Merge pull request #10 from chriselrod/pstates

Add docs discussing disabling turbo

Author: chriselrod

docs/make.jl

@@ -14,6 +14,7 @@ makedocs(;
     pages=[
         "Home" => "index.md",
         "Usage" => "usage.md",
+        "Turbo" => "turbo.md",
         "Memory Required for Large Matrices" => "memory-required.md",
         "Public API" => "public-api.md",
         "Internals (Private)" => "internals.md",

docs/src/turbo.md

@@ -0,0 +1,56 @@
+```@meta
+CurrentModule = BLASBenchmarks
+```
+
+# Disabling CPU Turbo
+
+Most recent CPUs have the ability to turbo, increasing their clock speeds for brief durations of time as thermal envelope and longer term power-use limitations allow. This is great for performance, but bad for benchmarking.
+
+If you're running Linux, it's probably easy to enable or disable turbo settings without having to reboot into your bios.
+The Linux Kernel Documentation is fairly thorough in discussing [CPUFreq](https://www.kernel.org/doc/html/v4.12/admin-guide/pm/cpufreq.html) and [intel_pstate](https://www.kernel.org/doc/html/v4.12/admin-guide/pm/intel_pstate.html) scaling drivers.
+
+To check those on my system, I can run:
+```sh
+> cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_driver
+intel_pstate
+intel_pstate
+intel_pstate
+intel_pstate
+intel_pstate
+intel_pstate
+intel_pstate
+intel_pstate
+```
+This tells me it is using `intel_pstate` in active mode.
+
+The documentation on `intel_pstate` mentions the `no_turbo` attribute:
+
+
+> If set (equal to 1), the driver is not allowed to set any turbo P-states (see Turbo P-states Support). If unset (equalt to 0, which is the default), turbo P-states can be set by the driver.
+
+This attribute is writable, so running
+```sh
+echo "1" | sudo tee /sys/devices/system/cpu/intel_pstate/no_turbo
+```
+disables turbo on this system. This, and closing programs that would compete for system resources (e.g., internet browsers; you can run `(h)top` to see if any processes are consuming non-negligible resources), should hopefully make benchmarking reasonably consistent and reliable.
+
+Finally, when I'm done benchmarking, I can reenable turbo by running:
+```sh
+echo "0" | sudo tee /sys/devices/system/cpu/intel_pstate/no_turbo
+```
+
+If your system does not use the `intel_pstate` driver, check for
+```sh
+/sys/devices/system/cpu/cpufreq/boost
+```
+discussed [here](https://www.kernel.org/doc/html/v4.12/admin-guide/pm/cpufreq.html#frequency-boost-support) in the kernel documentation. If the file is present, you should be able to disable boost with
+```sh
+echo "0" | sudo tee /sys/devices/system/cpu/cpufreq/boost
+```
+and then reenable with
+```sh
+echo "1" | sudo tee /sys/devices/system/cpu/cpufreq/boost
+```
+
+In either case, you may find it convenient to place these snippets in `#! /bin/bash` scripts for conveniently turning your systems boost on and off as desired.
+