Difference between revisions of "OS: Complete Teori Tuning Kernel Scheduler"

Revision as of 09:28, 21 March 2013

Sumber: http://doc.opensuse.org/documentation/html/openSUSE/opensuse-tuning/cha.tuning.taskscheduler.html

Sistem operasi modern biasanya menjalankan tugas-tugas yang berbeda pada waktu yang bersamaan. Misalnya, Anda dapat mencari dalam sebuah file teks sambil menerima e-mail dan menyalin file besar ke hard drive eksternal. Tugas-tugas mudah tersebut memerlukan proses tambahan yang harus dijalankan oleh sistem. Untuk memberikan setiap tugas dengan sumber daya yang diperlukan , kernel Linux membutuhkan alat untuk mendistribusikan sumber daya sistem yang tersedia untuk tugas individu. Dan ini adalah tugas scheduler .

Bagian berikut menjelaskan istilah yang paling penting yang terkait dengan scheduling proses. Disini juga akan diperkenalkan informasi tentang kebijakan task scheduler , algoritma penjadwalan, deskripsi tugas scheduler yang digunakan oleh Linux, dan referensi ke sumber informasi lain yang relevan.

Pendahuluan

Kernel Linux mengontrol cara task (atau proses) dikelola dalam sistem yang berjalan. Task scheduler, kadang-kadang disebut proses scheduler, adalah bagian dari kernel yang memutuskan task mana yang akan di jalankan berikutnya. Task scheduler adalah salah satu komponen inti dari sistem operasi multitasking (seperti Linux), yang bertanggung jawab untuk memanfaatkan sumber daya sistem sebaik-baiknya untuk menjamin bahwa beberapa task dapat berjalan secara bersamaan.

Preemption

Teori di balik task scheduling sangat sederhana. Jika ada proses runnable dalam suatu sistem, setidaknya satu proses harus senantiasa berjalan. Jika ada lebih banyak runnable proses daripada prosesor dalam sistem, tidak semua proses dapat berjalan sepanjang waktu.

Oleh karena itu, beberapa proses perlu dihentikan sementara, atau ditangguhkan, sehingga proses lain dapat berjalan kembali. Task scheduler memutuskan proses mana dalam antrian akan berjalan selanjutnya.

Seperti telah disebutkan, Linux, seperti semua varian Unix lainnya, adalah sistem operasi multitasking. Ini berarti bahwa beberapa task dapat berjalan pada waktu yang sama. Linux menyediakan apa yang disebut preemptive multitasking, di mana Task scheduler memutuskan kapan proses dihentikan. Suspensi secara paksa ini disebut preemption. Semua turunan Unix telah menyediakan preemptive multitasking sejak awal.

Timeslice

Periode waktu yang digunakan suatu proses yang akan berjalan terlebih dulu didefinisikan di awal. Hal ini disebut proses timeslice dan mewakili jumlah waktu prosesor yang diberikan kepada setiap proses. Dengan menetapkan timeslices, scheduler membuat keputusan global untuk sistem berjalan, dan mencegah proses individu untuk mendominasi sumber daya prosesor.

Prioritas Process

Task scheduler mengevaluasi proses berdasarkan prioritas mereka. Untuk menghitung prioritas sebuah proses, Task Scheduler menggunakan algoritma yang komplek. Akibatnya, setiap proses diberi nilai yang berdasarkan nilai tersebiut proses itu "diperbolehkan" untuk berjalan di prosesor.

Klasifikasi Proses

Proses biasanya diklasifikasikan menurut tujuan dan perilaku mereka. Meskipun batas tersebut tidak selalu jelas, umumnya ada dua kriteria yang digunakan untuk menyortir mereka. Kriteria yang independen dan tidak mengecualikan satu sama lain.

Salah satu pendekatan adalah dengan mengklasifikasi sebuah proses apakah sebagai I/O-bound atau processor-bound.

I/O-bound

I/O adalah singkatan dari perangkat Input / Output , seperti keyboard, mouse, atau cakram optik dan harddisk. I/O-bound proses menghabiskan sebagian besar waktu untuk mengajukan dan menunggu permintaan. Mereka berjalan sangat sering, tapi untuk interval waktu yang singkat, tidak untuk memblokir proses-proses lain yang menunggu permintaan I/O.

processor-bound

Di sisi lain, prosesor-bound task menggunakan waktu mereka untuk mengeksekusi kode, dan biasanya dijalankan sampai mereka ditunda eksekusinya oleh Task scheduler. Mereka tidak menghalangi proses yang menunggu permintaan I / O, dan, karenanya, dapat dijalankan lebih jarang tetapi untuk interval waktu yang lebih lama.

