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Diffstat (limited to 'Documentation/scheduler/sched-energy.rst')
| -rw-r--r-- | Documentation/scheduler/sched-energy.rst | 33 |
1 files changed, 5 insertions, 28 deletions
diff --git a/Documentation/scheduler/sched-energy.rst b/Documentation/scheduler/sched-energy.rst index 8fbce5e767d9..70e2921ef725 100644 --- a/Documentation/scheduler/sched-energy.rst +++ b/Documentation/scheduler/sched-energy.rst @@ -82,7 +82,7 @@ through the arch_scale_cpu_capacity() callback. The rest of platform knowledge used by EAS is directly read from the Energy Model (EM) framework. The EM of a platform is composed of a power cost table per 'performance domain' in the system (see Documentation/power/energy-model.rst -for futher details about performance domains). +for further details about performance domains). The scheduler manages references to the EM objects in the topology code when the scheduling domains are built, or re-built. For each root domain (rd), the @@ -281,7 +281,7 @@ mechanism called 'over-utilization'. From a general standpoint, the use-cases where EAS can help the most are those involving a light/medium CPU utilization. Whenever long CPU-bound tasks are being run, they will require all of the available CPU capacity, and there isn't -much that can be done by the scheduler to save energy without severly harming +much that can be done by the scheduler to save energy without severely harming throughput. In order to avoid hurting performance with EAS, CPUs are flagged as 'over-utilized' as soon as they are used at more than 80% of their compute capacity. As long as no CPUs are over-utilized in a root domain, load balancing @@ -359,32 +359,9 @@ in milli-Watts or in an 'abstract scale'. 6.3 - Energy Model complexity ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ -The task wake-up path is very latency-sensitive. When the EM of a platform is -too complex (too many CPUs, too many performance domains, too many performance -states, ...), the cost of using it in the wake-up path can become prohibitive. -The energy-aware wake-up algorithm has a complexity of: - - C = Nd * (Nc + Ns) - -with: Nd the number of performance domains; Nc the number of CPUs; and Ns the -total number of OPPs (ex: for two perf. domains with 4 OPPs each, Ns = 8). - -A complexity check is performed at the root domain level, when scheduling -domains are built. EAS will not start on a root domain if its C happens to be -higher than the completely arbitrary EM_MAX_COMPLEXITY threshold (2048 at the -time of writing). - -If you really want to use EAS but the complexity of your platform's Energy -Model is too high to be used with a single root domain, you're left with only -two possible options: - - 1. split your system into separate, smaller, root domains using exclusive - cpusets and enable EAS locally on each of them. This option has the - benefit to work out of the box but the drawback of preventing load - balance between root domains, which can result in an unbalanced system - overall; - 2. submit patches to reduce the complexity of the EAS wake-up algorithm, - hence enabling it to cope with larger EMs in reasonable time. +EAS does not impose any complexity limit on the number of PDs/OPPs/CPUs but +restricts the number of CPUs to EM_MAX_NUM_CPUS to prevent overflows during +the energy estimation. 6.4 - Schedutil governor |