-time, non-preemptable and non-real-time● SMP support
● 2001 - Linux 2.4 O(N) scheduler● Each task gets a slice (epoch)
● 2003 - Linux 2.6 O(1) scheduler● Two queues: active vs. Expired● Heuristics-based / no “formal” algorithm
● 2007 - ~ Linux 2.6.21● Con Kolivas' Rotating Staircase Deadline Scheduler (RSDL)
● 2007 - Linux 2.6.23● Ingo Molnar's Completely Fair Scheduler (CFS)
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3. Completely Fair Scheduler (CFS)
● Tasks put in red-black tree● Self-balancing – i.e. no path more than twice other path● O (log n) operations
● task_struct->sched_entity->rb_node● kernel/core/sched.c:schedule()● put_prev_task()● pick_next_task()● Priorities provide decay factors
● Higher priority = lower decay factor● i.e. lower priority uses up their time more quickly
● Group scheduling – control group mechanism (cgroups)● Grouped processes share a single process' virtual time● Use of cgroupfs – /acct in Android
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4. Sched Classes
● See kernel/sched/sched.h● struct sched_class
● Classes● CFS - SCHED_OTHER (i.e. SCHED_NORMAL in the sources)● RT - SCHED_FIFO / SCHED_RR● Deadline - SCHED_DEADLINE(since 3.14)● Idle - Not!!! SCHED_IDLE● Stop – CPU stopping
● sched_setscheduler()
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5. CPU power management
● Need to integrate CPU power management and scheduler more closely● Existing Linux support for power management (governors)
● cpuidle– What happens when the CPU is IDLe
● cpufreq drivers– HW counters- / FW-based decision (intel_pstate / longrun)– Hard-coded values based on ranges within driver DVFS – “Dynamic Voltage and Frequency Scaling”
● Trivial– Performance - highest frequency– Powersave - lowest frequency– Userspace - User-space app makes decision
● Non-trivial– Based on system load
● Stats for non-trivial governors come from scheduler stats (/proc/stat)● On-demand - Immediate jump on load increase● Conservative - Gradual scale-up on increase
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5.1. Making frequency scaling choices
● SCHED_DEADLINE● We have precise info, because of how this works
● SCHED_FIFO / SCHED_RR● Put CPU at max right away
● SCHED_NORMAL● Use stats
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6. Load tracking
● How to track how much “load” a process is putting on the system?
● Not just CPU time consumed● Process waiting for CPU is contributing to load● Since 3.8
● PELT - Per-Entity Load Tracking– Each load instance is .y times the previous one:
● L0 + L1*y + L2*y2 + L3*y3 + ...
● New proposal (used in Pixel)● WALT - Window-Assisted Load Tracking
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7. Android problems w/ Linux scheduler
● Even “on-demand” isn't good enough● It takes too many milliseconds for timer to tick
and stats to be updated● Since decision is based on stats, delay is user-
noticeable
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7.1. Android Initial Solution
● Android devs wrote their own governor:● “interactive”
● Detects transition out of idle● Shortens timeout to scale up to much shorter
time (“1-2 ticks”)● Out of tree
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7.2. Intermediate solutions discussed
● Trigger cpufreq when scheduler updates stats● Instead of trigger using a timeout
● Introduce new governor: schedutil (merged in 4.7)● Use the info straight from the scheduler stats
update
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7.3. Recent Work
● Problems:● bigLITTLE● EAS – Energy Aware Schedulign
● SchedTune Used Pixel phone● Provide user-space knobs to control schedutil governor● Android doesn't need to make suppositions, it knows what's happening● Implemented as control-group controller● Each cgroup has tunable “schedtune.boost”
● sched-freq● Use CFS runqueue info to scale CPU freq
● WALT – Window-Assisted Load Tracking
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8. User-space vs. Linux scheduler
● Control knobs● /dev/cpuset/ - which tasks run on which CPUs● /dev/cpuctl/ - restrict CPU time for bg tasks● /dev/stune/ - EAS stune boosting
● init.rc● Power HAL:
● Power “hints”● Set interactive
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9. Framework● System Services:
● Activity Manager– Causes apps to be started through Zygote– Feed lifecycle events– Manages foreground vs. background, etc.
● Scheduling Policy:– Modifies process scheduler based on request
● Defined thread groups:● Default● BG non-interactive● Foreground● System● Audio App -- SCHED_FIFO● Audio Sys -- SCHED_FIFO● Top App
● See● frameworks/base/core/java/android/os/Process.java● frameworks/base/core/jni/android_util_Process.cpp
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10. Summing Up
● Quite a few moving parts● Linux does bulk of work● Android gives hints to Linux● Still ongoing work to get best performance with
least battery usage