Pendekatan lain adalah untuk membagi proses sebagai proses interaktif, batch atau real-time.

Proses Interaktif

Proses interaktif menghabiskan banyak waktu yang menunggu permintaan I / O, seperti operasi keyboard atau mouse. Task scheduler harus membangunkan proses tersebut dengan cepat atas permintaan pengguna, jika tidak pengguna akan menemukan lingkungan tidak tanggap. Penundaan biasanya sekitar 100 ms. Aplikasi office, editor teks atau program manipulasi gambar merupakan contoh aplikasi yang membutuhkan proses interaktif.

Proses Batch

Proses Batch sering berjalan di latar belakang dan tidak perlu responsif. Mereka biasanya mendapat prioritas lebih rendah dari Task scheduler. Konverter multimedia, mesin database pencarian, atau file log analisis adalah contoh dari proses batch.

Proses Real-Time

Proses Real-time tidak harus diblokir oleh proses prioritas rendah, dan Task scheduler menjamin waktu respon singkat kepada mereka. Aplikasi untuk mengedit konten multimedia adalah salah satu contoh-nya.

O(1) Scheduler

Versi kernel Linux 2.6 memperkenalkan Task scheduler baru, yang disebut O (1) scheduler (lihat Big notasi O), itu digunakan sebagai default Task scheduler sampai versi Kernel 2.6.22. Tugas utamanya adalah untuk menjadwalkan task dalam jumlah waktu yang tetap, tidak peduli berapa banyak proses terdapat dalam sistem.

Scheduler menghitung timeslices secara dinamis. Namun, untuk menentukan proses timeslice yang tepat adalah tugas yang kompleks: timeslices Terlalu lama menyebabkan sistem menjadi kurang interaktif dan responsif, sementara yang terlalu pendek membuat prosesor membuang banyak waktu di overhead switching proses yang terlalu sering. proses timeslice standar biasanya agak rendah, misalnya 20ms . scheduler menentukan proses timeslice berdasarkan prioritas dari proses, yang memungkinkan proses dengan prioritas yang lebih tinggi untuk menjalankan lebih sering dan untuk waktu yang lama.

Sebuah proses tidak harus memanfaatkan semua proses timeslice sekaligus. Misalnya, dengan proses timeslice dari 150ms tidak harus berjalan selama 150ms dalam satu kali. Hal ini dapat berjalan di lima slot jadwal yang berbeda untuk 30ms . task interaktif biasanya memperoleh manfaat dari pendekatan ini karena mereka tidak perlu seperti proses timeslice besar sekaligus saat mereka harus responsif selama mungkin.

Scheduler juga menetapkan prioritas proses secara dinamis. Scheduler memonitor perilaku proses dan, jika diperlukan, menyesuaikan prioritas. Sebagai contoh, proses yang diskors untuk waktu yang lama dinyalakan dengan meningkatkan prioritas.

Completely Fair Scheduler (CFQ)

Since the Linux kernel version 2.6.23, a new approach has been taken to the scheduling of runnable processes. Completely Fair Scheduler (CFS) became the default Linux kernel scheduler. Since then, important changes and improvements have been made. The information in this chapter applies to openSUSE with kernel version 2.6.32 and higher (including 3.x kernels). The scheduler environment was divided into several parts, and three main new features were introduced:

Modular Scheduler Core

   The core of the scheduler was enhanced with scheduling classes. These classes are modular and represent scheduling policies.

Completely Fair Scheduler

   Introduced in kernel 2.6.23 and extended in 2.6.24, CFS tries to assure that each process obtains its “fair” share of the processor time.

Group Scheduling

   For example, if you split processes into groups according to which user is running them, CFS tries to provide each of these groups with the same amount of processor time.

As a result, CFS brings more optimized scheduling for both servers and desktops.

How CFS Works

CFS tries to guarantee a fair approach to each runnable task. To find the most balanced way of task scheduling, it uses the concept of red-black tree. A red-black tree is a type of self-balancing data search tree which provides inserting and removing entries in a reasonable way so that it remains well balanced. For more information, see the wiki pages of Red-black tree.

