sum: read ahead for perf for 1st run

Cargo.lock

@@ -4185,6 +4185,7 @@ version = "0.9.0"
 dependencies = [
  "clap",
  "fluent",
+ "rustix",
  "uucore",
 ]
 

src/uu/sum/Cargo.toml

@@ -21,6 +21,7 @@ doctest = false
 
 [dependencies]
 clap = { workspace = true }
+rustix = { workspace = true }
 uucore = { workspace = true }
 fluent = { workspace = true }
 

src/uu/sum/src/sum.rs

@@ -89,6 +89,8 @@ fn open(name: &OsString) -> UResult<Box<dyn Read>> {
             ));
         }
         let f = File::open(path).map_err_context(String::new)?;
+        #[cfg(any(target_os = "linux", target_os = "android", target_os = "freebsd"))]
+        let _ = rustix::fs::fadvise(&f, 0, None, rustix::fs::Advice::Sequential);
         Ok(Box::new(f) as Box<dyn Read>)
     }
 }