1==================================
2Benchmarking tips
3==================================
4
5
6Introduction
7============
8
9For benchmarking a patch we want to reduce all possible sources of
10noise as much as possible. How to do that is very OS dependent.
11
12Note that low noise is required, but not sufficient. It does not
13exclude measurement bias. See
14https://www.cis.upenn.edu/~cis501/papers/producing-wrong-data.pdf for
15example.
16
17General
18================================
19
20* Use a high resolution timer, e.g. perf under linux.
21
22* Run the benchmark multiple times to be able to recognize noise.
23
24* Disable as many processes or services as possible on the target system.
25
26* Disable frequency scaling, turbo boost and address space
27  randomization (see OS specific section).
28
29* Static link if the OS supports it. That avoids any variation that
30  might be introduced by loading dynamic libraries. This can be done
31  by passing ``-DLLVM_BUILD_STATIC=ON`` to cmake.
32
33* Try to avoid storage. On some systems you can use tmpfs. Putting the
34  program, inputs and outputs on tmpfs avoids touching a real storage
35  system, which can have a pretty big variability.
36
37  To mount it (on linux and freebsd at least)::
38
39    mount -t tmpfs -o size=<XX>g none dir_to_mount
40
41Linux
42=====
43
44* Disable address space randomization::
45
46    echo 0 > /proc/sys/kernel/randomize_va_space
47
48* Set scaling_governor to performance::
49
50   for i in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
51   do
52     echo performance > /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
53   done
54
55* Use https://github.com/lpechacek/cpuset to reserve cpus for just the
56  program you are benchmarking. If using perf, leave at least 2 cores
57  so that perf runs in one and your program in another::
58
59    cset shield -c N1,N2 -k on
60
61  This will move all threads out of N1 and N2. The ``-k on`` means
62  that even kernel threads are moved out.
63
64* Disable the SMT pair of the cpus you will use for the benchmark. The
65  pair of cpu N can be found in
66  ``/sys/devices/system/cpu/cpuN/topology/thread_siblings_list`` and
67  disabled with::
68
69    echo 0 > /sys/devices/system/cpu/cpuX/online
70
71
72* Run the program with::
73
74    cset shield --exec -- perf stat -r 10 <cmd>
75
76  This will run the command after ``--`` in the isolated cpus. The
77  particular perf command runs the ``<cmd>`` 10 times and reports
78  statistics.
79
80With these in place you can expect perf variations of less than 0.1%.
81
82Linux Intel
83-----------
84
85* Disable turbo mode::
86
87    echo 1 > /sys/devices/system/cpu/intel_pstate/no_turbo
88