When a task enters into the run queue (a planned time line of processes to be executed next), the scheduler records the current time. While the process waits for processor time, its “wait” value gets incremented by an amount derived from the total number of tasks currently in the run queue and the process priority. As soon as the processor runs the task, its “wait” value gets decremented. If the value drops below a certain level, the task is preempted by the scheduler and other tasks get closer to the processor. By this algorithm, CFS tries to reach the ideal state where the “wait” value is always zero. 14.4.2. Grouping Processes¶

Since the Linux kernel version 2.6.24, CFS can be tuned to be fair to users or groups rather than to tasks only. Runnable tasks are then grouped to form entities, and CFS tries to be fair to these entities instead of individual runnable tasks. The scheduler also tries to be fair to individual tasks within these entities.

Tasks can be grouped in two mutually exclusive ways:

By user IDs
By kernel control groups.

The way the kernel scheduler lets you group the runnable tasks depends on setting the kernel compile-time options CONFIG_FAIR_USER_SCHED and CONFIG_FAIR_CGROUP_SCHED. The default setting in openSUSE® 12.3 is to use control groups, which lets you create groups as needed. For more information, see Chapter 10, Kernel Control Groups. 14.4.3. Kernel Configuration Options¶

Basic aspects of the task scheduler behavior can be set through the kernel configuration options. Setting these options is part of the kernel compilation process. Because kernel compilation process is a complex task and out of this document's scope, refer to relevant source of information (for example http://en.opensuse.org/Configure,_Build_and_Install_a_Custom_Linux_Kernel). [Warning] Kernel Compilation

If you run openSUSE on a kernel that was not shipped with it, for example on a self-compiled kernel, you loose the entire support entitlement.

Terminology

Documents regarding task scheduling policy often use several technical terms which you need to know to understand the information correctly. Here are some of them:

Latency

   Delay between the time a process is scheduled to run and the actual process execution.

Granularity

   The relation between granularity and latency can be expressed by the following equation:

   gran = ( lat / rtasks ) - ( lat / rtasks / rtasks )

   where gran stands for granularity, lat stand for latency, and rtasks is the number of running tasks.

14.4.4.1. Scheduling Policies¶

The Linux kernel supports the following scheduling policies:

SCHED_FIFO

   Scheduling policy designed for special time-critical applications. It uses the First In-First Out scheduling algorithm.

SCHED_BATCH

   Scheduling policy designed for CPU-intensive tasks.

SCHED_IDLE

   Scheduling policy intended for very low prioritized tasks.

SCHED_OTHER

   Default Linux time-sharing scheduling policy used by the majority of processes.

SCHED_RR

   Similar to SCHED_FIFO, but uses the Round Robin scheduling algorithm.

14.4.5. Changing Real-time Attributes of Processes with chrt¶

The chrt command sets or retrieves the real-time scheduling attributes of a running process, or runs a command with the specified attributes. You can get or retrieve both the scheduling policy and priority of a process.

In the following examples, a process whose PID is 16244 is used.

To retrieve the real-time attributes of an existing task:

saturn.example.com:~ # chrt -p 16244
pid 16244's current scheduling policy: SCHED_OTHER
pid 16244's current scheduling priority: 0

Before setting a new scheduling policy on the process, you need to find out the minimum and maximum valid priorities for each scheduling algorithm:

saturn.example.com:~ # chrt -m
SCHED_OTHER min/max priority : 0/0
SCHED_FIFO min/max priority : 1/99
SCHED_RR min/max priority : 1/99
SCHED_BATCH min/max priority : 0/0
SCHED_IDLE min/max priority : 0/0

In the above example, SCHED_OTHER, SCHED_BATCH, SCHED_IDLE polices only allow for priority 0, while that of SCHED_FIFO and SCHED_RR can range from 1 to 99.

To set SCHED_BATCH scheduling policy:

saturn.example.com:~ # chrt -b -p 0 16244
saturn.example.com:~ # chrt -p 16244
pid 16244's current scheduling policy: SCHED_BATCH
pid 16244's current scheduling priority: 0

For more information on chrt, see its man page (man 1 chrt). 14.4.6. Runtime Tuning with sysctl¶

The sysctl interface for examining and changing kernel parameters at runtime introduces important variables by means of which you can change the default behavior of the task scheduler. The syntax of the sysctl is simple, and all the following commands must be entered on the command line as root.

