mirror of https://github.com/google/benchmark.git
101 lines
3.6 KiB
Markdown
101 lines
3.6 KiB
Markdown
# Reducing Variance
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<a name="disabling-cpu-frequency-scaling" />
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## Disabling CPU Frequency Scaling
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If you see this error:
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```
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***WARNING*** CPU scaling is enabled, the benchmark real time measurements may be noisy and will incur extra overhead.
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```
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you might want to disable the CPU frequency scaling while running the
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benchmark, as well as consider other ways to stabilize the performance of
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your system while benchmarking.
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See [Reducing Variance](reducing_variance.md) for more information.
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Exactly how to do this depends on the Linux distribution,
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desktop environment, and installed programs. Specific details are a moving
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target, so we will not attempt to exhaustively document them here.
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One simple option is to use the `cpupower` program to change the
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performance governor to "performance". This tool is maintained along with
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the Linux kernel and provided by your distribution.
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It must be run as root, like this:
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```bash
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sudo cpupower frequency-set --governor performance
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```
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After this you can verify that all CPUs are using the performance governor
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by running this command:
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```bash
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cpupower frequency-info -o proc
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```
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The benchmarks you subsequently run will have less variance.
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<a name="reducing-variance" />
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## Reducing Variance in Benchmarks
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The Linux CPU frequency governor [discussed
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above](user_guide#disabling-cpu-frequency-scaling) is not the only source
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of noise in benchmarks. Some, but not all, of the sources of variance
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include:
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1. On multi-core machines not all CPUs/CPU cores/CPU threads run the same
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speed, so running a benchmark one time and then again may give a
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different result depending on which CPU it ran on.
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2. CPU scaling features that run on the CPU, like Intel's Turbo Boost and
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AMD Turbo Core and Precision Boost, can temporarily change the CPU
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frequency even when the using the "performance" governor on Linux.
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3. Context switching between CPUs, or scheduling competition on the CPU the
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benchmark is running on.
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4. Intel Hyperthreading or AMD SMT causing the same issue as above.
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5. Cache effects caused by code running on other CPUs.
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6. Non-uniform memory architectures (NUMA).
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These can cause variance in benchmarks results within a single run
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(`--benchmark_repetitions=N`) or across multiple runs of the benchmark
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program.
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Reducing sources of variance is OS and architecture dependent, which is one
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reason some companies maintain machines dedicated to performance testing.
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Some of the easier and and effective ways of reducing variance on a typical
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Linux workstation are:
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1. Use the performance governor as [discussed
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above](user_guide#disabling-cpu-frequency-scaling).
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1. Disable processor boosting by:
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```sh
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echo 0 | sudo tee /sys/devices/system/cpu/cpufreq/boost
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```
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See the Linux kernel's
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[boost.txt](https://www.kernel.org/doc/Documentation/cpu-freq/boost.txt)
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for more information.
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2. Set the benchmark program's task affinity to a fixed cpu. For example:
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```sh
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taskset -c 0 ./mybenchmark
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```
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3. Disabling Hyperthreading/SMT. This can be done in the Bios or using the
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`/sys` file system (see the LLVM project's [Benchmarking
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tips](https://llvm.org/docs/Benchmarking.html)).
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4. Close other programs that do non-trivial things based on timers, such as
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your web browser, desktop environment, etc.
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5. Reduce the working set of your benchmark to fit within the L1 cache, but
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do be aware that this may lead you to optimize for an unrelistic
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situation.
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Further resources on this topic:
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1. The LLVM project's [Benchmarking
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tips](https://llvm.org/docs/Benchmarking.html).
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1. The Arch Wiki [Cpu frequency
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scaling](https://wiki.archlinux.org/title/CPU_frequency_scaling) page.
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