fix a units error, log more about parallelism determination

Author: iximeow

phd-tests/runner/src/main.rs

@@ -88,6 +88,12 @@ fn guess_max_reasonable_parallelism(
     // than the test runner itself. Overprovisioning CPUs will make everyone
     // sad but should not fail tests, at least...
     let lim_by_cpus = online_cpus / cpus_per_runner;
+    into!(
+        "parallelism by cpu count: {} ({} / {})",
+        lim_by_cpus,
+        online_cpus,
+        cpus_per_runner
+    );
 
     let ctl = bhyve_api::VmmCtlFd::open()?;
     let reservoir =
@@ -109,7 +115,9 @@ fn guess_max_reasonable_parallelism(
             .try_into()
             .expect("physical page count is reasonable");
 
-        let installed_mb = page_size * total_pages;
+        const MiB: usize = 1024 * 1024;
+
+        let installed_mb = page_size * total_pages / MiB;
         // /!\ Arbitrary choice warning /!\
         //
         // It would be a little rude to spawn so many VMs that we cause the
@@ -119,7 +127,7 @@ fn guess_max_reasonable_parallelism(
         // fraction.
         vmm_mem_limit = installed_mb / 4;
 
-        eprintln!(
+        warn!(
             "phd-runner sees the VMM reservior is unconfigured, and will use \
              up to 25% of system memory ({}MiB) for test VMs. Please consider \
              using `cargo run --bin rsrvrctl set <size MiB>` to set aside \
@@ -129,6 +137,12 @@ fn guess_max_reasonable_parallelism(
     }
 
     let lim_by_mem = vmm_mem_limit / memory_mib_per_runner;
+    into!(
+        "parallelism by memory: {} ({} / {})",
+        lim_by_mem,
+        vmm_mem_limit,
+        memory_mib_per_runner
+    );
 
     Ok(std::cmp::min(lim_by_cpus as u16, lim_by_mem as u16))
 }