To read a value from a kernel variable, enter

sysctl variable

To assign a value, enter

sysctl variable=value

To get a list of all scheduler related sysctl variables, enter

sysctl -A | grep "sched" | grep -v"domain"

saturn.example.com:~ # sysctl -A | grep "sched" | grep -v "domain"
kernel.sched_child_runs_first = 0
kernel.sched_min_granularity_ns = 1000000
kernel.sched_latency_ns = 5000000
kernel.sched_wakeup_granularity_ns = 1000000
kernel.sched_shares_ratelimit = 250000
kernel.sched_tunable_scaling = 1
kernel.sched_shares_thresh = 4
kernel.sched_features = 15834238
kernel.sched_migration_cost = 500000
kernel.sched_nr_migrate = 32
kernel.sched_time_avg = 1000
kernel.sched_rt_period_us = 1000000
kernel.sched_rt_runtime_us = 950000
kernel.sched_compat_yield = 0

Note that variables ending with “_ns” and “_us” accept values in nanoseconds and microseconds, respectively.

A list of the most important task scheduler sysctl tuning variables (located at /proc/sys/kernel/) with a short description follows:

sched_child_runs_first

   A freshly forked child runs before the parent continues execution. Setting this parameter to 1 is beneficial for an application in which the child performs an execution after fork. For example make -j<NO_CPUS> performs better when sched_child_runs_first is turned off. The default value is 0.

sched_compat_yield

   Enables the aggressive yield behavior of the old 0(1) scheduler. Java applications that use synchronization extensively perform better with this value set to 1. Only use it when you see a drop in performance. The default value is 0.

   Expect applications that depend on the sched_yield() syscall behavior to perform better with the value set to 1.

sched_migration_cost

   Amount of time after the last execution that a task is considered to be “cache hot” in migration decisions. A “hot” task is less likely to be migrated, so increasing this variable reduces task migrations. The default value is 500000 (ns).

   If the CPU idle time is higher than expected when there are runnable processes, try reducing this value. If tasks bounce between CPUs or nodes too often, try increasing it.

sched_latency_ns

   Targeted preemption latency for CPU bound tasks. Increasing this variable increases a CPU bound task's timeslice. A task's timeslice is its weighted fair share of the scheduling period:

   timeslice = scheduling period * (task's weight/total weight of tasks in the run queue)

   The task's weight depends on the task's nice level and the scheduling policy. Minimum task weight for a SCHED_OTHER task is 15, corresponding to nice 19. The maximum task weight is 88761, corresponding to nice -20.

   Timeslices become smaller as the load increases. When the number of runnable tasks exceeds sched_latency_ns/sched_min_granularity_ns, the slice becomes number_of_running_tasks * sched_min_granularity_ns. Prior to that, the slice is equal to sched_latency_ns.

   This value also specifies the maximum amount of time during which a sleeping task is considered to be running for entitlement calculations. Increasing this variable increases the amount of time a waking task may consume before being preempted, thus increasing scheduler latency for CPU bound tasks. The default value is 20000000 (ns).

sched_min_granularity_ns

   Minimal preemption granularity for CPU bound tasks. See sched_latency_ns for details. The default value is 4000000 (ns).

sched_wakeup_granularity_ns

   The wake-up preemption granularity. Increasing this variable reduces wake-up preemption, reducing disturbance of compute bound tasks. Lowering it improves wake-up latency and throughput for latency critical tasks, particularly when a short duty cycle load component must compete with CPU bound components. The default value is 5000000 (ns).
   [Warning]

   Settings larger than half of sched_latency_ns will result in zero wake-up preemption and short duty cycle tasks will be unable to compete with CPU hogs effectively.

sched_rt_period_us

   Period over which real-time task bandwidth enforcement is measured. The default value is 1000000 (µs).

sched_rt_runtime_us

   Quantum allocated to real-time tasks during sched_rt_period_us. Setting to -1 disables RT bandwidth enforcement. By default, RT tasks may consume 95%CPU/sec, thus leaving 5%CPU/sec or 0.05s to be used by SCHED_OTHER tasks.

sched_features

   Provides information about specific debugging features.

sched_stat_granularity_ns

   Specifies the granularity for collecting task scheduler statistics.

sched_nr_migrate

   Controls how many tasks can be moved across processors through migration software interrupts (softirq). If a large number of tasks is created by SCHED_OTHER policy, they will all be run on the same processor. The default value is 32. Increasing this value gives a performance boost to large SCHED_OTHER threads at the expense of increased latencies for real-time tasks.

