Resource: Q2.12 Name: Emulation of Asymmetrically Performing CPUs Date: 30-05-2012 Speaker: Rob Lee

1. Emulation of
Asymmetrically
Performing CPUs
Linaro Connect Q2.2012

2. Asymmetric CPU Performance?
● ARM's new big.LITTLE architecture is an
asymmetric multiprocessor system in which
two types of cores (which use the same
instruction set).
○ Higher Performance cores (Cortex A15)
Best performance
○ More Power Efficient cores (Cortex A7)
■ Best power efficiency, lower performance
● Similar asymmetric systems are also
possible by varying the manufacturing
processes of identical cores.

3. Emulation of Asymmetric CPU
Performance?
● Linux scheduler is work is underway to allow
intelligent scheduling for these type of
asymmetrical CPU systems and so testing of
these changes is needed.
● Actual hardware with this cpu configuration
isn't (readily) available.
● The performance differences that exists on
these asymmetric cpu systems can be
emulated which can hopefully provide a test
platform for some the scheduler work.

4. Host System Types for Running
Emulation
1. Multicore systems whose cpu frequencies
are coupled.
a. Cortex-A9
2. Multicore systems whose cpu frequencies
are independent from each other
a. modern X86, etc.
b. If in doubt, check
/sys/devices/system/cpu/cpu0/cpufreq/affected_cpus
.

5. Emulation Methods
1. Cycle Stealer - Steal the user programmed
percentage of cpu processing cycles in order
to degrade performance.
2. Frequency limiting- For systems whose cpu
frequencies are not couples, limit the
frequency of chosen cpus to degrade
performance.

6. Cycle Stealer Pros and Cons
Pros:
● Can be run on any multicore system
Cons:
● Subject to the overhead and accuracy of the
implementation.
● Current Implementation Cons:
○ Slices of time will be eaten. The smaller the slices
the more overhead. Only large slices of 10s of
milliseconds don't overly impact the overhead
○ Currently implemented userspace system doesn't
account for interrupt execution time

7. Frequency Limiter Pros and Cons
Pro:
● Simple, low overhead system.
Cons:
● Requires a system that supports
independent clock frequency changes.
● Most platforms only offer a few discrete
operating points. To achieve a very precise
level of performance degradation, you may
need to add additional CPUFreq operating
points.

8. CODE
git://git.linaro.org/people/rob_lee/asymm_cpu_emu.git
External Dependencies:
● schedtool
○ needed by cycle stealer to setting task priority

9. DEMO
● Cycle Stealer
a. In one terminal, run cycle_stealer_mgr.sh as root
b. In another terminal, run 'mpstat 2 -P ALL' to monitor
amount of cpu idle time.
● Frequency Limiter
a. In one terminal, run 'asymm_freq_limit.sh'
b. In another terminal run
■ cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_max_freq
c. test
■ taskset 2 ./busy.sh
■ cat /sys/devices/system/cpu/cpu1/cpufreq/scaling_cur_freq
● veryify frequency is limited.