Fix overhead measures based on real-life data

Author: emilk

README.md

@@ -30,7 +30,7 @@ fn my_function() {
 }
 ```
 
-The Puffin macros write data to a thread-local data stream. When the outermost scope of a thread is closed, the data stream is sent to a global profiler collector. The scopes are pretty light-weight, costing around 60 ns on an M1 MacBook Pro.
+The Puffin macros write data to a thread-local data stream. When the outermost scope of a thread is closed, the data stream is sent to a global profiler collector. The scopes are pretty light-weight, costing around 50-200 ns.
 
 You have to turn on the profiler before it captures any data with a call to `puffin::set_scopes_on(true);`. When the profiler is off the profiler scope macros only has an overhead of 1 ns on an M1 MacBook Pro (plus some stack space).
 

puffin_egui/src/lib.rs

@@ -533,10 +533,17 @@ impl ProfilerUi {
             let frame = frames.frames.first();
 
             let num_scopes = frame.meta.num_scopes;
-            let overhead_ms = num_scopes as f64 * 50.0e-6; // Around 50 ns per scope on an Apple M1.
-            if overhead_ms > 0.5 {
+            let realistic_ns_overhead = 200.0; // Micro-benchmarks puts it at 50ns, but real-life tests show it's much higher.
+            let overhead_ms = num_scopes as f64 * 1.0e-6 * realistic_ns_overhead;
+            if overhead_ms > 1.0 {
+                let overhead = if overhead_ms < 2.0 {
+                    format!("{:.1} ms", overhead_ms)
+                } else {
+                    format!("{:.0} ms", overhead_ms)
+                };
+
                 let text = format!(
-                    "There are {num_scopes} scopes in this frame, which adds up to ~{overhead_ms:.1} ms of overhead.\n\
+                    "There are {num_scopes} scopes in this frame, which adds around ~{overhead} of overhead.\n\
                     Use the Table view to find which scopes are triggered often, and either remove them or replace them with profile_function_if!()");
 
                 ui.label(egui::RichText::new(text).color(ui.visuals().warn_fg_color));