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authorIngo Molnar <mingo@elte.hu>2007-08-28 06:53:24 -0400
committerIngo Molnar <mingo@elte.hu>2007-08-28 06:53:24 -0400
commitf6cf891c4d7128f9f91243fc0b9ce99e10fa1586 (patch)
treeab26a8e708756c3fbafbb15ce48acea4f80ced08 /include/asm-generic
parent5f01d519e60a6ca1a7d9be9f2d73c5f521383992 (diff)
sched: make the scheduler converge to the ideal latency
de-HZ-ification of the granularity defaults unearthed a pre-existing property of CFS: while it correctly converges to the granularity goal, it does not prevent run-time fluctuations in the range of [-gran ... 0 ... +gran]. With the increase of the granularity due to the removal of HZ dependencies, this becomes visible in chew-max output (with 5 tasks running): out: 28 . 27. 32 | flu: 0 . 0 | ran: 9 . 13 | per: 37 . 40 out: 27 . 27. 32 | flu: 0 . 0 | ran: 17 . 13 | per: 44 . 40 out: 27 . 27. 32 | flu: 0 . 0 | ran: 9 . 13 | per: 36 . 40 out: 29 . 27. 32 | flu: 2 . 0 | ran: 17 . 13 | per: 46 . 40 out: 28 . 27. 32 | flu: 0 . 0 | ran: 9 . 13 | per: 37 . 40 out: 29 . 27. 32 | flu: 0 . 0 | ran: 18 . 13 | per: 47 . 40 out: 28 . 27. 32 | flu: 0 . 0 | ran: 9 . 13 | per: 37 . 40 average slice is the ideal 13 msecs and the period is picture-perfect 40 msecs. But the 'ran' field fluctuates around 13.33 msecs and there's no mechanism in CFS to keep that from happening: it's a perfectly valid solution that CFS finds. to fix this we add a granularity/preemption rule that knows about the "target latency", which makes tasks that run longer than the ideal latency run a bit less. The simplest approach is to simply decrease the preemption granularity when a task overruns its ideal latency. For this we have to track how much the task executed since its last preemption. ( this adds a new field to task_struct, but we can eliminate that overhead in 2.6.24 by putting all the scheduler timestamps into an anonymous union. ) with this change in place, chew-max output is fluctuation-less all around: out: 28 . 27. 39 | flu: 0 . 2 | ran: 13 . 13 | per: 41 . 40 out: 28 . 27. 39 | flu: 0 . 2 | ran: 13 . 13 | per: 41 . 40 out: 28 . 27. 39 | flu: 0 . 2 | ran: 13 . 13 | per: 41 . 40 out: 28 . 27. 39 | flu: 0 . 2 | ran: 13 . 13 | per: 41 . 40 out: 28 . 27. 39 | flu: 0 . 1 | ran: 13 . 13 | per: 41 . 40 out: 28 . 27. 39 | flu: 0 . 1 | ran: 13 . 13 | per: 41 . 40 this patch has no impact on any fastpath or on any globally observable scheduling property. (unless you have sharp enough eyes to see millisecond-level ruckles in glxgears smoothness :-) Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Signed-off-by: Mike Galbraith <efault@gmx.de>
Diffstat (limited to 'include/asm-generic')
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