Linux Scheduler © DJ Foreman 3/8/20091

Objectives Response time Throughput for batch No starvation Accommodate high AND low priority © DJ Foreman 3/8/2009 2

General Process Classifications Two schemes for classifying: –Traditional classes IO bound CPU bound –Alternative classes Interactive Batch Real-time –Video, robotics, sensor-based How to determine? © DJ Foreman 3/8/2009 3

Linux Scheduling Classes FIFOFIFO real-time process RRRound Robin real-time process Normalnon-real-time process © DJ Foreman 3/8/2009 4

Real-time Details © DJ Foreman 3/8/20095

Real-time Rules Priority 0(highest) - 99(lowest) –Program adjustable: sched_setparam() sched_setscheduler() FIFO - non-interruptible except –Higher priority –Blocked –Yields RR –Adds a time-slice to each thread –Suspends at end of slice © DJ Foreman 3/8/2009 6

Non-real-time Scheduling (Conventional processes) © DJ Foreman 3/8/20097

Rules Every process has a “static” priority –100(highest) – 139 (lowest) –Variable via nice() and setpriority() Base quantum (not a constant!) –If sp<120 20*(140-static priority) –If sp>=120 5*(140-static priority) Dynamic priority (also ) –Max(100, min(sp-bonus+5,139)) –0<=bonus<=10 © DJ Foreman 3/8/2009 8

Rules (continued) Bonus –0-5 is a penalty –6-10 is a prize –Related to average sleep time OF THIS PROCESS (not of all Processes) Based on past history Some sample values: © DJ Foreman 3/8/ Average sleep timeBonus 0<=N<100ms0 300<=N<400ms3 600<=N< <=N<10009

Priority Arrays Two arrays: struct prio_array { intnr_active; unsigned long bitmap[BITMAP_SIZE]; structlist_headqueue[MAX_PRIO]; } With: 140 priority levels (max) wordsize=32 bits BITMAP_SIZE is 5 (last 20 bits ignored) © DJ Foreman 3/8/

The Bitmaps Active queues Time-slice-expired queues 1 bit/queue (140 queues) Based on the Intel “bsfl” instruction –Select least significant 1-bit © DJ Foreman 3/8/

Priority arrays – revisited Two arrays (active/expired): *listheads prio_array[2][140]; typedef struct listheads { // 2-way list of this priority PD*next; PD*prev;} struct PD { PD *next; PD*prev; int PID; int prio; // etc } © DJ Foreman 3/8/

Scheduling Pick highest priority non-empty queue/list Tasks in queue/list scheduled © DJ Foreman 3/8/

References Lindsey, R., "What's New in the 2.6 Scheduler", Linux Journal, March 2004 Love, R., Linux Kernel Development, Indianapolis, IN, Sams Publishing, 2004 Understanding the Linux Kernel, Bovet & Cesati, O’Reilly Press, 2006 © DJ Foreman 3/8/