Run system_server benchmarks for 5 seconds

Especially for short running methods, this gives us better results and also
gives us more data for things like simpleperf to work on.

Since we don't know ahead of time how many times each benchmark will run, we had
to switch the mean and standard deviation formulas to one that supports running
updates. This has the advantage of making it constant space.

Bug: 140743821
Change-Id: I3e6b6b44d3bd573bad8afc8831226194147b108a

1 files changed, 15 insertions, 17 deletions

diff --git a/startop/apps/test/src/SystemServerBenchmarkActivity.java b/startop/apps/test/src/SystemServerBenchmarkActivity.java
index c370924c943c..61d43226c41e 100644
--- a/startop/apps/test/src/SystemServerBenchmarkActivity.java
+++ b/startop/apps/test/src/SystemServerBenchmarkActivity.java
@@ -35,7 +35,8 @@ import android.widget.TextView;
 import java.util.Arrays;
 
 class Benchmark {
-    public static final int NUM_ITERATIONS = 1000;
+    // Time limit to run benchmarks in seconds
+    public static final int TIME_LIMIT = 5;
 
     public Benchmark(ViewGroup parent, CharSequence name, Runnable thunk) {
         Context context = parent.getContext();
@@ -57,28 +58,25 @@ class Benchmark {
 
                 @Override
                 protected Object doInBackground(Object... _args) {
-                    long[] results = new long[NUM_ITERATIONS];
+                    long startTime = System.nanoTime();
+                    int count = 0;
 
                     // Run benchmark
-                    for (int i = 0; i < results.length; i++) {
-                        results[i] = -System.nanoTime();
+                    while (true) {
+                        long elapsed = -System.nanoTime();
                         thunk.run();
-                        results[i] += System.nanoTime();
-                    }
+                        elapsed += System.nanoTime();
 
-                    // Compute mean
-                    long sum = Arrays.stream(results).sum();
-                    resultMean = (double) sum / results.length;
+                        count++;
+                        double elapsedVariance = (double) elapsed - resultMean;
+                        resultMean += elapsedVariance / count;
+                        resultStdev += elapsedVariance * ((double) elapsed - resultMean);
 
-                    // Compute standard deviation
-                    double variance = 0;
-                    for (long i : results) {
-                        double t = (double) i - resultMean;
-                        variance += t * t;
+                        if (System.nanoTime() - startTime > TIME_LIMIT * 1e9) {
+                            break;
+                        }
                     }
-                    variance /= results.length - 1;
-
-                    resultStdev = Math.sqrt(variance);
+                    resultStdev = Math.sqrt(resultStdev / (count - 1));
 
                     return null;
                 }