Add some tips on benchmarking.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303769 91177308-0d34-0410-b5e6-96231b3b80d8

docs/Benchmarking.rst

@@ -0,0 +1,87 @@
+==================================
+Benchmarking tips
+==================================
+
+
+Introduction
+============
+
+For benchmarking a patch we want to reduce all possible sources of
+noise as much as possible. How to do that is very OS dependent.
+
+Note that low noise is required, but not sufficient. It does not
+exclude measurement bias. See
+https://www.cis.upenn.edu/~cis501/papers/producing-wrong-data.pdf for
+example.
+
+General
+================================
+
+* Use a high resolution timer, e.g. perf under linux.
+
+* Run the benchmark multiple times to be able to recognize noise.
+
+* Disable as many processes or services as possible on the target system.
+
+* Disable frequency scaling, turbo boost and address space
+  randomization (see OS specific section).
+
+* Static link if the OS supports it. That avoids any variation that
+  might be introduced by loading dynamic libraries. This can be done
+  by passing ``-DLLVM_BUILD_STATIC=ON`` to cmake.
+
+* Try to avoid storage. On some systems you can use tmpfs. Putting the
+  program, inputs and outputs on tmpfs avoids touching a real storage
+  system, which can have a pretty big variability.
+
+  To mount it (on linux and freebsd at least)::
+
+    mount -t tmpfs -o size=<XX>g none dir_to_mount
+
+Linux
+=====
+
+* Disable address space randomization::
+
+    echo 0 > /proc/sys/kernel/randomize_va_space
+
+* Set scaling_governor to performance::
+
+   for i in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
+   do
+     echo performance > /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
+   done
+
+* Use https://github.com/lpechacek/cpuset to reserve cpus for just the
+  program you are benchmarking. If using perf, leave at least 2 cores
+  so that perf runs in one and your program in another::
+
+    cset shield -c N1,N2 -k on
+
+  This will move all threads out of N1 and N2. The ``-k on`` means
+  that even kernel threads are moved out.
+
+* Disable the SMT pair of the cpus you will use for the benchmark. The
+  pair of cpu N can be found in
+  ``/sys/devices/system/cpu/cpuN/topology/thread_siblings_list`` and
+  disabled with::
+
+    echo 0 > /sys/devices/system/cpu/cpuX/online
+
+
+* Run the program with::
+
+    cset shield --exec -- perf stat -r 10 <cmd>
+
+  This will run the command after ``--`` in the isolated cpus. The
+  particular perf command runs the ``<cmd>`` 10 times and reports
+  statistics.
+
+With these in place you can expect perf variations of less than 0.1%.
+
+Linux Intel
+-----------
+
+* Disable turbo mode::
+
+    echo 1 > /sys/devices/system/cpu/intel_pstate/no_turbo

docs/index.rst

@@ -90,6 +90,7 @@ representation.
    CodeOfConduct
    CompileCudaWithLLVM
    ReportingGuide
+   Benchmarking
 
 :doc:`GettingStarted`
    Discusses how to get up and running quickly with the LLVM infrastructure.