14.4.7. Debugging Interface and Scheduler Statistics¶

CFS comes with a new improved debugging interface, and provides runtime statistics information. Relevant files were added to the /proc file system, which can be examined simply with the cat or less command. A list of the related /proc files follows with their short description:

/proc/sched_debug

   Contains the current values of all tunable variables (see Section 14.4.6, “Runtime Tuning with sysctl”) that affect the task scheduler behavior, CFS statistics, and information about the run queue on all available processors.

   saturn.example.com:~ # less /proc/sched_debug
   Sched Debug Version: v0.09, 2.6.32.8-0.3-default #1
   now at 2413026096.408222 msecs
     .jiffies                                 : 4898148820
     .sysctl_sched_latency                    : 5.000000
     .sysctl_sched_min_granularity            : 1.000000
     .sysctl_sched_wakeup_granularity         : 1.000000
     .sysctl_sched_child_runs_first           : 0.000000
     .sysctl_sched_features                   : 15834238
     .sysctl_sched_tunable_scaling            : 1 (logaritmic)

   cpu#0, 1864.411 MHz
     .nr_running                    : 1
     .load                          : 1024
     .nr_switches                   : 37539000
     .nr_load_updates               : 22950725
   [...]
   cfs_rq[0]:/
     .exec_clock                    : 52940326.803842
     .MIN_vruntime                  : 0.000001
     .min_vruntime                  : 54410632.307072
     .max_vruntime                  : 0.000001
   [...]
   rt_rq[0]:/
     .rt_nr_running                 : 0
     .rt_throttled                  : 0
     .rt_time                       : 0.000000
     .rt_runtime                    : 950.000000

   runnable tasks:
     task  PID   tree-key    switches prio exec-runtime    sum-exec sum-sleep
   --------------------------------------------------------------------------
   R cat 16884 54410632.307072    0   120  54410632.307072 13.836804 0.000000

/proc/schedstat

   Displays statistics relevant to the current run queue. Also domain-specific statistics for SMP systems are displayed for all connected processors. Because the output format is not user-friendly, read the contents of /usr/src/linux/Documentation/scheduler/sched-stats.txt for more information.

/proc/PID/sched

   Displays scheduling information on the process with id PID.

   saturn.example.com:~ # cat /proc/`pidof nautilus`/sched
    nautilus (4009, #threads: 1)
    ---------------------------------------------------------
    se.exec_start                      :    2419575150.560531
    se.vruntime                        :      54549795.870151
    se.sum_exec_runtime                :       4867855.829415
    se.avg_overlap                     :             0.401317
    se.avg_wakeup                      :             3.247651
    se.avg_running                     :             0.323432
    se.wait_start                      :             0.000000
    se.sleep_start                     :    2419575150.560531
    [...]
    nr_voluntary_switches              :               938552
    nr_involuntary_switches            :                71872
    se.load.weight                     :                 1024
    policy                             :                    0
    prio                               :                  120
    clock-delta                        :                  109

14.5. For More Information¶

To get a compact knowledge about Linux kernel task scheduling, you need to explore several information sources. Here are some of them:

   For task scheduler System Calls description, see the relevant manual page (for example man 2 sched_setaffinity).

   General information on scheduling is described in Scheduling wiki page.

   General information on Linux task scheduling is described in Inside the Linux scheduler.

   Information specific to Completely Fair Scheduler is available in Multiprocessing with the Completely Fair Scheduler

   Information specific to tuning Completely Fair Scheduler is available in Tuning the Linux Kernel’s Completely Fair Scheduler

   A useful lecture on Linux scheduler policy and algorithm is available in http://www.inf.fu-berlin.de/lehre/SS01/OS/Lectures/Lecture08.pdf.

   A good overview of Linux process scheduling is given in Linux Kernel Development by Robert Love (ISBN-10: 0-672-32512-8). See http://www.informit.com/articles/article.aspx?p=101760.

   A very comprehensive overview of the Linux kernel internals is given in Understanding the Linux Kernel by Daniel P. Bovet and Marco Cesati (ISBN 978-0-596-00565-8).

   Technical information about task scheduler is covered in files under /usr/src/linux/Documentation/scheduler.