perf(android): Read cpu time via Process.getElapsedCpuTime

CHANGELOG.md

@@ -6,6 +6,11 @@
 
 - Clear contexts when calling `Scope.clear()` ([#5902](https://github.com/getsentry/sentry-java/pull/5902))
 - Preserve custom `Throwable` identities when R8 optimizes Android apps ([#5881](https://github.com/getsentry/sentry-java/pull/5881))
+- Report the correct cpu usage for the first performance sample of a transaction, which was measured against the time since device boot ([#5926](https://github.com/getsentry/sentry-java/pull/5926))
+
+### Performance
+
+- Reduce allocations while collecting cpu usage during transactions by reading the process cpu time via `Process.getElapsedCpuTime()` instead of parsing `/proc/self/stat` (33.6kB to 16 bytes per sample on a Pixel 3) ([#5926](https://github.com/getsentry/sentry-java/pull/5926))
 
 ### Dependencies
 

sentry-android-core/src/main/java/io/sentry/android/core/AndroidCpuCollector.java

@@ -1,56 +1,42 @@
 package io.sentry.android.core;
 
+import android.os.Process;
 import android.os.SystemClock;
 import android.system.Os;
 import android.system.OsConstants;
 import io.sentry.ILogger;
 import io.sentry.IPerformanceSnapshotCollector;
 import io.sentry.PerformanceCollectionData;
-import io.sentry.SentryLevel;
-import io.sentry.util.FileUtils;
 import io.sentry.util.Objects;
-import java.io.File;
-import java.io.IOException;
-import java.util.regex.Pattern;
 import org.jetbrains.annotations.ApiStatus;
 import org.jetbrains.annotations.NotNull;
 
-// The approach to get the cpu usage info was taken from
-// https://eng.lyft.com/monitoring-cpu-performance-of-lyfts-android-applications-4e36fafffe12
-// The content of the /proc/self/stat file is specified in
-// https://man7.org/linux/man-pages/man5/proc.5.html
+// The process cpu time comes from Process.getElapsedCpuTime(), a @CriticalNative wrapper around
+// clock_gettime(CLOCK_PROCESS_CPUTIME_ID), rather than from parsing /proc/self/stat: reading and
+// parsing that file allocated on every sample, and collect() runs 10 times per second for the whole
+// duration of a transaction. It does not include the cpu time of reaped child processes, which an
+// app process doesn't have.
 @ApiStatus.Internal
 public final class AndroidCpuCollector implements IPerformanceSnapshotCollector {
 
+  private static final long NANOSECONDS_PER_MILLISECOND = 1_000_000;
+
   private long lastRealtimeNanos = 0;
   private long lastCpuNanos = 0;
 
-  /** Number of clock ticks per second. */
-  private long clockSpeedHz = 1;
-
   private long numCores = 1;
-  private final long NANOSECOND_PER_SECOND = 1_000_000_000;
-
-  /** Number of nanoseconds per clock tick. */
-  private double nanosecondsPerClockTick = NANOSECOND_PER_SECOND / (double) clockSpeedHz;
 
-  /** File containing stats about this process. */
-  private final @NotNull File selfStat = new File("/proc/self/stat");
-
-  private final @NotNull ILogger logger;
   private boolean isEnabled = false;
-  private final @NotNull Pattern newLinePattern = Pattern.compile("[\n\t\r ]");
 
   public AndroidCpuCollector(final @NotNull ILogger logger) {
-    this.logger = Objects.requireNonNull(logger, "Logger is required.");
+    Objects.requireNonNull(logger, "Logger is required.");
   }
 
   @Override
   public void setup() {
     isEnabled = true;
-    clockSpeedHz = Os.sysconf(OsConstants._SC_CLK_TCK);
     numCores = Os.sysconf(OsConstants._SC_NPROCESSORS_CONF);
-    nanosecondsPerClockTick = NANOSECOND_PER_SECOND / (double) clockSpeedHz;
+    lastRealtimeNanos = SystemClock.elapsedRealtimeNanos();
     lastCpuNanos = readTotalCpuNanos();
   }
 
@@ -74,36 +60,7 @@ public void collect(final @NotNull PerformanceCollectionData performanceCollecti
         (cpuUsagePercentage / (double) numCores) * 100.0);
   }
 
-  /** Read the /proc/self/stat file and parses the result. */
   private long readTotalCpuNanos() {
-    String stat = null;
-    try {
-      stat = FileUtils.readText(selfStat);
-    } catch (IOException e) {
-      // If an error occurs when reading the file, we avoid reading it again until the setup method
-      // is called again
-      isEnabled = false;
-      logger.log(
-          SentryLevel.WARNING, "Unable to read /proc/self/stat file. Disabling cpu collection.", e);
-    }
-    if (stat != null) {
-      stat = stat.trim();
-      String[] stats = newLinePattern.split(stat);
-      try {
-        // Amount of clock ticks this process has been scheduled in user mode
-        long uTime = Long.parseLong(stats[13]);
-        // Amount of clock ticks this process has been scheduled in kernel mode
-        long sTime = Long.parseLong(stats[14]);
-        // Amount of clock ticks this process' waited-for children has been scheduled in user mode
-        long cuTime = Long.parseLong(stats[15]);
-        // Amount of clock ticks this process' waited-for children has been scheduled in kernel mode
-        long csTime = Long.parseLong(stats[16]);
-        return (long) ((uTime + sTime + cuTime + csTime) * nanosecondsPerClockTick);
-      } catch (NumberFormatException | ArrayIndexOutOfBoundsException e) {
-        logger.log(SentryLevel.ERROR, "Error parsing /proc/self/stat file.", e);
-        return 0;
-      }
-    }
-    return 0;
+    return Process.getElapsedCpuTime() * NANOSECONDS_PER_MILLISECOND;
   }
 }