Let's be gentlemen's and share our knowledge for this rather popular benchmark.
Since I've started the thread, I will start with my share of tweaks I've been using (partially) for the last 2 years.
Operating System
* Preferred OS, in order of performance
Linux
MacOSX
Windows NT5 (XP / 2003)
Windows NT6 (Vista / 7 / 8 / 8.1)
* General OS Tweaks
For Windows platforms there are no specific requirements, other than having as few processes and services running. Do not try achieving this by using DIAGNOSTIC MODE, it will generally score lower than in normal mode with startup items disabled etc.
Command Line Parameters
* Using High Priority for unrar.exe
You can achieve this in various ways (up to your general knowledge). Since 99.99% of the users are using Windows for this benchmark, they could use:
start /high /b unrar bench test.rar
Make sure that launcher.exe was started with Administrator rights for this to work.
* Setting unrar.exe intensity
unrar bench test.rar -ri#
# is a number from 0 to 15
You can also set a sleep time (in milliseconds), which seems to also help
unrar bench test.rar -ri#
$ is a number representing the amount of milliseconds to sleep. It's up to you to test values, personally I've tried up to 10000.
* Changing the Instruction Set
It is important NOT to run using the "All Supported" option. General rule is to select the instruction set available before the newest supported. If your cpu supports SSE4, then SSE3 would be the preferred choice. The only exception here would be on CPUs with only SSE2 and i386, where you would definitely want to choose SSE2. Note that it's good practice to also test the latest instruction set, but it will rarely score any better.
On AMD processors use XOP if available (Bulldozer and newer). Note that XOP only works under:
Windows 7 SP1 / R2 SP2 / 8 / 8.1 OR Linux with kernels newer than 2.6.30
* Changing the Threads used
THIS IS NO LONGER ALLOWED, NOTHING TO SEE HERE.
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UPDATE1: 07/SEP/2014 Thread changing removed.