1. Linux Scheduler
© DJ Foreman 3/8/2009

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

3. 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

4. Linux Scheduling Classes
FIFO FIFO real-time process
RR Round Robin real-time process
Normal non-real-time process
© DJ Foreman 3/8/2009

5. Real-time Details
© DJ Foreman 3/8/2009

6. 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

7. Non-real-time Scheduling (Conventional processes)
© DJ Foreman 3/8/2009

8. 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

9. 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:
Average sleep time
Bonus
0<=N<100ms
300<=N<400ms 3
600<=N<700 6
900<=N<1000 9
© DJ Foreman 3/8/2009

10. Priority Arrays Two arrays: struct prio_array { int nr_active;
unsigned long bitmap[BITMAP_SIZE];
struct list_head queue[MAX_PRIO]; }
With: 140 priority levels (max)
wordsize=32 bits
BITMAP_SIZE is 5 (last 20 bits ignored)
© DJ Foreman 3/8/2009

11. The Bitmaps Active queues Time-slice-expired queues
1 bit/queue (140 queues)
Based on the Intel “bsf/bsr” instruction
Select least significant 1-bit
© DJ Foreman 3/8/2009

12. 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 3/8/2009

13. Scheduling Pick highest priority non-empty queue/list
Tasks in queue/list scheduled
© DJ Foreman 3/8/2009

14. 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 3/8/2009