Messages in this thread Patch in this message | | | From | maeda.naoaki@jp ... | Date | Fri, 21 Apr 2006 11:27:53 +0900 | Subject | [RFC][PATCH 5/9] CPU controller - Documents how the controller works |
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5/9: cpurc_docs
Documentation that describes how the CPU resource controller works.
Signed-off-by: Kurosawa Takahiro <kurosawa@valinux.co.jp> Signed-off-by: MAEDA Naoaki <maeda.naoaki@jp.fujitsu.com>
Documentation/ckrm/cpurc-internals | 166 +++++++++++++++++++++++++++++++++++++ 1 files changed, 166 insertions(+)
Index: linux-2.6.17-rc2/Documentation/ckrm/cpurc-internals =================================================================== --- /dev/null +++ linux-2.6.17-rc2/Documentation/ckrm/cpurc-internals @@ -0,0 +1,166 @@ +CPU resource controller internals + + There are 3 components in the CPU resource controller: + + (1) load estimation + (2) hungry detection + (3) timeslice scaling + + We need to estimate the class load in order to check whether the + share is satisfied or not. Class load also gets lower than the + share when all the tasks in the class tends to sleep. We need to + check whether the class needs to schedule more or not by hungry + detection. If a class needs to schedule more, timeslices of tasks + are scaled by timeslice scaling. + +1. Load estimation + + We calculate the class load as the accumulation of task loads in the + class. We need to calculate the task load first, then calculate the + class load from the task loads. + + Task load estimation + + Task load is estimated as the ratio of: + * the timeslice value allocated to the task (Ts) + to: + * the time that is taken for the task to run out the allocated timeslice + (Tr). + If a task can use all the CPU time, Ts / Tr becomes 1 for example. + + The detailed procedure of the calculation is as follows: + (1) Record the timeslice (Ts) and the time when the timeslice is + allocated to the task (by calling cpu_rc_record_allocation()). + * The timeslice value is recorded to task->last_slice ( = Ts). + * The time is recorded to task->ts_alloced. + (2) Calculate the task load when the timeslice is expired + (by calling cpu_rc_account()). + Tr is calculated as: + Tr = jiffies - task->ts_alloced + Then task load (Ts / Tr) becomes: + Ts / Tr = task->last_slice / (jiffies - task->ts_alloced) + + The load value is scaled by CPU_RC_LOAD_SCALE. + If the load value equals to CPU_RC_LOAD_SCALE, it indicates 100% + CPU usage. + + task->ts_alloced task scheduled now + v v v + |---------------===========================| + + |<------------------------>| + Ts ( = task->last_slice) + + |<---------------------------------------->| + Tr ( = now - task->ts_alloced) + + |<------------->| + the time that the task isn't scheduled + + + Note that task load calculation is also needed for strict + accuracy when a task forks or exits, because timeslice is + changed on fork and exit. But we don't do that in order to + simplify the code and in order not to introduce overhead on fork + and exit. Probably we can get enough accurate number without + calculating the task load on fork/exit. + + Class load estimation: + + Class load is the accumulation of load values of tasks in the class in + the duration of CPU_RC_SPREAD_PERIOD. + Per-CPU class load is recalculated each time the task load is calculated + in the cpu_rc_account() function. + Then on CPU_RC_RECALC_INTERVAL intervals, the class load value per-CPU + value is calculated as the average of the per-CPU class load. + + Task load is accumulated to the per-CPU class load as if the class uses + Ts/Tr of the CPU time from task->ts_alloced to now (the time the timeslice + expired). + + So the time that the task has used the CPU from (now - CPU_RC_SPREAD_PERIOD) + to now (Ttsk) should be: + + if task->ts_alloced < now - CPU_RC_SPREAD_PERIOD: + Ts/Tr * CPU_RC_SPREAD_PERIOD + (We assume that the task has used the CPU at the constant rate of Ts/Tr.) + + now-CPU_RC_SPREAD_PERIOD now + v v + |---------------------------------------| + |==================================================| load: Ts/Tr + ^ + task->ts_alloced + + else: + Ts + + now-CPU_RC_SPREAD_PERIOD now + v v + |---------------------------------------| + |============================| load: Ts/Tr + ^ + task->ts_alloced + + Also, we assume that the class uses the CPU at the rate of the class load + from (now - CPU_RC_SPREAD_PERIOD) to the last time the per-CPU class load + was calculated (stored in struct cpu_rc::stat[cpu].timestamp). If + cpu_rc::stat[cpu].timestamp < now - CPU_RC_SPREAD_PERIOD, we assume that + the class doesn't use the CPU from (now - CPU_RC_SPREAD_PERIOD) to + task->ts_alloced. + + So the time that the class use the CPU from (now - CPU_RC_SPREAD_PERIOD) + to now (Tcls) should be: + if cpu_rc::stat[cpu].timestamp < now - CPU_RC_SPREAD_PERIOD: + 0 + else: + cpu_rc::stat[cpu].load * (cpu_rc::stat[cpu].timestamp - (now - CPU_RC_SPREAD_PERIOD)) + + The new per-CPU class load that will be assigned to cpu_rc::stat[cpu].load + is calculated as: + (Ttsk + Tcls) / CPU_RC_SPREAD_PERIOD + +2. Hungry detection + + When the class load is less than the share, there are 2 cases: + (a) the share is enough and tasks in the class have time for sleep + (b) tasks in other classes overuse the CPU + + We should not scale the timeslice in case (a) even if the class load + is lower than the share. In order to distinguish case (b) from + case (a), we measure the time (Tsch) from when a task is activated + (stored in task->last_activated) till when the task is actually + scheduled. If the class load is lower than the share but tasks + in the class are quickly scheduled, it can be classified to case (a). + If Tsch / timeslice of a task is lower than the share, the class + that has the task is marked as "maybe hungry." If the class load of + the class that is marked as "maybe hungry" is lower than the + share, it is treated as hungry and the timeslices of tasks in + other classes will be scaled down. + + +3. Timeslice scaling + + If there are hungry classes, we need to adjust timeslices to satisfy + the share. To scale timeslices, we introduce a scaling factor + used for scaling timeslices. The scaling factor is associated with + the class (stored in the cpu_rc structure) and adaptively adjusted + according to the class load and the share. + + If some classes are hungry, the scaling factor of the class that is + not hungry is calculated as follows (note: F is the scaling factor): + F_new = F * share / class_load + + And the scaling factor of the hungry class is calculated as: + F_new = F + CPU_RC_TSFACTOR_INC_LO (CPU_RC_TSFACTOR_INC_LO is defined as 2) + + When all the classes are not hungry, the scaling factor is calculated + as follows in order to recover the timeslices: + F_new = F + CPU_RC_TSFACTOR_INC_HI (CPU_RC_TSFACTOR_INC_HI is defined as 5) + + Note that the maximum value of F is limited to CPU_RC_TSFACTOR_MAX. + The timeslice assigned to each task is: + timeslice_scaled = timeslice_orig * F / CPU_RC_TSFACTOR_MAX + + where timeslice_orig is the value that is calculated by the conventional + O(1) scheduler. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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