5/9: cpurc_docs
Documentation that describes how the CPU resource controller works.
Signed-off-by: Kurosawa Takahiro <[email protected]>
Signed-off-by: MAEDA Naoaki <[email protected]>
Documentation/res_groups/cpurc-internals | 167 +++++++++++++++++++++++++++++++
1 files changed, 167 insertions(+)
Index: linux-2.6.17-rc3/Documentation/res_groups/cpurc-internals
===================================================================
--- /dev/null
+++ linux-2.6.17-rc3/Documentation/res_groups/cpurc-internals
@@ -0,0 +1,167 @@
+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 resource group load in order to check whether
+ the share is satisfied or not. Resource group load also gets lower than
+ the share when all the tasks in the resoruce group tends to sleep. We need to
+ check whether the resource group needs to schedule more or not by hungry
+ detection. If a resource group needs to schedule more, timeslices of tasks
+ are scaled by timeslice scaling.
+
+1. Load estimation
+
+ We calculate the resource group load as the accumulation of task loads in the
+ resource group. We need to calculate the task load first, then calculate the
+ resource group 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.
+
+ Resource group load estimation:
+
+ Resource group load is the accumulation of load values of tasks in
+ the resource group in the duration of CPU_RC_SPREAD_PERIOD.
+ Per-CPU resource group load is recalculated each time the task load is
+ calculated in the cpu_rc_account() function.
+ Then on CPU_RC_RECALC_INTERVAL intervals, the resource group load value
+ per-CPU value is calculated as the average of the per-CPU resource group load.
+
+ Task load is accumulated to the per-CPU resource group load as if the resource
+ group 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 resource group uses the CPU at the rate of
+ the resource group load from (now - CPU_RC_SPREAD_PERIOD) to the last time
+ the per-CPU resource group 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 resource group doesn't use the CPU from (now - CPU_RC_SPREAD_PERIOD) to
+ task->ts_alloced.
+
+ So the time that the resource group use the CPU from
+ (now - CPU_RC_SPREAD_PERIOD) to now (Trgrp) 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 resource group load that will be assigned to
+ cpu_rc::stat[cpu].load is calculated as:
+ (Ttsk + Trgrp) / CPU_RC_SPREAD_PERIOD
+
+2. Hungry detection
+
+ When the resource group load is less than the share, there are 2 cases:
+ (a) the share is enough and tasks in the resource group have time for sleep
+ (b) tasks in other resource groups overuse the CPU
+
+ We should not scale the timeslice in case (a) even if the resource group 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 resource group load is lower than the share but tasks
+ in the resource group are quickly scheduled, it can be classified to case (a).
+ If Tsch / timeslice of a task is lower than the share, the resource group
+ that has the task is marked as "maybe hungry." If the resource group load of
+ the resource group that is marked as "maybe hungry" is lower than the
+ share, it is treated as hungry and the timeslices of tasks in
+ other resource groups will be scaled down.
+
+
+3. Timeslice scaling
+
+ If there are hungry resource groups, 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 resource group (stored in the cpu_rc structure) and adaptively adjusted
+ according to the resource group load and the share.
+
+ If some resource groups are hungry, the scaling factor of the resource group
+ that is not hungry is calculated as follows (note: F is the scaling factor):
+ F_new = F * share / resource_group_load
+
+ And the scaling factor of the hungry resource group is calculated as:
+ F_new = F + CPU_RC_TSFACTOR_INC_LO (CPU_RC_TSFACTOR_INC_LO is defined as 2)
+
+ When all the resource groups 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 [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
