60 lines
1.7 KiB
Text
60 lines
1.7 KiB
Text
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Real-Time group scheduling.
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The problem space:
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In order to schedule multiple groups of realtime tasks each group must
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be assigned a fixed portion of the CPU time available. Without a minimum
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guarantee a realtime group can obviously fall short. A fuzzy upper limit
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is of no use since it cannot be relied upon. Which leaves us with just
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the single fixed portion.
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CPU time is divided by means of specifying how much time can be spent
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running in a given period. Say a frame fixed realtime renderer must
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deliver 25 frames a second, which yields a period of 0.04s. Now say
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it will also have to play some music and respond to input, leaving it
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with around 80% for the graphics. We can then give this group a runtime
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of 0.8 * 0.04s = 0.032s.
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This way the graphics group will have a 0.04s period with a 0.032s runtime
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limit.
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Now if the audio thread needs to refill the DMA buffer every 0.005s, but
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needs only about 3% CPU time to do so, it can do with a 0.03 * 0.005s
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= 0.00015s.
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The Interface:
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system wide:
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/proc/sys/kernel/sched_rt_period_ms
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/proc/sys/kernel/sched_rt_runtime_us
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CONFIG_FAIR_USER_SCHED
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/sys/kernel/uids/<uid>/cpu_rt_runtime_us
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or
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CONFIG_FAIR_CGROUP_SCHED
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/cgroup/<cgroup>/cpu.rt_runtime_us
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[ time is specified in us because the interface is s32; this gives an
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operating range of ~35m to 1us ]
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The period takes values in [ 1, INT_MAX ], runtime in [ -1, INT_MAX - 1 ].
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A runtime of -1 specifies runtime == period, ie. no limit.
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New groups get the period from /proc/sys/kernel/sched_rt_period_us and
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a runtime of 0.
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Settings are constrained to:
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\Sum_{i} runtime_{i} / global_period <= global_runtime / global_period
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in order to keep the configuration schedulable.
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