shuf: update BENCHMARKING.md

Signed-off-by: Christian Menges <christian.menges@outlook.com>

src/uu/shuf/BENCHMARKING.md

@@ -4,23 +4,46 @@
 benchmark: with and without repetition.
 
 When benchmarking changes, make sure to always build with the `--release` flag.
-You can compare with another branch by compiling on that branch and than
+You can compare with another branch by compiling on that branch and then
 renaming the executable from `shuf` to `shuf.old`.
 
+## Generate sample data
+
+Sample input can be generated using `/dev/random`:
+
+```shell
+cat /dev/random | base64 | fold | head -n 50000000 > input.txt
+```
+
+To avoid distortions from IO, it is recommended to store input data in tmpfs.
+
 ## Without repetition
 
-By default, `shuf` samples without repetition. To benchmark only the
+By default, `shuf` samples without repetition. 
-randomization and not IO, we can pass the `-i` flag with a range of numbers to
+
-randomly sample from. An example of a command that works well for testing:
+To benchmark only the randomization and not IO, we can pass the `-i` flag with 
+a range of numbers to randomly sample from. An example of a command that works 
+well for testing:
 
 ```shell
 hyperfine --warmup 10 "target/release/shuf -i 0-10000000"
 ```
 
+To measure the time taken by shuffling an input file, the following command can
+be used::
+
+```shell
+hyperfine --warmup 10 "target/release/shuf input.txt > /dev/null"
+```
+
+It is important to discard the output by redirecting it to `/dev/null`, since
+otherwise, a substantial amount of time is added to write the output to the
+filesystem.
+
 ## With repetition
 
 When repetition is allowed, `shuf` works very differently under the hood, so it
-should be benchmarked separately. In this case we have to pass the `-n` flag or
+should be benchmarked separately. In this case, we have to pass the `-n` flag or
 the command will run forever. An example of a hyperfine command is
 
 ```shell