Optimize recursive ls

src/uu/ls/BENCHMARKING.md

@@ -0,0 +1,29 @@
+# Benchmarking ls
+
+ls majorly involves fetching a lot of details (depending upon what details are requested, eg. time/date, inode details, etc) for each path using system calls. Ideally, any system call should be done only once for each of the paths - not adhering to this principle leads to a lot of system call overhead multiplying and bubbling up, especially for recursive ls, therefore it is important to always benchmark multiple scenarios.
+This is an overwiew over what was benchmarked, and if you make changes to `ls`, you are encouraged to check
+how performance was affected for the workloads listed below. Feel free to add other workloads to the
+list that we should improve / make sure not to regress.
+
+Run `cargo build --release` before benchmarking after you make a change!
+
+## Simple recursive ls
+
+-   Get a large tree, for example linux kernel source tree.
+-   Benchmark simple recursive ls with hyperfine: `hyperfine --warmup 2 "target/release/coreutils ls -R tree > /dev/null"`.
+
+## Recursive ls with all and long options
+
+-   Same tree as above
+-   Benchmark recursive ls with -al -R options with hyperfine: `hyperfine --warmup 2 "target/release/coreutils ls -al -R tree > /dev/null"`.
+
+## Comparing with GNU sort
+
+Hyperfine accepts multiple commands to run and will compare them. To compare performance with GNU sort
+duplicate the string you passed to hyperfine but remove the `target/release/coreutils` bit from it.
+
+Example: `hyperfine --warmup 2 "target/release/coreutils ls -al -R tree > /dev/null"` becomes
+`hyperfine --warmup 2 "target/release/coreutils ls -al -R tree > /dev/null" "ls -al -R tree > /dev/null"`
+(This assumes GNU sort is installed as `sort`)
+
+This can also be used to compare with version of ls built before your changes to ensure your change does not regress this

src/uu/ls/Cargo.toml

@@ -15,6 +15,7 @@ edition = "2018"
 path = "src/ls.rs"
 
 [dependencies]
+cached = "0.23.0"
 clap = "2.33"
 lazy_static = "1.0.1"
 number_prefix = "0.4"