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+==================================
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+Benchmarking tips
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+==================================
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+
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+
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+Introduction
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+============
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+
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+For benchmarking a patch we want to reduce all possible sources of
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+noise as much as possible. How to do that is very OS dependent.
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+
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+Note that low noise is required, but not sufficient. It does not
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+exclude measurement bias. See
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+https://www.cis.upenn.edu/~cis501/papers/producing-wrong-data.pdf for
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+example.
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+
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+General
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+================================
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+
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+* Use a high resolution timer, e.g. perf under linux.
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+
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+* Run the benchmark multiple times to be able to recognize noise.
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+
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+* Disable as many processes or services as possible on the target system.
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+
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+* Disable frequency scaling, turbo boost and address space
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+ randomization (see OS specific section).
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+
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+* Static link if the OS supports it. That avoids any variation that
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+ might be introduced by loading dynamic libraries. This can be done
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+ by passing ``-DLLVM_BUILD_STATIC=ON`` to cmake.
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+
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+* Try to avoid storage. On some systems you can use tmpfs. Putting the
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+ program, inputs and outputs on tmpfs avoids touching a real storage
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+ system, which can have a pretty big variability.
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+
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+ To mount it (on linux and freebsd at least)::
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+
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+ mount -t tmpfs -o size=<XX>g none dir_to_mount
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+
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+Linux
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+=====
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+
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+* Disable address space randomization::
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+
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+ echo 0 > /proc/sys/kernel/randomize_va_space
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+
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+* Set scaling_governor to performance::
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+
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+ for i in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
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+ do
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+ echo performance > /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
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+ done
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+
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+* Use https://github.com/lpechacek/cpuset to reserve cpus for just the
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+ program you are benchmarking. If using perf, leave at least 2 cores
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+ so that perf runs in one and your program in another::
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+
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+ cset shield -c N1,N2 -k on
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+
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+ This will move all threads out of N1 and N2. The ``-k on`` means
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+ that even kernel threads are moved out.
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+
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+* Disable the SMT pair of the cpus you will use for the benchmark. The
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+ pair of cpu N can be found in
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+ ``/sys/devices/system/cpu/cpuN/topology/thread_siblings_list`` and
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+ disabled with::
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+
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+ echo 0 > /sys/devices/system/cpu/cpuX/online
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+
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+
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+* Run the program with::
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+
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+ cset shield --exec -- perf stat -r 10 <cmd>
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+
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+ This will run the command after ``--`` in the isolated cpus. The
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+ particular perf command runs the ``<cmd>`` 10 times and reports
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+ statistics.
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+
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+With these in place you can expect perf variations of less than 0.1%.
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+
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+Linux Intel
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+-----------
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+
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+* Disable turbo mode::
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+
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+ echo 1 > /sys/devices/system/cpu/intel_pstate/no_turbo
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Rafael Espindola