Try to reduce compiler build times

-j1 and -t12 builds take a long time (roughly 11 minutes) on production
hardware, which means that the two builds currently run by perf add up to 22
minutes. This still has roughly 2-6% variance on roughly identical compilers,
which is quite high. Try a -j2 -t3 build which hopefully ends up using separate
physical (not just virtual) cores for each rustc, and as such introducing
less variance from parallelism issues.

Author: Mark-Simulacrum

collector/src/bootstrap-rustc.rs

@@ -3,7 +3,7 @@ use std::env;
 use std::process::Command;
 
 fn run() -> i32 {
-    let server = Client::new(num_cpus::get()).expect("made jobserver");
+    let server = Client::new(num_cpus::get_physical() / 2).expect("made jobserver");
 
     let mut cmd = Command::new(env::var_os("RUSTC_PERF_REAL_RUSTC").unwrap());
     cmd.args(env::args_os().skip(1));

collector/src/execute/rustc.rs

@@ -96,7 +96,7 @@ fn record(
         // if multiple rustcs are competing for jobserver tokens, we introduce
         // quite a bit of variance. Instead, we configure -j1 here, and then
         // full all vCPU parallelism for each rustc.
-        .arg("-j1")
+        .arg("-j2")
         .arg("compiler/rustc"),
     )
     .context("building rustc")?;